fi EIGENMANN FRONTISPIECE Entrance of Ariguanabo River, Cuba, to its underground channel at San Antonio de los Banos. Blmd-nsn rocks at base of Point Loma, San Diego, California. CAVE VERTEBRATES OF AMERICA A STUDY IN DEGENERATIVE EVOLUTION BY CARL H. EIGENMANN PROFESSOR OF ZOOLOGY, INDIANA UNIVERSITY WASHINGTON, D. C. PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON JUNE, 1909 CARNEGIE INSTITUTION OF WASHINGTON PUBLICATION No. 104 "Xnitonnfi )0rrs» 3. 8. Cushlng Co. — P.. i u 11 k .^ ^inlth Co. \>iru t. M:i- - .. I ^ \ CONTENTS. PAGE Preface . vii Introductory ........ ....... vii Acknowledgments ............ . . viii Conclusions of General Import ............. ix A General Consideration of Caves and the Cave Fauna ......... I Caves and the Cave Fauna ....... .3 Caves in their Relation to the Rest of the Universe ......... 3 The Nature of the Cave Environment ........... 5 The Blind Vertebrates and Cave Vertebrates of North America . ..... 9 The Origin and Dispersal of Cave Animals . .......... 12 The Origin of the Food Supply of Caves ........... 17 Age of Caves in their Relation to the Variety of Cave Fauna . . . . . . .17 Divergence in Epigean and Convergence in Subterranean Fishes . . . . . .18 Conclusions ........ .21 Blind and Cave Vertebrates and their Eyes ........... 23 Mammals ........ 25 Eyes of the Common Mole ... ....... 25 The Cave Rat and its Eyes ....... ..... 26 The Cave Salamanders ............... 28 The eyes of ' Typlilomolge rathbuiu . . • • • • • . • • • .31 Sclera and Choroid .............. 32 Pigment Layer Exclusive of the Irideal Parts ......... 33 Iris and Ora Serrata .............. 33 The Retina 35 The eyes of TypUotriton spcheits Stejneger .......... 36 Conclusions as to the eye of Typlilotriton spelaus ......... 40 Summary in regard to TypUotriton . . . . . . . . . . . .41 The Blind Reptiles 42 Amphisbtzna puiictata .............. 42 Methods ................ 42 General Account of the Eye ............ 42 Minute Anatomy of the Eye ............ 43 Rlrinenra floridana ............... 48 Habits of Rliiiieiirij .............. 48 General Account of the Eye of Rhiiieitra .......... 49 Minute Anatomy of the Eye of ' Rhineiira .......... 50 Typhlops liimbricalis ............... 54 General Account of the Eyes of Snakes .......... 54 Eyes of Typhlops vermicidaris ............ 55 Eyes of Typhlops himbriialis ............ 56 Conclusions as to the eyes of Blind Reptiles .......... 59 Amphisb/zna ............... 59 Rhineura ................ 59 Typhlops ................ 60 Eyes of Cyclostome polistotrema stoitti . . . . . . . . . . . .61 Fishes .................. 62 General Remarks on the Eyes of Fishes ........... 62 The eyes of Zygonectfs notatus ............. 64 Typhlogobius : The Point Loma Blind Fish and its Relatives ....... 65 The Eyes of the Blind Catfish, Anieiurus nigrilabris ......... 69 iii IV CONTENTS. Fishes — continued I-AGR The Amblyopsidae 7° Relationships of the Amblyopsidae 7° Distribution of the Ami ilyopsidas ... 71 Amblyopsis spelaits . . 7 1 Troglichtliys rflsa- .............. 72 Typltlichthys ... 72 TypMichthys siiHerraiifiis • • 73 Typhlichtliys osborni 74 Typhlichthys -wyamiiittf 75 ............. 75 >- (iiipilliffrus 75 • • 76 The color of the Amblyopsidse ... ..... 76 General habits <>l .7//// 80 Respiration ................ 81 Feeding habits of. 4ni/>lyi>/>sis . . . . . 81 Habits of t'/n>fi>s,'iis/?>- . 85 Reactions t.. Light . 87 Breeding habits of Anihlyvpsis ............. 92 Rivalry of Males and Secondary Sexual Differences .... .... 93 The Egg and Central Development of Amblyopsis . . . . 94 The Migration of the Anus .... 95 The Tactile Organs 96 The Ear of Aiiihlynpsis .............. ico Dots . l»i/>ly<>p*ii • " hear "? . . . . . . . • . .102 The Brain nl ' Ain/'l\'i>psis 106 < onclusions on the Amblyopsidze . 109 The eyes of the Amblyopsidae . . . . . • • • • • • • .no Cholo%aster papillifenu ...... . ... no c7/«/«;w >Av iii,rii\.\i:ii ..... . . . . 116 (."//"/( V' v/'v' turniilns ..... ... 117 Typhlichthys suiterraneus ............. 120 Troglichthys rostz .............. 126 sl/>i/i/\'n/>\j.>i s/iftirns . . . . . • • . • • • • • 134 Summary of the Eyes of the Amblyopsidae . .......... 145 I >i velopment and Later History of the Eye of Amblyopsis ........ 147 Growth of the Eye from Time of its Appearance . . . 157 History of the Lens .............. 158 Histmy "I tin- Si 1. r.il (', i Milages 158 History of the Optic Nerve ............. 159 History of the Development, Maturity, and Degeneration of the Eye ..... 160 ( 'omparativt: K.it. ni ( intn^, nctic and 1'hyliigenetic Degeneration of the parts of the Eye 164 The KutiiiT i if the Eye 166 Retardation and Cutting off of Late Stages of the Development of the Eye .... 166 Retardation and Cessation in the Development of the Eye ..... 167 Thi I .. i -s r>f .-/wM'()/>v/v .UK! the law of Biogenesis ........ 170 CoiH-hision ......... . 173 (icnt-ral Snmniarial Accmini nl the eyes of the Amblyopsidae ....... 175 rinli-tic I ' the eye of the Amblyopsidse . ....... 175 Ri -Milts ni I'hvli'tu Degeneration on the Different Parts of the Eyes of the Amblyopsidse . 177 Out. ration ............. 180 I'l.n , I'hylctic Degeneration in the Amblyopsidae ...... i.So The i nd Fishes . 183 185 lion of Stygicola and Ltiafugu ......... 187 1'iini Secondary Sexual Characters 187 •iilintii.il lit .s'/i-sv ,•!.! .mil /,„, i/'n^i .......... 188 CONTENTS. V The Cuban Blind Fishes — continued PAGE Nature of the Habitat of Stygicola and Lucifuga . . . . . . . . .188 Abundance of Stygicola and Lucifuga . . . . . . . . . . 197 Origin of the Cuban Blind Fishes ........... 197 Physical environment of Stygicola and Lucifuga and their Reactions to it . . . .198 Biological environment of Stygicola and Lucifuga ........ 201 General habits of Lucifuga and Stygicola .......... 204 Breeding habits of Lucifuga and Stygicola .......... 204 The Ovaries of Lucifiiga and Stygicola .......... 206 The Eyes of Lucifttgti 208 The Eyes of Stygicola .............. 220 On the Ovary and Ova in Lucifiiga and Stygicola .......... 226 Conclusions in Regard to Lucifiiga and Stygicola ......... 232 The causes of Individual and Phyletic Degeneration .......... 233 LIST OF PLATES. FACING PAGE Frontispiece. Entrance to Ariguanabo River, Cuba. Blind-fish rocks at base of Point Loma, San Diego, California ............. Title Plate A. Twin and Shawnee Caves ............ 6 1. Chologaster papilliferus, Spelerpes inaciilicamici, Spelerpes stejnegeri, and Typhlotriton spelceus ............... 12 2. Spelerpes longicauda and Typhloiuolge rathbinii ........ 28 3. Rhineura floridana ............. 48 4. Eye of Typhlops litmbricalis ........... 54 5. Amblyopsis ............... 70 6. Chologaster agassizii, Troglichthys rose?, and Typhlichthys subterraiieus ... 72 7. Views of Amblyopsis, early stages .......... 92 8. Tactile organs of Amblyopsis and Chologaster . .98 9. Heads oiZygonectes not at its, Chologaster agassizii, Chologaster papillifcr us, Typhlichthvs subterraiieus, Troglichthys rosce, and Amblyopsis spelaus . . . . .no 10. Photographs of the eyes of Amblyopsis and Troglichthys ...... 132 11. Carboneria Beach near Matanzas. Cave of the Insurrectos, near the Carboneri.i . . 186 12. Young of Lucifiiga in Ashton Cave. Cave Isabella, showing roots .... 190 13. Stygicola. (Preserved specimens) .......... 196 14. Living Stygicolas ............. 200 1 5 . Views of Lucifiiga ............. 200 16. Sections of eye of Lucifuga ............ 208 17. Two sections through right eye of Lucifuga ........ 208 18. Sections of eye of Lucifiiga, showing contents of lens, capsule, and layers of retina . 208 19. Eyes of Lucifiiga, showing pigment layer and retina and folding of sclera . . . 208 20. Eyes of Lucifuga, showing differences in size and structure . . . . . .216 21. Sections through left and right eye-cavities of Lucifuga ...... 216 22. Sections of eyes of Lucifuga, showing pigment layer and cells and oblique and rectus muscles . . . . . . . . . . . . . . .216 23. Eye of old Lucifuga, showing pigment mass and fibrillar network about eye . . 216 24. Eye of Lucifuga .............. 222 25. Eye of Stygicolas and Liicifitgas ........... 222 26. Eye of Stygicola .............. 222 27. Ovaries of Lucifuga and Stygicola .......... 232 28. Sections of ovaries ............. 232 29. Sections of ovaries ............. 232 KEY TO DESCRIPTION OF PLATES AND TEXT FIGURES. 1. Pigment epithelium. pi. Densest pigmented seetion of the pigment epithe- lium, just below the nucleus. 2. Roils and cones. ;. ( 'uter nuclear layer. 4. Outer reticular layer. 5. Horizontal cells. 6. Inner nuclear layer. 7. Spongiublastic layer. 8. Inner reticular layer. 9. Ganglionic layer. 10. Optic-liber layer. a. o. Ophthalmic artery. ,i»i . Ameloid bodies of the pigment epithelium. b. Brille. bac. Rod. ci. p. Ciliary process. cj. Conjunctiva. cj. s. Conjunctiva! sac. chr. or chd. < Imp ml. chr. I. Choroidal lymph. chr.f. Choroidal fissure. en. Cones. en. »/. Cone nuclei. cor. or crn. Cornea. cfs. or tfl. sng. Blood-corpuscles in normal vessels. (/>,*. x. Stagnant blood-corpuscles. l i mie. /•'. c/. I'Wnix conjunctiva. . ' ill. ii I "iv pit. lid. or //i1/. Hyaloid membrane. //. gl. Hardcr's gland. i. Iris. ;'. i. ( )ul< i layer of iris. I. 2. Inner layrt nl ins. ltd |i..l.lli'i| I rlls. /. I /.', /.-, /.:: I'irsl, secornl, and third labial sc;di . Ins. or /. Lens. /. c. Lens capsule. A/. Miillerian nuclei. in. m. Middle member of cone. nisi:, or mil. Kye muscle. ;;/. Nucleus. nl. con. Cone nuclei. nl. f. Nuclear fragments. n/. £. Nuclei of the ganglionic cells. «/. /. or /7/.1 Elongate nuclei of the pars ciliaris. /;/. Musi. Miillerian nuclei. ;/. t'/1. < >ptic nerve. ;;. .t. Nasal scale. e T. Torralbas, Prof. Carlos de la Torre, Mr. Pascual Ferreiro, Dr. Felix Garcia, and the Director of the Cuban Agricultural Station, Prof. F. S. Earl, assisted me materially in various ways. The assistance of my friend, Mr. Francesco Martinez, lias been invaluable. s at the margin of the cave region of Cuba, and in the interval between our trips he ferreted out unsuspected raves, determined their rich- ne-s in blind fishes, and put himself at our disposal in guiding us to his various finds. Prof. D. \Y. Dermis of Karlham College, Richmond, Indiana, made the micro- photOgraphs in a manner to leave nothing desired (plates 9, 10, 16-23). PREFACE. IX Mr. Lewis H. Wild, under the direction of Prof. J. Reighard, made a series of photographs of entire eggs and embryos (plate 7). Mr. Samuel Garman sent me my first specimens of the blind fish, Troglichthys. Dr. B. W. Evermann of the Bureau of Fisheries and the late Prof. W. Norman secured me specimens of Typhlomolge. Prof. Wm. Roux, Dr. F. R. Lillie, and others kindly consented to the repub- lication of articles issued in the journals under their editorship. I desire also to express my high appreciation of the interest taken by the authori- ties of Indiana University, especially by President William Lowe Bryan, in the various trips and plans necessary to bring this work to a successful conclusion. The present work forms No. 97 of the Contributions from the Zoological Lab- oratory of the Indiana University. Finally, I wish to express my indebtedness to her who as Rosa Smith guided me to the blind-fish rocks at the base of Point Loma, and who as Mrs. R. S. Eigen- mann collected for me at the same place, has acted as editor of the various papers that have appeared, and through the twelve years during which my leisure has largely gone to the blind vertebrates has ever been ready with advice, encour- agement, and assistance. CONCLUSIONS OF GENERAL] IMPORT. (1) The bleached condition of animals living in the dark, an individual envi- ronmental adaptation, is transmissible and finally becomes hereditarily fixed. (See page 80.) (2) Ornamental secondary sexual characters not being found in blind fishes are, when present, probably due to visual selection. (See page 94.) (3) Individual degeneration of the eye may begin in even earlier stages of development until nearly the entire development becomes affected, that is, func- tional adaptations are transmissible. (See pages 172 and 235.) A GENERAL CONSIDERATION OF CAVES AND THE CAVE FAUNA CAVES A XL) THE CAVE FAUNA. CAVES IN THEIR RELATIONS TO THE REST OF THE UNIVERSE. The environment favorable to animal life is limited to a thin layer of water, earth, and air. From its deepest to its most elevated point this layer does not much exceed 10 miles ' in thickness. At no particular point does it exceed much more than half this thickness ; and usually the layer is but a few feet thick. About half the total thickness is below sea-level and the other half above it. The places where the ocean has a depth of 5 miles are few, but in these places the greatest depth of possible environment is found. The favorableness of the environment diminishes rapidly with the depth. The depth of the possible environment at any point on land above the surface is very limited, and beneath the surface it depends on conditions ; solid rocks may limit it to the surface and soil may permit mam- mals, and especially insects, to burrow several feet beneath the surface. Under- ground watercourses, which are caves in the formation, may enable animals to live several hundred feet beneath the surface of the ground. The animals thrown out by artesian wells attest this. Typhlomolge is occasionally thrown out of the artesian well 190 feet deep at San Marcos, Texas. The plant environment stops at the surface of the ground ; 2 animal life diminishes rapidly within a few feet of the surface unless trees cover the ground. Animal environment definitely stops at the tops of trees, though the air above them may be temporarily visited. While the depth of the environment at any point is only a few feet on land, because the surface of the land itself rises to a few miles above sea-level, the total depth of the environment above sea-level is considerable. The fauna rapidly diminishes in either direction from sea-level, and were it not that the extreme limits of the environment, above and below, furnish rare, sometimes peculiarly adapted forms, sometimes relicts, the numbers of individuals and types found would not repay the exploration of the ocean depths and mountain heights. Since the environment varies within the limits of the possible existence of living matter, from the extreme of wetness and dryness, of heat and cold, of depth and height, of light and dark, etc., we may divide the environment into many distinct units within which the conditions are similar or alike. It is profitable at present to call attention only to discontinuous and continuous units of environment. Similar or identical conditions may stretch uninterruptedly in one or more directions indefi- nitely, permitting the free movement of its inhabitants from one part to another. The continuous unit of environment of greatest extent is furnished by the ocean at considerable depths. Light and temperature conditions and seasonal fluctuations are reduced to the minimum and are nearly uniform under the whole surface of the ocean, furnishing an ideal of the type of the continuous environment. This particular environment is continuous not only as to space, but also as to time. The surface of the ocean forms an equally continuous area, but because tem- perature and light conditions differ greatly in different parts of the globe we must here deal not with a single but with several distinct units of environment, each large in extent. If we assume the conditions in the north polar sea to be identical with 1 Highest mountain, deepest ocean. - Some fungi are found in caves. 3 4 BLIND VERTEBRATES AND THEIR EYES. those of the south polar sea, these form a discontinuous unit of environment, a unit whose parts do not form a portion of a continuous area and whose inhabitants can not migrate from one part to the other. If we assume the conditions in the equatorial Atlantic to be the same as those of the equatorial Pacific, we are again dealing with a discontinuous unit — discontinuous because the inhabitants of one part can not migrate to the other. If we examine these two units more closely, it becomes evident that the Arctic and Antarctic oceans have always formed a discontinuous unit. Arctic conditions have never prevailed between the two. On the other hand, the equatorial Atlantic and the equatorial Pacific were formerly connected in Colombia and formed one continu- ous environment. The land area and the fresh waters near the equator from Para to the Andes form a continuous unit of environment, and the Galapagos Islands to the west of it form a discontinuous unit, each separate island forming a continuous unit of a smaller order. It is evident that there are degrees of discon- tinuity, depending in part on the length of time the discontinuity has existed, and in part on the space separating the nearest parts of the unit. Caves are discontinuous units of environment whose elements have always U-eii separate. It is possible that in some areas a large complex of different under- ground channels exists. An east to west fault has lowered the southern part of Texas, or has raised the northern part, many feet. The dividing line is an abrupt escarpment across the State. This fault has favored the formation of underground watercourses, and inasmuch as river valleys do not cut down to the underground channels, it is possible that they form a network of channels or a continuous unit which permits the ready migration of its inhabitants from one part to another. The lower area on the southern slope of Cuba, between Canas on the west and an undetermined point east of Union, is drained by underground rivers. No valleys cut down to these rivers, and since this part of Cuba has sunk in recent times, the land being only a few feet above sea-level, it is possible that we again have a complex of underground channels permitting the migration of its inhabit- ants. However, it is also possible that the streams run in separate courses. The absence of Lucifnga from the eastern caves favors this hypothesis. At best we have here several degrees of continuity. The large streams cut the cave region of Kentucky, Indiana, and Missouri into sections, their beds lying deeper than the caves. These caves are, therefore, part of a discontinuous environment. These facts must be constantly borne in mind in considering the origin and dispersal of cave faunas. It is quite out of the question in this connection to give even a partial list of North American caves, or an account of the North American cave regions. The region to which Mammoth Cave belongs reaches from near Bloomington, Indiana, through Kentucky into Tennessee and embraces many thousand square miles of territory. (July the larger streams whose rapidly deepening channels have made the caves possible How on (he surface. "One may travel on horseback all day, through certain parts of Kentucky, without crossing a single running stream ; all the rainwater that falls being carried down through the sinkholes into caverns rhere are the gathering beds that feed the few large open streams of the ,vhich ('liven River is an example. It is reported that there are 4,000 oo known caverns in Rdmondson County (Kentucky) alone." * Foran'' tli Vmerica see 1 l<»v,v, OMirat.-d American Caverns, Cin- ive Fauna of North America, Memoirs of the Nat. Acad. Sci. vol. 4, i CAVE ENVIRONMENT. THE NATURE OF THE CAVE ENVIRONMENT. Each cave is a distinct unit of environment and needs special consideration. In the present work we can deal only with the general features of this environment. The chief element for consideration is the absence or reduction of the amount of light and the relative constancy of other physical conditions. On this basis a cave may be divided into three regions: (i) the twilight region just within the cave, bounded by the distance to which light penetrates from without — this part shades generally from epigean conditions to the real cave conditions; (2) the region of fluctuating temperatures ; (3) the inner cave region . These different sections occupy greatly variable parts of different caves. In Mammoth Cave the twilight region is large enough to contain a tennis court and reaches some distance beyond the "iron door." Some Cuban caves are entirely of the twilight character, usually containing an abundant fauna, consisting largely of occasional, regular, or accidental visitors from the outside. The second region in Mammoth Cave reaches to the Mammoth Dome. On a cold winter day I found ice stalagmites on the floor of the entrance gallery just before it enters the dome. In certain of the ice caves the entire portion beyond the twilight area may belong to this section. In caves of the tropics, on the other hand, it may not exist at all. The third part is the cave par excellence — the inner section, but little influenced by external conditions. Here there is absolute darkness at all times, both day and night, summer and winter following each other without very decided change in temperature. The temperature differs in the various parts of the same cave and also changes slightly with the seasons. In the center of the Shawnee Cave at Mitchell the fluc- tuations in temperature during a week do not equal the error of the recording ther- mograph arising from unequal trimming of the paper, the absorption of water, etc. The total fluctuation during a year is 2.2° C. It is remarkable that this record of cave temperature is taken in a cave open at both ends with a current of air flowing through it at times. The instrument is placed where it would be least affected by these currents, that is, in a large room near the center of the cave about 15 feet above water-level. Glacieres, or ice caves, are found in various places. They exist wherever the prevailing direction of the winds and nature of the cave causes a strong inflow of air during the winter, reducing the temperature to below the freezing point. The summer winds do not blow in the same direction, and convection currents are pre- vented by the nature of the cave.1 Between June, 1906, and February, 1908, the fluctuations in the temperature in the water where it leaves Shawnee Cave ranged from a maximum of 17.3° C. to 7.4°, or through about 10° C.2 1 A very extensive list and excellent account of glacieres is given by Balch in his Glacieres or Freezing Caverns, 1900. Concerning the cause of glacieres, he says, on page 148: "The cold air of winter sinks into and permeates the cave, and in course of time freezes up all the water which, in the shape of melting snow or cold winter rain or spring water, finds its way in; and once ice is formed it remains long after ice in the surrounding open country has melted away, because heat penetrates with difficulty into the cave." 2 This range becomes interesting when compared with the range of temperatures in a lake. Professor Birge gives the ranges of the water at the surface and at the depth of 18 m. for Lake Mendota: Surface, 1895 . . . o° to 24° Bottom, 1895 • • • i-5° t° I7-I° Surface, 1896 . . . o° to 26° Bottom, 1896 ... 2° to 16° G BLIND VERTEBRATES AND THEIR EYES. Conditions of moisture, while practically uniform in some parts of caves, fluc- tuate in others more than any other element of environment. The maximum degree of moisture is naturally found in the pools and streams. On the other hand, in the upper parts of Mammoth and Wyandotte Caves the dust lies undisturbed for years. In Mammoth Cave the tracks of oxen made in 1860 are now shown to visitors, and I am told that in Wyandotte the still older tracks of the moccasined Indians are perceptible to-day. There are, however, parts of caves where the moisture dripping through from above is considerably increased after a rain, and the River Styx in Mammoth Cave rises 60 feet above low- water mark. The creek in Shawnee Cave sometimes fills parts of the cave to the ceiling. The conditions of the water also change very greatly. At ordinary times it mav be very clear ; after rain it may carry a large amount of sediment. In its low condition it may How very quietly, in its high condition be a torrent. The water, then, fluctuates in amount, clearness, and swiftness, with meteoric conditions. Charts of simultaneous records on two self-registering barometers show the close agreement in changing barometric pressures inside a cave and outside it. One of the instruments was placed about 90 feet above the exit of the cave, the other near the middle of Shawnee Cave. Records chosen on account of peculiarities in the rise and fall of the pressure at certain times leave no room for doubt that baro- metric changes similar to those of the outside take place in the caves. The following table shows the temperatures for air and water in Donaldson and Shawnee Caves in 1906 and 1907 : / i iiif'i-riiliircs for air and iMler in Donaldson and Shawnee Caves. Maximum Maximum Maximum Time. Tempera- ture of air in center of Donaldson tempera- ture of water at its exit from Time. Tempera- ture of air in center of Donaldson tempera- ture of water at its exit from Minimum tempera- ture at Time. Tempera- ture of air in center of Donaldson tempera- ture of water at its exit from Minimum tempera- ture at Cave. Shawnee Cave. Shawnee < same pl.U 1 Cave. Shawnee Cave. same place. 1906. 1907. 1907. July 12.7 I2.| Jan II. S n.62 9-5 July ii. 9 i ust... I2.S Feb. 11.5 "•3 8.9 August . . . 12.7 16.1 I3-1 September I3.2 Man li 11.5 [2.6 9-9 September 12.7 17-3 ' 13 October . . 12.7 n.8 ; April.... i 1.5 I 2. 1 IO.2 October . . 12.2 J3-4 12-3 Move i II 10.3 M.n ". S 1 n.6 November II.9 "•S "-S Deccmbrr 12.2 10. June 11.7 IS-I iz-S December II.7 12. 1 7-4 Tlic higher temperatures .•. : I l,i i only a few hours after.a heavy rain. During the first Todays in September, IQO?, the temperature of r |. 6, 13.0, and 15.3 on successive days. During the last 10 days of the month it ranged from 15° to IS-S°. » From the ist to the isth the temperature was between 10.6 and 1 1.6. Currents in water and air differ materially in different caves and at times in In the Cuban blind fish caves there is neither appreciable air- current nor water-current, so that the evaporation from the quiet surface of the water forms a covering crusl of • arbonaleof lime and magnesium. In the blind-fish ives at Mitchell, Indiana, a small current of water flows during normal conditions. The stream becomes a raging torrent in high water. 1 "ir ma] ed, ( i ) by the ilmv of water; (2) by the epigean s in the atmospheric pressure; and (4) by differences in temperature.1 iture »f l he water in the Mitchell Caves will be published within a vear. EIGENMANN AIR-CURRENTS IN CAVES. 7 In Mammoth Cave a very perceptible air-current flows into the top of the dome from Little Bat Avenue. It probably descends to the bottom of the dome and then ascends at the side to flow out at Sparks Avenue. This current was flowing at the rate of 8,640 feet per hour on November 30, 1902. It is probably caused by a thin fall of water which descends from the roof of the dome to the bottom. By far the most violent air-current may be caused by a change in the atmos- pheric pressure in the air without. These currents are perceptible only in caves of considerable extent, and become violent when the opening is insignificant com- pared with the size of the cave. When the weight of superincumbent air is lightened, the compressed air in the cave expands and there is an outrush of air through the opening. If, on the other hand, the barometric pressure increases when the superincumbent air column gains in weight, there is an inrush of air. I have been at the entrance of Mammoth Cave when the internal and external pressures were so equalized that the anemom- eter would show ingoing and outgoing currents alternating irregularly every few minutes. In 1902 I was also at the entrance l when the anemometer showed the following rates per hour for air going in : November 29, 9 a. m., 46,350 feet ; 6 p. m., 39,840 feet; November 30, 7 a.m., 50,290 feet; 9h 40™ a.m., 55,830 feet; and i2h 30™ p. m., 7,800 feet. Mr. A. M. Banta reports from Mammoth Cave that on January 31, 1903, "At the gate the air-currents were surprisingly fitful. The current was running in 40 seconds, stopped 15 seconds, flowed out 8 seconds, stopped 10 seconds, and then ran in for 2 minutes, when we left." His records give the following rates per hour of air going in during February, 1903 : February 18, 12 m., 76,464 feet ; 5h 30™ p. m., 77,396 feet; 6h 20™ p.m., 79,896 feet; February 19, 10 a.m., 76,692 feet; 12 m., 68,904 feet; and February 21, 9 a. m., 56,556 feet. I know of no direct record of currents due to changing temperature on the outside. Until direct observation with an anemometer had been made the general impression among the guides at Mammoth Cave was that air rushed in during one part of the year and out during the other. On cold winter days at Mitchell frost on the bushes showed that a gentle current of the damp cave air was flowing out from the upper part of the cave. The strength of the convection currents is undoubtedly dependent in large measure upon the shape of the cave and the nature of the open- ing. But the influence of water-currents or winds might at any time be sufficient to change the direction of the convection currents. Nothing very definite can be said about the size of the environment afforded by a cave.2 While it is known that some caves are much larger than others, it is never certain how large the unexplored or unexplorable part of a cave may be, how far the smaller cracks lead, and in how far they may establish intercommunica- tion between neighboring caves. 1 A wall partially closes the entrance avenue so that the air passes in and out through a narrow gate where the currents were measured. 2 Hovey (The Mammoth Cave of Kentucky 1897, p. 64) makes the longest course in Mammoth Cave from the entrance to Grogham Hall about 4.5 miles;' the total length of all the known channels is several times that. The width and height may vary greatly from the many cracks where one has to crawl to Chief City between 450 feet (Hovey) to 541 feet (Call) long, and an average width of 175 feet (Hovey) to 190 (Call), with a maximum width of 287 feet. Blatchley says of Marengo (p. 157), "Marengo Cave has been advertised far and near as containing 7 miles of underground passages. Our measurements showed its total length to be 3,850 feet, or 0.7 of one mile. The main channels of Wyandotte Cave we determined to be 4.21 miles long." Very many of the "caves are but a few inches in diameter and too small to be entered. S BLIND VERTEBRATES AND THEIR EYES. The Mitchell Caves can be traced for over 2 miles. Given that they are 3 kilometers long, their average width is perhaps 8.3 meters. This would give an area of 25,000 square meters. As a stream flows their entire length a direct comparison can be made with epigean conditions by taking a stream of similar size and length above ground, with territory equaling the width of the cave. The fauna of the epigean area of equal si/e is incomparably richer than the subterranean one.1 The biological environment of cave animals is comparatively simple. While much has been written on them, the only account of the interrelation of the animals of any cave has recently appeared in a publication by one of my students, Mr. A. M. Banta (publication No. 67 of the Carnegie Institution of Washington). 1 For a discussion of the age of caves see page 17. BLIND AND CAVE VERTEBRATES. 9 THE BLIND VERTEBRATES AND CAVE VERTEBRATES OF NORTH AMERICA. The blind vertebrates do not belong to one class nor do those within one class belong to one family. The blind fauna is very diverse in character and origin, but not all families of vertebrates are represented. A certain predisposition in habit and structure must be present to enable a species to dispense with light and to live in caves. A large blind epigean animal might secure its food and meet its mate, but it could not escape its enemies. Large blind forms are therefore impos- sible. While the size of a sun-fish (Lepomis) might not preclude it from entering caves, the fact that it detects its prey by sight excludes it entirely from the possibly blind. There is, on the other hand, no reason why members of the nocturnal Siluridrc, for instance, should not become blind. No large mammals are blind, nor have large mammals permanently taken up their abode in caves. Bears visit caves, and raccoons, minks, and ground hogs also enter them. The latter two confine their underground wanderings mostly to small caves or to caves of their own making. None of these animals permanently live in caves ; they are all twilight animals and depend on light for their continued exist- ence ; they have normal eyes and are not otherwise modified for life in caves. Blatchley reports that a number of cats have established themselves in Wyan- dotte Cave, where they bring forth and rear their young. Nothing is known about their adaptations. They have exterminated the cave rats and are said to place themselves in a narrow passage of the cave and capture bats passing through. Neotoma pennsylvanica, a wood rat widely distributed in eastern America, has entered caves. It was formerly found in Wyandotte Cave, but has been extermi- nated there. In various caves white-footed mice are found, but they are not blind. The common mole (Scalops aquatic us), the long- tailed mole (Parascalops breweri), and the star-nosed mole (Condylura cristata) burrow in the ground and are partly or entirely blind. They are not found in caves. Bats, which are twilight animals, but have minute eyes, do not depend on their eyes to secure food; they fly at night because their food is then abroad. There are in North America and the West Indies a large number of bats partly or totally blind. Many, if not all of those of the temperate region, winter in caves; a smaller number spend only the day there. They do not secure much, if any, of their food in caves and simply use them as shelters in a more systematic manner than bears do. There are no blind birds, and no birds, as far as I know, permanently live in caves. The phoebe utilizes the entrances as it uses all other similarly sheltered places to nest. In Cuba a small owl is sometimes found in caves, but I know of none that makes it a permanent home. Many owls are adjusted to existence in twilight, but that they are dependent on their eyes is shown by the increase in size of their eyes. Other animals, depending on their eyes but living in the dusk, have similarly enlarged eyes. This is especially well shown by marine fishes liv- ing at twilight depth. There are no cave reptiles, nor do reptiles temporarily enter caves for shelter, as do mammals. One turtle found a little distance inside of one cave was evidently accidental. I have never seen a snake in a cave, but once secured a copperhead at the entrance to one. But there are numerous blind lizards and snakes that JO BLIND VERTEBRATES AND THEIR EYES. burrow in the ground. Anicllai 1 wards as well as forwards. On the ground they progress •• i ions, mil by lalnal movrnu-nis, as in other limbless reptiles; the tail e Tin- food of these lizards consists of small insects and '•d in this account; Man- American, of which only a li of the Tropic of Cancer, and i i Impkisbana) in the West Indies." — Boul- tng^r- ' ' iSSq. ' in the Vmericas, as far north as Cuba: Tvplilops lion- ind 7>/./;/»/<.v l,-i,,,is, Guatemala and Mrxiro. illy IHVII drsnilird from I!ia-il. it bj tni in ol theii .-ves or olfartorv organs, but i 'i barbels. MARINE BLIND FISHES. 11 The fishes, blind or partly blind, living in the depth of the ocean bordering the American continents are Ipnops nnirrayi Giinther and Ipnops agassizii Garman. The former lives at depths varying from 955 to 2,158 fathoms and is very widely distributed. The second one is known horn the type specimens from Lat. 2° 34' N., 92° 6' W-, at a depth of 1,360 fathoms. Ipnops stands alone in a family and is the only vertebrate in which no eyes have been found. The Brotulidre have several members blind, or with very minute eyes, in various parts of the globe. With the exception of the fresh-water species of Cuba, the only ones found in the neighborhood of America are Aphyonus moll is Goocle and Bean, 955 fathoms, and Alexeterion parfaiti Vaillant, 2,736 meters. Other deep-sea blind fishes are Aphyonus gclatinosits Giinther between Australia and New Guinea, 1,400 fathoms; Mancalias shufeldtii Goode and Bean-, 372 fathoms; Paroneirodes glomerosus Goode and Bean, 1,260 fathoms; Tauredophidium hextii Goode and Bean, Bay of Bengal, 1,310 fathoms; Typ/iloniis nasus Giinther, north of Aus- tralia and Celebes, 2,150 and 2,440 fathoms. 12 BLIND VERTEBRATES AND THEIR EYES. THE ORIGIN AND DISPERSAL OF CAVE ANIMALS. It has been shown that many cave animals have good eyes. Epigean animals with degenerate or no eyes are not rare, hence the origin of the cave fauna and of the blind fauna are two distinct questions. This was first recognized by H. Gar- man and indorsed by Kigenmann and by Hamann. Other writers have usually confused the two questions, and indeed they may become one when they concern an animal- that has become blind concomitantly with its cave colonization. A consideration of the forms that are not found in caves, and the reasons why they are not found there, is in this connection possibly more illuminating than the direct consideration of the cave forms. Caves may have become populated by one of the four following processes : (1) Animals may by accident have been carried into caves. (2) Animals may, step by step, have colonized caves, becoming adapted to the environment as successive generations gradually entered deeper and deeper recesses of the caves. (3) Animals which had elsewhere become adjusted to do without light may have gathered voluntarily in caves. (4) Animals may have developed along with the development of the caves. First process : This process was imagined by Lankester to operate as follows : Supposing a number of some species of arthropod or fish to be swept into a cavern or to be carried from less to greater depths in the sea, those individuals with perfect eyes would follow the glimmer of light and eventually escape to the outer air or the shallower depths, leaving behind those with imperfect eyes to breed in the dark place. A natural selection would thus be effected. In every succeeding generation this would be the case, and even those with weak but still seeing eyes would in the course of time escape, until only a pure race of eyeless or blind animals would be left in the cavern or deep sea. While this is a possible mode of origin of cave animals, and even of blind ones, it is highly improbable that many or even any animals depending, as he supposes, on their eyes have thus come to first colonize the cave. Fishes are annually swept into caves, but these' are not able to permanently establish themselves in them. To do this the fish must have peculiar habits, special methods of feeding and mating, before an accidental colonization can become successful, and if they are so adapted for a cave existence, they would probably voluntarily colonize the caves, without waiting for an accident.1 The Amblyopsidse are a small family of fishes, ies being known. They form a very small part of the large fish fauna about the North American caves. But since 6, possibly 7, of the species of this family cave dwellers, and only one of the numerous other fishes is permanently at home in the caves, we must suppose, if the theory under consideration is the correct one, that thr accident of being carried into caves happened to 6 or 7 out of 8 of the Amblyopsidse, and to only i of all the oilier fishes about the caves. The absurdity of this supposition i: *-lf-cvident. A comparison of the abysmal fauna with the pelagic and shore faunas would probably give us similar results. ibly to be made between the aquatic cave animals that will bediscussed under the Non-a<|uatit rave animals are later immigrants of caves. These must twilight fauna about the entrance of the rave or they must have become other- no dillk-ulty in accounting for the presence of Myriopoda on this score i illy found under bark and under rocks. Myriopods are everywhere abundant in irlh A (if any animals) may have accidentally been carried into caves with sticks of wood or trun EIGENMANN MrsERBielmg 4el. ' A. Chologaster papilliferus. B, Bb. Spelerpes stejnegeri. I I 2 mm. Wilson's Cave, Sarcoxie, Missouri. C, Cc. Spelerpes maculicauda. I 30.5 mm. Wilson's Cave, Sarcoxie, Missouri. D, Dd. Typhlotriton spelaeus. 1 34 mm. Marble Cave, Missouri. ORIGIN OF CAVE FAUNA. 13 Second process : The second theory is that of Herbert Spencer : The existence of these blind cave animals can be accounted for only by supposing that their remote ancestors began making excursions into the cave, and, finding it profitable, extended them, generation after generation, farther in, undergoing the required adaptations little by little. — Popu- lar Science Monthly, XLITI, 487 and 488. I can offer no objection to this theory. It presupposes the existence of caves, and it is perfectly possible that many cave animals have arisen in this way. The abundant twilight fauna in the entrance of caves argues in favor of it. Spelerpes maculicaiida and other salamanders, which are so frequently found a short dis- tance within caves and even in remote recesses, seem to be present colonizers that bear out Spencer's view, though it is possible that these should be grouped with the animals next to be considered. Spelerpes maculkauda has not yet been affected, as far as its structure is con- cerned, by its habits. It is a nascent cave form that may result in the future in a single blind species of wide distribution, or a number of species in the groups of caves that are geographically separated from each other. There can be no question whatever, in its case, about an accidental carrying into caves, for if it enter caves by accident it must be continually meeting accidents through a very wide region. Third process: This view was first expressed by Garman (Science, Oct. 28, 1892, p. 240): The originals of the cave species [non-aquatic, especially] of Kentucky were probably already adjusted to a life in the earth before the caves were formed * *. The writer * independently came to the same conclusion. This theory makes the cave simply the collecting ground of animals adapted to a cave existence, and leaves the origin of this adaptation an open question. Gar- man imagined that the animals become adjusted to cave existence in crevices of rocks. Since these crevices are but caves on a small scale, his suggestion simply tends to account for the aggregation of the animals found in the caves of Kentucky, not for their becoming cave animals in the first instance. If at this point we might call mutation to our aid, we would have a satisfactory explanation. If mutants arose among any species of animals adapted to cave existence, they would find their way into caves or crevices if such existed. What would happen if there were none need not concern us. But while mutation might account for the positive adaptive modifications in cave animals, it does not account for the negative or degenerative changes, and the more venerable theory of special creation is of equal potency. Fourth process: It is certain that in some cases cave animals have developed concomitantly with the caves. It seems quite possible that in more cases than we have thought the adaptation of an animal to a very complex environment can only be explained as the result of concomitant development of environment and 1 In answer to the statement made by Eigenmann, Krause [Promethius, No. 457, p. 652, 1898] said: "Nicht weil sie in dunklen Hohlen leben, seien ihre Gesichtsorgane verkummert, nicht die Entziehung des Lichtes babe diese Organe zuriickgehen lassen, sondern umgekehrt, weil sie sich schon in der Oberwelt dem Leben ohne Licht angepasst batten, waren sie wohl vorbereitet gewesen, in den Hohlen, von volliger Dunkelheit umgeben, so — glanzend, kbnnte man beinahe sagen — zu reiissiren. * * * Nun, wer's glaubt, mag ja auch bei dem Glauben selig werden konnen, dass die Hohlen gleichsam zum Tummelplatz und Elysium der Blinden aller Thierklassen erschaffen seien. Wir haben diese Sirenenklange aus dem mystischen Dunkel der Gegner des Lichtes und der Entwickelungslehre schon ofters gehort; sie stehen in Harmonic mit den immer starker hevortretenden Bestre- bungen, dem Lamarckismus, Darwinismus und selbst dem Weismannismus ein Bein zu stellen." This quotation is possibly sufficient to indicate the general tenor of the rest of his article. 14 BLIND VERTEBRATES AND THEIR EYES. animal. Certain parasitic insects are in the habit of boring through the hard mud walls of the nests of mud wasps to deposit their eggs. It seems difficult to explain the origin of so complex a habit and of the organ sufficient to pierce the hard wall. A mutation to account for it seems inconceivable. It is, however, quite possible that the hard wall is a partial adaptation against these very enemies, and that the habit of building heavier and heavier walls, and the development of more and more efficient organs for piercing them were developed as armor plate and armor- piercing slu-lls are interrelated developments. From the hills about Horse Cave, Kentucky, one sees valleys about 250 feet deep stretching out in four directions. Of the river that is responsible for them nothing is to be seen. It is 185 feet beneath the bottom of the valley at the town of Horse Cave. The hills are capped with over 70 feet of sandstone. The river has had a continuous existence from the time it formed the valley in the sand- stone capping, through its later history when it continued the process of valley formation in the limestone underlying it, and later still when it hollowed out its underground channel in the limestone. There is nowhere any indication that there has been a cataclysm in the history of the river. It lies south of the glacial area. What is true' of the river may be true of the inhabitants still within it. There is no reason to think that the ancestors of the blind fishes may not have lived in the stream when it flowed over the sandstone capping the hills.1 Some fishes of any stream stay in the light, others in the shade, others under rocks. The ances- tors of the blind fishes probably lived in the shade under rocks and became ad- justed to thi1 dark or dusk, existing there long before the caves were formed. When placed in open pools Amblyopsis still has that habit. What more natural than that this lish should descend farther and farther with the river after it began its subterranean course — not suddenly, but gradually? At first only part of its water found its way underground ; but when all its water could flow beneath the surface under normal conditions, a part flowed above ground after every freshet, just as the water of Lost River of Indiana does at present. It could not sink beneath the ground at all until Greene River, into which it empties, had cut a considerable distance U-nealh the surface of the limestone, and thus gave the water in the lime- stone rifts a chance to flow out and be replaced with fresh water from the river \- tin stream sank beneath the surface, naturally those fishes depending on light for food and courtship left it, and only those either negatively hcliotropic or positively steivotropic remained. The lilind aquatic fauna looked at from this standpoint is not a new acquisi- li:>n of the presenl cave stream, but a relict of the fauna of the river when it still >ve ground. The cave and its fauna have developed hand in hand. I 'u' i ot the cave fishes and other aquatic cave dwellers do not so much •\planation (they were present long ago) as does the absence of all of the '(her lonns that must have been present when the stream flowed in its epigean I'he prime requisite for a candidate for underground existence is a nega- tive reaction to light ui\v stercotropism, or both. •videnl that a fish depending on its sight to procure its food ; form. Sun fishus. which are annually carried into the loped eaves, belong to this class of fishes. They are always rs that (lurini; tin- ul.n ml cpcnh tin- . oinlilinns in tin- caves of Kentucky were such thai the pri-M-ni fauna r\iMnl there. ORIGIN OF CAVE FAUNA. 15 poor when found in the caves, and will never be able to establish themselves in them. On the other hand, there are no reasons why fishes detecting their prey either by smell or touch should not be capable of colonizing caves. The cat-fishes and Amblyopsidae belong to the latter class. It is surprising that more cat-fishes have not established themselves in caves. Among the Amblyopsidae, even those with functional eyes depend on touch and vibrations for their food. Chologaslcr has well-developed tactile organs and poor eyes. It is found chiefly at the mouths of underground streams, but also in the underground streams themselves. The tactile organs are not different in kind from those of other fishes, and their high development is not more marked than their development in the barbels of the cat- fishes. The characters which distinguish Chologaster as a fish capable of secur- ing its food in the dark are emphasized in Typliliclithys, and the tactile organs are still more highly developed in Amblyopsis. The eyes of the last two genera are so degenerate that it is needless in this connection to speak of degrees of degen- eration. On account of the structure of their eyes and their loss of protective pigment, they are incapable of existence in open waters. With the partial and total adaptation to underground existence in the Amblyopsida? and their negative reaction to light, it is scarcely possible that for this family the idea of accidental colonization can be entertained for a moment. Their structure is not as much due to their habitat as their habitat is due to their structure and habit. Typhlogobius lives in the holes of shrimps, under rocks, on the coast of southern California. It is a living example of the origin of blind forms in dark places remote from caves. Here again the "accidental" idea is preposterous, since no fish could by accident be carried into the devious windings of the burrows they inhabit. Moreover, a number of related species of gobies occur in the neighbor- hood. They live ordinarily in the open, but always retreat into the burrows of crustaceans when disturbed. The origin of the blind species by the gradual change from an occasional burrow seeker to a permanent dweller in the dark, and the consequent degeneration of the eye, is evident here at once. Among insects the same process and the same results are noted. We have everywhere the connection of diurnal species with nocturnal, dark-loving, and blind forms, a tran- sition, the result of habit entered into with intent, but no evidence of such a con- nection as the result of accident ; also numerous instances of daylight species being swept into caves, but no instance of one establishing itself there. Attention has been called to the difference in the time of origin of the aquatic and non-aquatic cave-dwellers. The latter are later immigrants. They neces- sarily arrived after some channels had been cleared of water through the stream burrowing into still lower channels. The non-aquatic forms are derived, in part at least, by migrations from the twilight forms that may have developed with the twilight region, and in part they are active immigrants of stereotropic or negatively phototropic forms like the Spckrpcs. Some of them, like the myrio- pods, may even be accidentally brought in with their food and habitat,1 but even here the active voluntary immigration is, at least, as probable as the accidental one. Species widely distributed over a continuous environment may have become distributed from one center of development. The same may be said of the species found in distant, discontinuous environments where it can be shown that the dis- continuity is of recent origin. The same can not be said of species distributed in 1 Decaying logs have been carried into and are found in various parts of the Mitchell caves. 16 BLIND VERTEBRATES AND THEIR EYES. isolated elements of a discontinuous environment that can not, in the nature of the case, at any time have formed parts of a continuous environment. Amblyopsis is found on both sides of the Ohio River. The caves of the two sides have certainly never formed part of the same complex. It is possible, though scarcely probable, that the caves south of the Ohio, inhabited by Typhlichthys at one time, formed a continuous environment. It seems evident that Amblyopsis could not have migrated from the caves south of the Ohio to those north of the Ohio. The different colonies probably had similar but independent histories. 'I 'he cave salamander, Spdcrpcs iiuinilianida, is widely distributed in the Missis- sippi Valley. It enters caves wherever they are found within its area of distribu- tion. It is becoming adapted to a cave existence in widely isolated places. What is at present taking place with Spdcrpcs may have taken place with Amblyopsis, except that Spdcrpcs found its caves ready made, while Amblyopsis was present during their making. The ancestry of the Amblyopsidae we may assume to have had a tendency to seek dark places, wherever found, and incipient blind forms would thus arise over their entire distribution. Certainly the fearless, conspicuous blind fishes, as at present developed, would have no chance of surviving in the open water. Their wide dispersal after their present characters had been assumed would 'be out of the question entirely, except through subterranean waters. The same would not be true of the incipient cave forms when they had reached the stage at present found in Chologaster. This genus has the habit of hiding under- neath objects in the darker sides of an aquarium. These dark-seeking crea- tures would be especially well fitted to become distributed in caves throughout their habitat. S. Carman's able argument for the single origin and dispersal of the blind fishes through epigean waters was based on the supposition that the cis Mississippi and trans-Mississippi forms were identical. The differences between these species arc such as to warrant the inference, not only that they have been independently segregated, but that they are descended from different genera. The external differences between these species are insignificant, but this is to be expected in an environment where all the elements that make for external color markings are lacking. The similarity between Typhlichthys and Anthlyopsis is so great that the former has been considered to be the young of the latter. For reasons that will be fully set forth there is every probability that the Cuban blind fishes developed with the caves which they inhabit. In conclusion it may be said : (i) That the cave fauna is in large part the result of the formation of the caves themselves, that environment and habitat developed purl passu. That to this original fauna have been added and are being added species (such as Spdcrpcs niiiciiliciiinlii) which, because they are negatively heliotropic or po-itiveh stcrcotropic, :,re gradually becoming adapted to the deeper and deeper "f eaves. (3) That to the fauna of the larger caves may also have been added animals vlnrli had become adjusted to cave existence in crevices, under banks or rocks, etc., that is, in small caves. (4) That accident has played little or no part in developing the cave fauna. ORIGIN OF FOOD SUPPLY. 17 THE ORIGIN OF THE FOOD SUPPLY OF CAVES. Cave existence, reduced to its simplest terms, is the securing of food and the meeting of mates in absolute darkness. Food is so scarce that no large preda- ceous animals have taken up their abode in caves, hence the largest cave animals, such as the cave fishes, have no enemies aside from parasites and disease germs. Of the cave fishes Chologaster reaches a length of but (>2 mm.; Typhlichthys, 55 mm.; Troglichthys, 55 mm.; Amblyopsis, 135 mm.; Litcifitga, 104 mm., and Stygicola, 152 mm. All are insignificant in size. The density of the population of any cave, other things equal, is inversely pro- portional to the size of the cave. No food is generated in caves by the growth of plants. Directly or indirectly all food consumed in a cave must be imported. It may come in through various openings ; usually there are only one or two open- ings of any consequence: (a) the "entrance" in a dry cave, (l>) the entrance and point of inflow of the stream in a wet cave. That cave is bust supplied with food per square yard which has the smallest area over which the limited supply must be distributed. There is, of course, a great difference in the amount of food carried in through different openings. An entrance sloping upward naturally will not admit as much decaying vegetation as one sloping downward. A narrow crack through which water may enter a cave will not admit as much as a large opening, through which in times of flood the water may carry tree trunks. These matters equalized, I may repeat that that cave is best supplied with food per square yard which has the smallest area over which the limited supply entering a given opening must be distributed. The density of the fauna varies as the amount of food, and hence, other things equal, inversely as the size of the cave. AGE OF CAVES IN RELATION TO THE VARIETY OF CAVE FAUNA. Desired lines of research are the relation of the abundance of the cave fauna to the age of the particular cave and the comparative degree of adjustment of the animals to caves of different ages. We have in North America a series of caves reaching from Howe's and other northern caves in the glaciated region to the Ohio Valley caves near the edge of glaciation, and the caves of Texas and Cuba never affected by glaciation.1 Howe's Cave in central New York is exceedingly poor in animals, the Texas caves are as correspondingly rich, but no detailed comparison has been made. It is also known that the Ohio Valley cave salamander, Spclrrpcs niticiillcanda, has well-developed eyes, that the Missouri salamander, Typhlotriton, has degenerate eyes, and that the Texas salamander, Typhlomol^r, has very much more degenerate eyes. The degree of degeneration seems here coordinate with the age of the cave. Also that the Missouri blind fish has more degenerate eyes than those of the Ohio Valley. In a general way the older caves appear to have more intimately adapted or more profoundly modified forms than the newer. But here again we lack entirely a detailed study. 1 Shaler, 1875, estimates the age of the Kentucky caves at between 750,000 and 2,000,000 years. He further maintains that, during the glacial epoch Kentucky was populated by an Arctic fauna and that the cave fauna was not derived from this, but from the present fauna of Kentucky, "since the glacial period." I agree with him that the present cave fauna of Indiana and Kentucky was derived from or developed concomitantly with the present epigean fauna, but am in doubt about the nature of the fauna during the glacial period. 18 BLIND VERTEBRATES AND THEIR EYES. DIVERGENCE IN EP1GEAN AND CONVERGENCE IN SUBTERRANEAN FISHES. The struggle for existence with the biological environment as the result of the geometric rate of increase tends to divergence in habit and form. It does this by preserving variants whenever such possess a character diverging sufficiently in amount to give the variant a personal advantage over his fellows — always provided the divergent character is transmissible. Whether we call the diverging individuals variants in the old sense, or mutants in the new, it is to the selection of those among them best adapted to utilize the foods of various sorts, to occupy localities of various kinds, to escape the enemies of various sorts, and to leave others similar to them in their place when they die, that we owe the specific divergence in structure, shape, color, food habits and breeding habits of a given family — say the American Characins. The entire process tends to the divergence and multiplicity of species. The Characins are a family of fresh-water fishes that, in America, range from the border of the United States to some distance south of Buenos Aires. They form about one-third of the entire South American fresh- water fauna, and have diverged in adaptation to diverse food, diverse habitat, and diverse enemies to fill nearly every niche open to fishes. The ends of three of the lines of adaptation to different food give us mud-eating forms, with long intestinal tract and no teeth; llcsh caters, with shear-like teeth, that make bathing dangerous to life and that cut their way out of nets; and conical-toothed forms, with sharp, needle-like teeth and comparatively huge fangs. Greater diversity could scarcely be imagined, and one is led to suspect that some of the forms are over-adapted. In their divergence in form they have reached almost every conceivable shape as we shall in a moment. The struggle for existence with any unit of physical environment, whether there be geometric rate of increase or not, tends to convergence in habit and form. There is no more striking instance of this than the acceptance of the annual or deciduous habit of most of the plants inhabiting the temperate zones with their -onal changes, nor is there a more striking illustration of the struggle with other individuals than the diversity of form and habit of various forest plants for ground and light space. Records of the simultaneous and similar changes in the form in the mass of species of any area during changing physical conditions are not want- ing. For instance, Scott says: The steps of modernization which may be observed in following out the history of many dif- ' if mammals are seen to keep curiously parallel, as may be noticed, for example, in tin- series <>! skulk figured by Kowalevsky, where we find similar changes occurring in such families .s the pi.^s, divr, antelopes, horses, elephants, etc. Indeed, one may speak with propriety of a I'lii-rco, or Wasatch, or White River type of skull, which will be found exemplified in widely separate orders. < >n some riffles of the San Juan River of Cuba I found a small fish that is very like other fishes inhabiting similar localities in the eastern United States. The lormer is a goby, a marine' form, I'liily/wns donnitator, which has become •d to conditions found about the ri files of streams; the others are darters, belonging to an entirely different family of fresh water fishes. The similarity of various •'darters" which live on the bottom of our streams to various DIVERGENCE AND CONVERGENCE. 19 gobies and blennies that occupy a similar position along the marine shores has repeatedly been noticed. In the tropics live many burrowing lizards and snakes. Rhineura, one of the lizards, lives and acts like an earthworm, and so like an earthworm has it become that only a close inspection reveals its true nature. Even the chickens following the plows in Florida and Cuba are said to be taken in by the similarity of some of the burrowing lizards to earthworms. The Characins again furnish striking illustrations. Diverging among them- selves, as has been noted above, they have approached, or paralleled, many mem- bers of the diverse families of North American fresh-water fishes. Our shads and fresh-water herrings have their counterparts in Elopomorphus, Potamorhina, and Psectrogaster ; our salmon are paralleled by Salminus and Catabasis ; our min- nows are paralleled by Tetragonopterus and its relatives. It will take but a slight flight of the imagination to detect the striking similarity of some of the Hydro- cyninae to our garpikes; our mullets are duplicated by Prochilodns ; our top- minnows are mimicked by Nannostomus ; and even our festive darters are dupli- cated by the species of Characidium, members of this most remarkable family. In a dark cave, all those differences between related species which would strike the eye, such as protective coloration, recognition marks, decorations of any sort, etc., are absent, and related species tend to look alike. It was not until after a detailed examination of many specimens that I could invariably distinguish Luclfitga and Stygicola, the Cuban blind fishes, from each other. On the surface the specimens of Troglichthys rosce very closely resemble Typh- lichthys subtcrranms from Mammoth Cave, differing slightly in the proportion and in the pectoral and caudal fins. These fins are longer in rosa. It is, however, quite evident from a study of their eyes that we have to deal here with a case of convergence of two very distinct forms. They have converged because of the similarity of their environment and especially owing to the absence of those ele- ments in their environment that lead to external protective adaptations. It would be difficult to distinguish specimens of similar size of Amblyopsis from either subterrancus or rosce, were it not that it possesses ventrals. The eye of T. subterraneus is surrounded by a very thin layer of tissue repre- senting the sclera and choroid. The two layers are not separable. In this re- spect it approaches the condition in the epigean, eyed member of the family, Cholo- gaster. For other reasons, that need not be given here, it is quite certain that Typhlichthys is the descendant of a Chologaster. The intensity of coloration and the structure of the eye are the chief points of difference. The eye of rosce is but about one-third the diameter of that of subterrancus, measuring 0.06 mm. or there- about. It is the most degenerate, as distinguished from undeveloped, vertebrate eye. The point of importance in the present instance is the presence of com- paratively enormous scleral cartilages.1 These have not degenerated in propor- tion to the degeneration of the eye and in some cases are several times as long as the eye, projecting far beyond it, or are puckered to make their disproportionate size fit the vanishing eye. This species is unquestionably descended from a species with well-developed scleral cartilages, for it is not conceivable that the sclera as found in Chologaster could, by any freak or chance, give rise during degeneration 1 Kohl mistook the nature of these structures, as he did of every other connected with these eyes, except the lens and ganglionic cells. 20 BLIND VERTEBRATES AND THEIR EYES. to scleral cartilages, and if it did they would not develop several sizes too large for the eye. At present no known epigean species of the Amblyopsida? possesses scleral cartilages. The ancestry of roses is hence unknown. Amblyopsis has the scleral cartilages, and the eye of roses passed through a condition similar to that possessed by Amblyopsis, but the latter species has ventral fins and is hence ruled out as a possible ancestor of rosa. The epigean ancestry of Amblyopsis is also unknown. The ancestry of Typhlichthys being quite distinct from that of roses, the latter species is referred to a separate genus, Troglichthys. Judging from the degree of degeneration of the eye, Troglichthys has lived in caves and has done without the use of its eyes longer than any other known vertebrate. (Ipnops, being a deep-sea form, is not considered.) The species of Typhlichthys differ from each other in only a few inconspicuous respects. (See page 53.) CONCLUSIONS ON CAVE ENVIRONMENT. 21 CONCLUSIONS. (1) The possible physical environment of animals is composed of units, each of which is distinguished by a combination of conditions peculiar to it. (2) A unit may embrace one continuous area. (3) A unit may have extended in the past over a continuous area, but may now be broken up into separate, though similar, parts between which the migra- tion of animals is not possible. (4) A unit may always have existed of separate and distinct parts (units of a smaller order) which together form a discontinuous unit. (5) An animal distributed over a continuous, or parts of a formerly continuous, unit may have arisen at a single center of dispersal. (6) An animal distributed over a discontinuous unit must have had separate places of origin or have originated at a time when the parts of the unit were con- tinuous. (7) Each cave consists of a twilight section, a fluctuating temperature section, and the cave par excellence. (8) The environment in the third section is chiefly characterized by (a) the absence of light ; (b) the constancy of meteorological conditions between seasons ; (c) the absence of food except such as is imported. (9) All classes of vertebrates, except birds, have blind members. (10) Some cave animals (aquatic) have developed pari passu with the devel- opment of an underground stream and are among the few inhabitants remaining to the stream of its inhabitants during its epigean period. (n) Some cave animals (non-aquatic) have gradually colonized caves after their formation. (12) Some cave animals became elsewhere adjusted to live in the dark and later migrated into caves. (13) Accident had little or nothing to do with the colonization of caves. (14) Some widely distributed cave species have independently arisen in dif- ferent places from a widely distributed epigean species. (15) Directly or indirectly all of the food supply of a cave must be imported. (16) Smaller caves have a relatively richer fauna, because the food supply is more abundant. (17) Older caves have a more varied and richer fauna. (18) Cave animals tend to converge in their evolution; epigean animals, to diverge. BLIND AND CAVE VERTEBRATES AND THEIR EYES. MAMMALS. EYES OF THE COMMON MOLE. Dr. J. R. Slonaker has found that the eye of the mole (Scalops aquaticus machrinus) lies embedded in the muscle beneath the skin, where it appears as an inconspicuous dark spot. It is situated well forward on the side of the snout. The eye is degenerate and is no longer capable of functioning in distinct vision. The most noticeable changes which have occurred are: 1. The great reduction in the size of the eye. 2. The much crowded condition of the retina as a result of the decrease in size of the eye as a whole. 3. The noticeable reduction in the size, or the complete absence, of the aqueous and vitreous chambers. 4. The varied modification of the shape and size of the lens, also the peculiar cell structure of the lens. All the structures of the normal mammalian eye are present in some form or other, (i) The conditions found in the adult and at birth have been studied. Very little difference is seen in these two stages excepting an increase in size. The eye muscles and the optic nerve are easily traced back to the skull. At birth the nerve presents in its course from the eye to the skull a peculiar arrange- ment. The course is marked by numerous cells and few or no fibers. At the eye there is a rapid change from this cell condition to the fiber condition of the nerve tract. The fibers have not apparently grown much beyond the limits of the eye. In the adult the fibers can be traced to the skull. The eye cleft is very small and of practically the same diameter in both hori- zontal and vertical sections through it. It meets'the eye at such an angle that it is impossible for rays of light, should any enter, to'pass through the eye along the axis of vision. All the elements of the normal retina are present, but, owing to the much crowded condition, the ganglion-cell layer is much increased in thickness. The lens, which is found in a great variety'of shapes and sizes, is composed of peculiar cartilage-like cells with well-defined 'nuclei. It is therefore incapable of functioning as a normal lens. It is very doubtful, therefore, whether the eye of the mole functions in any sense. At best it can do no more than distinguish between light and darkness. 25 26 BLIND VERTEBRATES AND THEIR EVES. THE CAVE RAT AND ITS EYES.1 The cave rat, Xn>tinna master, ranges eastward to southern New York and south to Alabama, and is not confined to caves. It lives in "cliffs, caves, and rock ledges of the mountains, descending into the lowlands, where limestone caves afford it security." In White's Cave, near Mammoth Cave, Kentucky, it has its nests near the entrance, in the twilight region. In Mammoth Cave I found it in Mammoth Dome, and it occurs also farther in, far removed from the twilight area. Rhoads (Jour. Cin. Soc. Nat. Hist., xix, No. 2, 55, 1897) says of it: Any suspicion of blindness or deficient eyesight, such as is exemplified in some of the lower orders of animal life in the cave, can not attach to this mammal. As in all the more strictly noc- turnal rodents, the eyes of this species are greatly developed ; nevertheless, they are able to make most intelligent use of them in broad daylight, if need be. In his "Origin of Species," sixth edition, vol. i, page 171, Darwin says that the eyes of Neotoma of Mammoth Cave are "lustrous and of large size; and these FIG. i. (a) Eye of Mammoth Cave Rat. (/») Eye of Common Gray Rat. animals, as I am informed by Professor Silliman, after having been exposed for aboul a month to a graduated light, acquired a dim perception of objects." The ratj .\foloniii, is still abundant in Mammoth Cave. Its tracks are numerous, and in plans little paths have been made by the rats where they run backward d forward along ledges of rock. Since, however, a track once made in a cave unchanged by wind or weather, the abundance of rats, as judged by their ;, may be misleading. A number of traps were set in the rotunda. During trap was sprung and one had the bait removed. No rats were caught in the traps and none were caught alive. The author discovered one rat rolling a mouse trap about which was too small for it to enter. When approached I, the rat turned about and stared at the light. It then ran to a pile of but did : 'ipt to hide; instead, the rat ran to one end of the pile, then alon-j; the top hack to when- it had stood, then stopped and again stared at the light. Dr. J. B. Slonakor's account, from which figures I and 2 are l"'l. Ac.ui . 25S| ,899. THE CAVE RAT. 27 An attempt to catch the rat sent it running back and forth along the ledges of rock at the side of the cave. Finally the rat appeared at the ground again, and despairing of catching it alive, it was killed. Its eyes seemed to be large and pro- truding very much as in the common rat. Without question the rat noticed the light. It had no hesitation in running from place to place. Later four of these rats were sent by express. Only one arrived alive; one had been partly eaten by the others. The living one was quite gentle. It permitted itself to be stroked. Occasionally it pushed an object away with a sideward motion of the forefoot. If provoked it snapped at the object. During daylight it sat quietly in a nest it formed for itself of cotton batting, which it pulled into a fluffy mass. At night it frequently moved about in its cage. Turning on an electric light near its face always produced a twitching of the eyelids, so there can be no doubt that the light was perceived. An object held some distance from the cage on one side or another was always perceived, but just how precise its vision was has not been determined. Its hearing was acute. FIG. 2. Retinas of Neotoma and Common Gray Rat Compared, (a) Mammoth Cave Rat. (6) Common Gray Rat. Its eyes were as prominent as those of the gray rat. If there was any difference, its eyes were larger in proportion to the size of the body weight than those of the gray rat. The lens in both cases was enormously large in proportion to the eye. The pupil was capable of very wide dilation. A microscopic comparison of the retinas also showed little difference. Bits of retina from corresponding parts of the eye of a cave rat and a gray rat were hardened by the same process, sectioned the same thickness, and stained alike. The results are given in figures i and 2. There is little difference except in the thickness of the retina, that of the cave rat being thicker. However, the difference may be due to the differences in the ages of the animals, the cave rat being fully grown, the gray rat only half grown. The thickness of the retinas are proportionate to the size of the eye. The increased thickness is largely due to the larger size of the cells of corresponding layers of the retina. For instance, the rods and cones are decidedly longer and larger in the cave rat. But with the exceptions given the two retinas are nearly alike. BLIND VERTEBRATES AND THEIR EYES. Til!' CAVE SALAMANDERS. The salamanders, of which there are many species in the United States, habitually live under rocks, logs, and the bark of decaying trees. These all shun the light except during the breeding season. Others habitually live in the water and are principally nocturnal in their habits, hiding under the banks, logs, or rocks in the water during daylight. The eyes of the cave salamanders of North America, of which there are four species, range in their structure from the perfectly normal to the most degenerate known among the Batrachia. Spelerpfx maculicauda (Cope) (plate i, fig. c) is common in the caves of the Mississippi Valley. As far as I have been able to determine, its eyes have not undergone any degeneration. It is abundant and so nearly allied to Spelerpes longicauda Green, an epigean species of very wide distribution, that formerly the two were considered identical (plate 2, fig. A). Spelerpes stejnegeri Eigenmann (plate i, fig. B) is found in the twilight regions of the raves of southwestern Missouri. Its eyes are also normal. Other species of Spelerpes ' are sometimes found in caves. TypMotriton s pel-ecus Stejneger (plate i, fig. D) is restricted to the western caves of the Mississippi Valley. It has so far been found in Marble Cave and a. FIG. i. (a) Head of Spelerpes n:>i<-ulit uial'i, 54 mm. long. (6) Head of Typhlolriton spelteus, 54 mm. long. (<) He-ad of Typhlomolge rathlntni, 47.5 mm. long. Kockhousc Cave, and smaller caves in the same neighborhood in southwestern Mi"ouri. It is found under rocks in and out of the water. This is the most interesting form, inasmuch as it is a much more typical cave animal than Spelerpes, but has not yet reached the degenerate condition of Typhlomolge. Its eyes are apparently normal in the larva, but in the adult have undergone marked degen- The eyelids are disappearing and the rods and cones are no longer preM-nt in the adult. The eyes of this species will be dealt with below. Typhlomolge nitlibnni Stejneger (plate 2, fig. B) is found in the underground streams near San Marcos, Texas. It has been taken from the artesian well at Jatvos and a surface well. It has also been noticed in one of the caves near that place, './.el's, in which the underground water can be reached. It is :i'd to have come out of some artesian wells south of San Antonio. It is a peren- all of its time in the water. Its remarkably long and slender ; to support its body when out of the water. Figure 3 shows found about the caves of Hloomington, Indiana. EIGENMANN PLATE 2 A, Aa. Spelerpes longicauda. 147.5mm. Carlisle, Pennsylvania. B, Bb. Typhlomolge rathbum. 88 mm. San Marcos, Texas. THE TEXAN CAVE SALAMANDER. 29 the heads of three cave salamanders of North America. The heads were sub- jected to the same treatment to prepare them for photography, and photographs were taken under approximately the same magnification. In February, 1896, the first recorded specimens of this species were cast up from an artesian well about 190 feet deep, bored by the U. S. Fish Commission. Other specimens have since been thrown up at the rate of 30 to 50 a year. The following notes on the habit of this cave salamander are by the late Pro- fessor Norman of the University of Texas. Unless disturbed, the salamanders appear at all times either resting, or very slowly and cautiously walking along. They move a few steps at a time, wait awhile, and go again. They have no particular pose when quiet except that they always rest on their 4 feet, holding themselves up from the bottom of the vessel and fre- quently retaining the exact position of the legs at the moment the motion is arrested. If the vessel contain, for example, watercress, they crawl in among the branches and stop as when walking on firm bottom, with the legs in such a posi- tion as fits easiest for gliding in among the twigs. They are never seen to move faster than a slow, easy walk, except when dis- turbed by external stimuli. Then one of three methods of locomotion may follow: (i) the walking speed may pass into a grotesque run by long strides and corre- sponding winds of the body ; or, (2) this passes into a combined movement of legs and tail, the last acting as fin; (3) at its greatest speed the legs are laid length- wise against the body, and the tail only is used for locomotion. The legs are exceedingly slender and weak. If the animal is placed on a table out of water, the body falls to the table, and at best the animal may wriggle a few inches ; but in water the weight of the salamander is so little that the legs are amply strong for its locomotion. Dr. Stejneger lost sight of this point when he guessed that the animal used its tail for locomotion and its legs as feelers. He stated as follows: Viewed in connection with the well-developed, finned swimming-tail, it can be safely assumed that these extraordinarily slender and elongated legs are not used for locomotion, and the convic- tion is irresistible that in the inky darkness of the subterranean waters they serve the animal as feelers. The motion in water is, for the most part, slow and cautious, the movement of the long legs being apparently calculated to produce the least commotion in the water. The motion suggests that of a cat creeping upon its prey, or the elephan- tine progression of the snapping turtle. The feet are lifted high in walking, and the body is kept from the bottom by the full length of the fore arm and leg. In ordinary progression the body slopes from nose to tail, which drags (plate 2, fig. B). The method of moving the limbs is as follows : Left hand and when this is nearly ready to place, or usually when placed, the right foot. When the right foot is placed, then the right hand and then the left foot. As the hand of one side is not raised till the foot of the same side is placed, the enormous strides of the long- legged creature cause it to step on its hand or even beyond. Its natural gait is a deliberate progression by means of its feet with three feet usually on the ground. Any attempt at great rapidity by this means of locomotion results in a most un- dignified and futile wriggle. When going slowly, the head is held sloping upward. When walking rapidly, it is held sloping down, so that the snout is near the ground. 30 BLIND VERTEBRATES AND THEIR EYES. No definite information has been obtained as to their habits in nature. They show no reaction to light, either as a response by motion to the direction of the rays or to the quantity of light. If kept in a vessel, half of which is dark and the other half light, the animal is found about as often in one as the other, and on emerging into light from the dark it indicates in no way an awareness of the difference. If in a tangle of plants, as watercress, they are found about the same as in any other part of the vessel. If they are headed against a current, the flowing water acts as stimulus, urging them on. If the current strikes them from behind, they move more rapidly in the direction of flow. The sense of touch is highly developed. There is, however, no experimental evidence that this is confined to any particular region. If the surface of the body is touched anywhere except at the blunt truncated snout, the animal responds at once by moving away. If the stimulus causes it to swim away, it may go (say 12 or 1 6 inches) till it strikes the side of the vessel, after which it soon comes to a standstill. If, however, it is struck, say with the flat side of a scalpel handle, sufliciently hard to move the entire animal even an inch backward, it may not react, and this may often be repeated before it reacts by moving away. A possible explanation of this fact is that in normal life it is every day striking itself against <>1 Mudes, especially the sides of the vessel (when in confinement). The animal is exceedingly sensitive to any motion of the water. Where one is kept in water about an inch deep, with its head near the surface, waves of water set going by a gentle puff of the breath act as a sure stimulus. But little evidence thus far shows in favor of a sense of smell. All attempts at feeding (except one) have been in vain. No attention was given to meat or other articles placed near it. Examination of a dead specimen showed chitinous remains of such Crustacea as Cyclops. If a glass rod or other object is held a little to one side and in front of the ani- mal, it will cautiously turn its head in the direction of the rod. If the latter is then made' to describe an arc about the side of the salamander, the head will follow it with a continuous motion, expressive of the greatest caution, as far as it can be followed without moving any of the limbs. A sudden jar, produced by tapping the rod on the bottom of the aquarium at such a time, causes the salamander to jerk its head back and rear back on its limbs as far as it can. The same effect is produced if the rod is introduced too rapidly. If a piece of crayfish tail is held by pincers in the fingers a short distance in I nmt or on one side of the head of the salamander, there is the same cautious motion forward till the snout comes in contact with it. There is then a momentary hesi- tation, fullowed by a sudden snap and seizure. The silamander may be pulled from side to side by the meat, after it has once secured a hold, without causing it to let go. All of its caution is apparently directed in approaching the food without disturbance. After it has secured a hold it will struggle to maintain it. EYES OF THE TEXAN CAVE SALAMANDER. 31 THE EYES OF TYPHLOMOLGE RATHBUNI.1 The U. S. Fish Commission, through Dr. B. W. Evermann, sent me four speci- mens of this salamander and a number of its eggs. Of these, one adult had been received in Washington, April 8, 1896, and three young, of different sizes, March i, 1896. A few eggs were laid about March 15, 1896. The late Professor Nor- man, of the University of Texas, and Professor Bray, of the same place, secured me an additional number. Later, I visited the caves and the artesian well at San Marcos, and have been able to observe the living specimens. The specimens sent by Professor Evermann were preserved in alcohol ; those sent by Professor Norman had been killed in Perenyi's fluid. The sections were stained chiefly in Biondi-Ehrlich's tricolor mixture. The following gives the dimension of the eyes in a number of individuals. Professor Norman sent only the heads, so the length of his specimens sent can be given only approximately. The sizes (in millimeters) were obtained by compar- ing the distance between the eyes, with the same distance in entire specimens. Dimensions of Ike Eyes of Typhlomolge in Millimeters. LENf.TH OF SPECIMEN. DISTANCE BETWEEN EYES. DIAMETER OF LEFT EYE. DlAMETEl; OF FUI1HT EYE. Longitudinal. Transverse. Longiludin.il. Transverse. mm. 3° mm. 1.44 °-336 0.232 0.368 0.240 47 70 90 I.Q2 3- I0 4.00 •432 •544 .496 .592 .320 •3S4 •432 .400 -432 .608 •544 •592 •3°4 .368 .384 .448 The eye of Typhlomolge is, in many respects, much more degenerate than that of its European caverniculous relative, Proteus. In Proteus the six muscles are all present; in Typhlomolge they have entirely disappeared. In the former all the layers normal to the retina are present; in the latter the conditions are much simpler. In Proteus the lens is still present and blood-vessels still enter the eye; in Typhlomolge no trace of the lens could be found, except in one individual, and blood-vessels no longer enter the eye. While some of the asymmetry may have been caused by reagents, it is evident that there is a great deal of fluctuation in the shape of the eye. The eye is irregular-oval in outline as seen from above, but the optic nerve enters it at the posterior half of its inner face. The eye increases materially in size from the smallest to the largest of specimens examined. This increase is not directly proportional to the increase in the length of the animal, so the young have relatively larger eyes (fig. 4). The eye lies immediately beneath the skin, to which it is attached by a connective tissue mass which is horizontally elongate. The axis of the eye makes an acute angle with the surface of the skin, the eye being directed outward and forward. The dermis over the eye does not differ from that in the neighboring tissues. The epidermis, in the largest individual, is perceptibly thinner over the eye, i.e. from the continuation of the axis of the eye to the surface of the epidermis. The measure- ment, in the largest individual, of the epidermis at a point over the eye and 320 fj. above and below this point gives the following: thickness over the eye 73 p., 320 fj. above the middle of the eye 96 p., 320 p. down from the eye 80 p. 1 See Trans. Am. Microsc. Soc. xxi. p. 49, 1900. 32 BLIND VERTEBRATES AND THEIR EYES. The same elements are found over the eye that are evident in other regions. There is no indication of a past free orbital rim; the dermis and epidermis are directly continuous over the eye. There are no eye muscles and no glandular structures connected with the eye. It is surrounded on all sides, except where it becomes assoi iatcd with the skin, by loose connective tissue meshes filled with fatty tissue, and is bound to the dermis by many fibers running in various directions, and among these a few pigment cells are found. SCLERA AND CHOROID. (a) largest spi'diiii'ii* : Cartilaginous elements are found in the sclera of but two eyes. Fn one individual, go mm. long, the left eye possesses a cartilage, while there is none in the right eye. It is in this case placed just above the entrance of eft. e P. FIG. 4. Outline Sketch of Part of Section of Head of Specimen of Typhlomulgc ralhbuni, QO mm. long, showing Position of Eye. the optic nerve and measures 96 /x in thickness, i6o/A vertically/and 204 p. antero- |>o-teriorly. In all other cases the sclera is a thin, flocculent layer not distinctly separable from the layers beneath it. It is thickest about the entrance of the optic nerve. ( )vcr the front of the eye there are a few denser strands, which may repre- -i 'lit the remains of the cornea. Over the sides of the eye of the largest individual tin- sclera measures from 4 /a to nothing. About the entrance of the optic nerve it attains a thickness of i \ /j., and contains many flat nuclei with a length up to 17 fi. The ' horoid reaches a thickness of 20 /x near the entrance of the optic nerve, and dwindles regularly from this point to the distal face of the eye. Blood-vessels are lound in it nexl to ihe pigmeiited epithelium of the eye. Otherwise it is a mass of pigment interlarded with streaks of colorless tissue containing nuclei. < >\vr the Ironl of the eye, next to the epithelium, there are a number of colorless cells with large, granular nuclei. ssentially the same conditions exist in younger specimens, but the parts are The ophthalmic artery, extending approximately parallel with the optic nerve during its di>tal course, is sometimes surrounded by pigment. EYES OF THE TEXAN CAVE SALAMANDER. 33 THE PIGMENT LAYER, EXCLUSIVE OF THE IRIDEAL PARTS. The pigment layer is a thin, compact layer, densely pigmented. In an indi- vidual 30 mm. long it is a bout 8 /n in thickness. As there are no rods and cones, the inner surface of this layer is similar to the outer, that is, the cells form a pa ve- rt 7 / I a. op. - i. 1 n.op FIG. 5. (j) Right Eve of Specimen of Typltlomvlst 30 mm. long. (6) Exit of Optic Nerve of Same, (f) Iris of Left Eye of Same Specimen, (d) Upper Half of Iris of Right Eye of Specimen "I Typhtamolge 70 mm. long. ment epithelium. In places, however, processes of the cells extend in among the cells of the nuclear layers, for a distance of 40 /A in some cases (fig. 5) Vertical Section through Cornea and Lids of Adult. the upper lid, which closes over the lower, is as thickly pigmented as any other part of the body. Stejneger says of the eyes that they are "small, only slightly raised, and covered by the continuous skin of the head, with only a shallow groove to indicate the open- ing between the lids, the underlying eyes visible as two ill-defined dusky spots." In sections the lids are seen to overlap one another some distance, forming an obscure, free orbital rim. Figure 9 b is a median section of the lids and cornea! epithelium of an eye 0.954 mm. in diameter, taken from an adult specimen 106 mm. in length. In this section the upper lid overlaps the lower lid 0.216 mm., or more than one-fifth the diameter of the eye. Passing from the median section toward the corners of the eye, the lower lid unites with the underlying tissue first. When observed from the top, the upper lid covers the eye entirely. The orbital slit is 0.17 mm. in length. The conjunctival pocket extends some distance forward and back- ward beyond the slit. The eye increases in size but little from the larval to the adult stage and its growth is not proportional to the growth in length of the ani- mal. (See comparative measurements of the eyes at the close of the chapter.) The following is a series of measurements (in millimeters) on the larvae of Typhlotriton : Locality. Length of specimen. Size of pupil. Length of cyt. Length from optic nerve to front of lens. Vertical diameter. I.3O o 80 78 0.640 I .so 1. 20 1.248 88 I 28 Sections of the adult and larva from Marble Cave were made in the usual manner. The six normal eye muscles are present in Typhlotriton. The m. recti form a sheath about the optic nerve in its distal part and spread out from it 38 BLIND VERTEBRATES AND THEIR EYES. near the eye. In the adult the sclera is a layer of uniform thickness except in the region of the entrance of the optic nerve. It is not usually separated from the adjoining parts of the eye, but in places is retracted a short distance from the choroid coat by the action of reagents. It is for the most part fibrous, with few compressed nuclei, and varies from i8p. to 40 ^ in thickness. In the larva a narrow cartilaginous band surrounds all but the ventral wall of the eye. In a specimen 35 mm. long the width of the band is about 30 p., its thickness 16 p.. In three adult specimens the- sclera of only one had any traces of cartilage. In the right eye of the adult specimen 103 mm. long a cartilage about 36 p. thick, 60 p. wide, and not more than 40 p, long is found on the upper face of the eye. The absence of this cartilage in the adult has probably no connection with the degeneration of the eye. Its presence is probably a larval characteristic which disappears as the gills disappear during the metamorphosis. )*'' ©oc (I .11 of Retina. exclusive of Pigment Cells, of Larva 35 mm. long. (6) Tangential Section through Rods and Conesabout on A itli Innermost Extent of Pigment (seen on Right) showing Relative Sizes and Abundance of Rods and Cones, (c) Section of Ret- ina of Larva 48 mm. long, (r/) Section of Retina of Larva QO mm. long, (c) Tangential Section showing Rods and Cones at about Inner Limit of Pigment (seen on Left). (/) Section of Retina of Adult ic6 mm. long, (g) Tangential Section at about Inner Limit of Pigment, (h) Section of Retina of Adult g? ram. long. The average thickness of the cornea is 40^. In the adult it is covered by a layer of stratified epithelium, 25 p. in thickness, consisting of three rows of cells. The cells of the inner row are columnar in shape, those of the middle row rounded, and those of the outer row very much flattened and elongated (fig. 9 b). In the adult the choroid coat is usually separated from the pigment layer, but adheres closely to the sclera. In general it is thicker at the back part of the eye, and quite decidedly so at the entrance of the optic nerve. The lens is normal. Its si/e is given in the table on page 40. The layers of the retina arc well developed in the larva. The retina of the larva differs from that of an Anihlyslonui larva in the greater thickness of its gangli- onic layer. This layer is, in the young larva of Typklotrilon, composed of 5 or This thickness may in part be an artifact, since the retina? examined an brunken away from the pigment epithelium and the ganglionic layer EYES OF TYPHLOTRITON FROM MISSOURI. 39 is in contact with the lens. In the larva 90 mm. long this layer has been reduced to not more than 3 series of cells. Aside from the differences noted above, the eye of the larval Typhlotriton is apparently normal in all of its histological details. The relative thickness in the different sizes of the larvae may be gathered from figures 10 a to d and from the comparative table at the end of this chapter. Figures 10 a to/ are drawn with the same magnification and show the relative thickness of the different layers in the retime of the larvae of different sizes and of the adult. The adult retina is reduced in thickness by the absence of the rods and cones and the (partial ?) atrophy of the outer reticular layer and by the thinning of the ganglionic layer. The ganglionic layer in the adult contains from two to five rows of cells. In this respect, the adult approaches the condition found in the Amblystoma more than the young does. The inner reticular layer is comparatively thick, that of the young being thicker than that of the adult. In the adult the inner nuclear layer is continuous with the outer nuclear layer. (See fig. io/.) The inner nuclear layer consists of about 7 series of cells in the smallest larva and of 4 to 7 in the largest. The cells in the preparations available can not be separated into bipolar and spongioblastic layers, nor are the horizontal cell layers distinguishable. The outer reticular layer is well differentiated, but quite thin in the larvae, and is irregular in outline, adapting itself to the overlying nuclei which encroach on its outlines. In the adult this layer is indistinguishable by the same methods that make it conspicuous in the larva. In places there appeared an open space where the outer reticular layer should be (fig. io h 4), but none of its structure remains. It is fair to suppose that the fibers forming this layer are resorbed during the metamorphosis. This layer seems to be the very first obliterated by the pro- cesses of degeneration both ontogenetic and phylogenetic in this as in other verte- brates with a degenerating eye. The greatest change during and shortly after metamorphosis takes place in the layer of the rods and cones. In the larva 35 mm. long, from the mouth of Rock- house Cave, the rods reach an extreme length of 50 //,. The relative sizes and number of these as compared with the much smaller cones may be gathered from figure 12. In the larva 90 mm. long the outer segments of the rods are much shorter and stain less conspicuously than in the younger. The nuclei of the outer nuclear layer are distinctly in 2 layers, whereas in the younger specimen they are in 3 less regular layers. The cones A are correspondingly fainter than in the young. It ® is surprising that whereas in the larva 90 mm. long we find the rods and cones well developed, they have Fm „ ((])0nlvConefoundinEvesof Adult, greatly degenerated or practically disappeared in the adult only a few mm. longer. In an adult specimen 97 mm. long the rods have retained their normal shape and position, but no differentiation into inner and outer segments was detected. In longer ones most of the nuclei of the outer series have become rounded at both ends. But one cone was found in eyes of the adult over TOO mm. long. It is shown in figure n a. In an adult specimen 103 mm. long filmy rods are still evident. They appear as conical spaces above the nuclei free from pigment rather than as possessing any 40 BLIND VERTEBRATES AND THEIR EYES. demonstrable structure. Just at the margin of the place where the pigment has been torn from the retina one of these is drawn out to a great length. The pigment in this individual extends in places down between the nuclei of the cones. This latter condition appears in a very exaggerated form in the eye of Typhlomolge. In tangential section this condition and the filmy rods give rise to the appearance represented in figure 10 g. Distinct signs of ontogenetic degeneration are also seen in other parts of the retina. For instance, many nuclei of the inner series of the outer nuclear layer are shriveled. In some eyes the ganglionic nuclei have for the greater part lost their granular structure and show a homogeneous pasty condition, only a few cells with granular nuclei being present (fig. io/). The same is true in large part of the inner nuclei of the inner nuclear layer. This condition of the ganglionic nuclei is not entirely confined to the adult but is also found in the larva. Some of the modifications in the shapes of the outer nuclei in the adult are shown in the figures. In figure 1 1 b the upper part of the nucleus is very much elon- gated. This form is of frequent occurrence. In figure n c is shown the common form where the nuclei are simple elliptical bodies, which give no evidence what- ever of any processes uniting them with the other elements of the retina. The Mtil- lerian fibers are profusely present and of very large size in both larva and adult. In both adult and young the optic nerve enters as a single strand and passes entirely through the layers. A heavy mass of pigment is found following the optic nerve to within a short distance of the brain. Average Measurements of the Eyes of Typlilotriton. Length of Specimen. 35 mm. 48 mm. 62 mm. oo mm. 97 mm. 103 mm. 106 mm. V( riiral diameter of eve 810 600 76 I 7" 16 68 4 20 342 Soo 672 42 2 72 20 s<> 16 25 3°° 112 80 16 64 960 720 36 5° 24 32 500 720 28 48 8 24 8 432 800 720 28 72 8 26 20 23 43° 1170 H34 72 13 20 22 29 5°4 1 i -in front of lens to back of eye. . . . < )uter nuclear layer with the rods. . . . ( Hlter retieuhir lavi-r . . Inner reticular layer i tangl ionic layer . . . J'i^nn nt l.i\'T * )ptir nerve 1 ens CONCLUSIONS AS TO THE EYE OF TYPHLOTR1TON SPEL^EUS. ( i ) Tin- eye lies just beneath the skin. The skin is but little thinner over the rye than elsewhere and shows no structural characters different from those of neighboring regions. (2) The eye mu-cles have vanished. The lens has vanished and its place has in part become filled by an ingrowth of choroidal tissue containing pigment. The vitrcal body is very small, if present at all. The vitreal cavity is a funnel or trumpet -haped space. I In pigmented layer of the retina is a pavement epithelium with indistinct boundaries and with occasional pigmented processes extending into or through the nuclear layers. CONCLUSIONS CONCERNING EYES OF SALAMANDERS. 41 (6) Rods and cones are not found. (7) The outer reticular layer has disappeared. (8) The inner and outer nuclear layers form one layer of cells indistinguishable from each other. (9) The inner reticular layer, as usual in degenerate eyes, is relatively well developed. (10) The ganglionic layer is well represented and connected with the brain by the well-developed optic nerve. (n) The epithelial part of the iris is at first simple, with an outer pigmented and an inner colorless layer. With age the margins of the iris become folded in- ward in such a way that the pigmented layer may be thrown into folds in the interior of the eye, while the colorless layer is but little affected. (12) Pigment granules, and rarely pigmented cells, are associated in the eye with the optic nerve. (13) The eye is more degenerate than that of the European Proteus. It is less degenerate than that of the North American blind fishes, Ainblyopsis, Typhlichlhys, and Troglichthys, but much more so than that of the species of Chologastcr. SUMMARY IN REGARD TO TYPHLOTRITON. (1) Typhlotriton is an incipient blind salamander living in the caves of south- western Missouri. (2) It detects its food by the sense of touch without the use of its eyes. (3) It is stereotropic. (4) Its eyes show the early stages in the steps of degeneration from those of salamanders living in the open to those of the degenerate Typhlomolge from the caves of Texas. The lids are in process of obliteration, the upper overlapping the lower so that the eye is always covered in the adult. The sclcra possesses a car- tilaginous band in the larval stages but not in the adult. The disappearance of the cartilage is probably an incident of the metamorphosis, not of the degeneration the eye is undergoing. The lens is normal. The retina is normal in the larva with a proportionally thicker ganglionic layer than in the related epigean forms. (5) Marked ontogenetic degenerations take place during and shortly after the metamorphosis, (a) The outer reticular layer disappears, (b) The rods and cones lose their complexity of structure, such as differentiation into inner and outer segments, and finally are lost altogether. 42 BLIND VERTEBRATES AND THEIR EYES. THE BLIND REPTILES. AMPHISB£NA PUNCTATA.1 Amphisba-na piinclala (Bell) is a blind, legless lizard which burrows in the ground. It is common in Cuba, to which place it is restricted. How deep it burrows can not be stated, but it is often turned up by the plow. The specimens obtained ranged from 103 to 245 mm. in length. The head is short, hard and pointed, and the tip of the upper jaw projects slightly beyond the tip of the lower. In shape, arrangement of the dermal plates, and color of the ventral surface of the body it closely resembles an earthworm. The dorsal surface is flesh-color with small brown spots. The tail is short and flattened dorso-ventrally. In a specimen 245 mm. in length, there were 225 annuli on the dorsal side, 202 on the ventral, and 15 on the tail. In this specimen the tail was one-thirteenth and the head one-thirty- ' fifth the length of the body. METHODS. The lizards were put alive into formalin. They were afterwards put into alco- hol. For decalcification, the heads were placed in 5 per cent nitric acid from 20 to 30 days. A shorter period did not give satisfactory results. Some heads were embedded in paraffin and others in paraffin and celloidin. In using the latter method the head was embedded in celloidin in the usual manner and hardened in chloroform. From chloroform the block was transferred to soft paraffin for 24 hours and thence to hard paraffin for 24 hours, after which it was embedded in paraffin. The best results were obtained from those embedded in paraffin and celloidin. Several methods of staining were used ; iron haematoxylin with eosin as a counter stain gave the best results. The more modern methods of treating the retina with silver could not be applied for lack of fresh specimens. On account of the extreme toughness of the cuticle it was impossible to get complete series of M (lions. For comparison the eye of Anolis carolincnsis has been examined. GENERAL ACCOUNT OF THE EYE. The eye of Amphisbcena appears indistinctly as a small black spot beneath the ocular plate (fig. 12). In a specimen 225 mm. in length, the eye is 352 p. beneath the surface, 420 \j. in width, and 360 p. in depth. The conjunctival sac is 116 //. in diameter. The conjunctiva is very thin over the cornea, but measures 4 w in thickness over the anterior part of the sac. Ko (Bell) ;. showing I • ~ ' r I'hc dermis and epidermis have the same structure over the eye as over the regions near by. This corresponds with vlint l-'.i^i-nmunn ("The Eyes of Rhineura floridana," 1902) found in Rhineura, although the eye of Rhineura is a much more degenerate organ than the eye of '.a, but to what extent the eye is degenerated from a more elaborate stated. Few organs arc stationary, and this one is probably 3 of reduction. The writer has been unable to obtain the young, s no means of finding out from the adult whether the eye is degenerat- 1 By Fcrnandus Payne. Sri- Hiol. Hull. \i. 60, 1906. AMPHISB^NA. 43 lens ing at present or not. In each specimen examined the eyes appeared in about the same state of degeneration. The eye measures 1,224 /* m circumference and the pupil 104 /j- in diameter. The uveal part of the iris on each side of the pupil measures 250 fj-. The pupil and iris occupy 49.3 per cent, or very nearly half, of the entire circumference. Harder's gland is very much larger than the eye. In a cross-section through the central part of the eye, the antero-posterior diameter of the gland is approxi- mately three times and the medio-lateral diameter four times the meclio-lateral diameter of the eye. It is divided into two distinct lobes, the anterior being much smaller than the posterior. The gland completely surrounds the eye except over the anterior face. Its secretion is poured into the conjunctival sac and from thence into the mouth cavity. The large size of the gland in Typhlops led Duvernoy to the conclusion that its function was not connected with the eye. As its secretion, in Amphisbcena, is poured into the conjunctival sac and thence into the mouth cavity, its function must have been, primarily at least, connected with the eye. No eye muscles are present in AinphisbcEiia. The eye is directed outward and for- ward and makes an angle of about 60° with a line drawn tangent to the dermal plate which covers it. Whether the eye is still used as a sense-organ is not certain, but since the parts are so well developed and the eye is not buried very deeply beneath the surface, it is probable that it is at least susceptible to light. The Sclera. -- The sclera (scl., fig. 14) has apparently undergone no FIG. degeneration whatever. It compares favorably with that of Anolis. In fact, there is but little difference in its structure in the two eyes. At the proximal part of the eye, the sclera measures 1 2 /* in thickness, while at the same place in Anolis it measures 15 /A. It is continuous over the front of the lens as the cornea, which together with the thin wall of the conjunctival sac at this place measures 7 p.. Scleral cartilages extend from about the middle of the eye back almost to the optic nerve. On each side of the sclera, and forming a part of it, are thin irregular layers of pigment in patches. MINUTE ANATOMY OF THE EYE The Choroid. — If the blood-vessels in the choroid still persist, the preparations do not show them. All that can be seen is a number of densely pigmentecl cells, around and between which are filaments of connective tissue (chr., fig. 14). At the entrance of the optic nerve, this layer measures 8 ^ in thickness, but gradually becomes less forward and vanishes entirely a short distance back of the enlarged end of the pigment layer. The pecten, present in Anolis, is not seen in Amphisbcena. 3. Diagram of Eye, showing Parts in their Relation and Dis- tance of Eye beneath Surface. pigment layer; 2, cones; 3, outer nuclear layer; 4, outer retic- ular layer; 6, inner nuclear layer; 8, inner reticular layer; 9, ganglion-cell layer; 10, fiber layer; lens, lens; scl., sclerotic; chr., choroid; cor., cornea; scl. c., scleral cartilage; n. op., optic nerve; I'it. vitreous cavity; con. cai1., conjunctival cavity; C., outer covering of eye; A/., Mullerian fiber; L., membrana limitans externa. 44 BUM' YI.KTEBRATES AND THEIR EYES. The Lens. -- The lens has retained its natural shape and position (lens, fig. 14). Tt is almost spherical and measures So fj. in diameter. In most of the sections an outer layer of cells extends around the anterior surface of the lens. The interior in nearly every case stained as a structureless mass, but in a few sections it appeared to be made up of large irregularly shaped cells with small nuclei. If any fibrous cells still persisted, they did not show. No capsule is present. FIG. 1 Horizontal Section of Eye showing Different Parts. Retina diagrammatic. For explanation of letters see fig. 13. I In Vilreous Body. - The vitreous body (ml., fig. 14) occupies the greater part the eyeball and has certainly undergone but little change. The aqueous cavity entirely disappeared. - Only the uvcal part of the iris remains. It is continuous with the pigment epithelium of the retina and has the same structure. In the thickest part it measures 68 p.. The cells are similar to those of the pigment layer, except that their radial diameter is much greater. The ciliary processes are no longer present. 'I In Of'tii Nerve. -The optic nerve can be traced from the eye, through and along the >ide of I larder's gland. While the nerve could be traced no farther on account of an incomplete series of sections, there is no doubt that the connection The nerve fibers enter the eye in a compact mass, pass through the layers of the retina until they reach the nerve fiber layer, where they AMPHISBvENA. 45 spread out and connect with the nerve cells of the ganglionic layer in the usual manner. The Retina. — While the retina has undergone considerable change, all of the layers are still present (fig. 15 a). It measures 78 p in thickness. In Anolis about half-way between the anterior and posterior parts of the eye it is 179 p. in thickness. If the macula lutea is still present, the preparations do not show it. The Pigment Layer. - The pigment layer (i, fig. 14), which bounds the retina externally, consists of a single stratum of rectangular cells separated by a small amount of clear intercellular substance. These cells have large oval nuclei free from pigment, almost transparent and with small nudeoli. At the back part of the eye, where the pigment layer measures 8 fj., the transverse diameter of the cells Flo. 15. ( structures. The fibers of Miiller pass through it as fine vertical hlaments. Occasionally there is a nucleus from the nuclear layer or from the gang] ionic layer which lies embedded in the edge of this layer. AMPHISB^ENA. 47 The Ganglionic Layer. — The ganglionic layer (9, fig. 15 a) consists of a single layer of nuclei 6 p- in diameter, with now and then another nucleus above or below the single layer. From the outer side of these nuclei, fibers which run out and penetrate the inner reticular layer can be traced for a short distance. On the opposite side are also fibers which continue as fibers of the nerve fiber layer. In Anolis this layer measures 23 /A and is made up of loosely connected nuclei, some of which are large and spherical, others are smaller and irregular, while still others stain very densely. The Nerve-fiber Layer. - - The nerve-fiber layer is 6 p. in depth, while in Anolis it is 26 fj.. The Fibers of M tiller. - - The Mullerian fibers can be traced from the membrana limitans interna to the outer nuclear layer. They commence at the inner surface of the retina by a broad conical foot which extends into the ganglionic layer. Through the inner reticular layer the fibers pass as fine filaments, but in the inner nuclear layer each fiber is characterized by an irregularly shaped nucleus, which stains densely and shows no structure. The membrana limitans externa is not visible. These fibers differ but little from those in Anolis, except that those in Anolis can be traced to the membrana limitans externa, which is plainly visible. 48 1JI.IXD VERTEBRATES AND THEIR EYES. RHINEURA FLORIDANA.1 HABITS OF RHINEURA. Rhineura Jloridana Baird is a legless, burrowing, blind Amphisbsenian lizard. It is abundant in some parts of Florida. The largest individual secured by the author measured 340 mm. The tail is very short, flattened dorso-ventrally, and the upper surface of its distal half is strongly rugose. Each of the transverse rings is here, with numerous tubercles. The mouth is small ; the tip of the lower jaw is some distance behind the tip of the upper jaw. In shape, color, and arrange- ment of its dermal plates it strikingly resembles an earthworm. This resemblance is heightened by its vermiform progression through the rhythmic movements of its annular plates. Its forward and backward locomotion in its burrows is entirely due to this vermiform movement. It burrows rapidly, and for this its small, hard, conical head is well adapted. The point of the snout is turned down and the head then thrust upward in a rooting fashion. An individual will readily dis- appear in from half a minute to two minutes. By placing it in a glass vessel partly filled with earth its burrowing can readily be seen from below. If placed on a bare surface, it for a time will wriggle actively from side to side, snake fashion, but without much effect as far as locomotion is concerned. The tail, under such cir- cumstances, is dragged behind, as if it had no vital connection with the head. Rarely there is a suggestion of a bracing with the tip of the tail against the floor. In one minute an individual moved 250 mm. In an attempt at rooting, after the snout had become wedged under the edge of an immovable object, the whole body to the tip of the tail was repeatedly lifted off the floor. Rhuu'itni is, as far as known, one of the two blind vertebrates that have been found in the fossil state. Baur described a species of Rhineura (R. htitcherii) and another Amphisbtenian (Hypsorhina nnti^iiit) from the Miocene beds of South Dakota. I'.aur says nothing concerning the dermal plates, so that nothing is definitely known about the eyes of this fossil Rhineura. Since all the genera of the family AmjJhisbaenidae have rudimentary eyes, the eyes were very probably degenerate before the genera became separated. It seems quite certain that any fossil members of an existing genus all of whose living species have degenerate eyes, must have had eyes that were to a greater or less extent degen- erate. The time suggested by this find of Baur during which the eyes of Rhineura have been degen- FIO.I; .1 ii, ,,i ,,i /;/,„„,„„ crating is surprisingly long, extending as it does slum i-i face Plates and Position of .. , . . ' . through about 5 to 10 per cent of the formation of sedimentary rocks. Rhincnni is a burrowing animal, and blind animals which burrow in the ground are not loimd in naturally made caves. The latter are largely populated by species that lend to hide in ( revices or natural cavities under rocks. It would seem from this that the cave fauna was incipient In-fore the existence of caves, and that the latter were coloni/ed as soon as they were large enough to admit their present inhabitants. 1 ' V :nl. Si i., l\ . |i. 5 ^, 1902. EIGENMANN Rhmeura flondana. A. Side and dorsal views of tail. B. Horizontal section of head, showing Harder's gland and position of eye. C. Horizontal section through right eye, showing solid strand of cells, extending from Harder's gland to near epidermis. D. Horizontal section of left eye, showing extent of pigmentation and lens. E. Distal part of another section of same eye, showing different layers of retina at their highest development. 2 mm. objective. F. Proximal part of another eye, showing cyst represented diagrammatically in text- fig. 1 9c. 2 mm. objective. RHINEURA 49 GENERAL ACCOUNT OF THE EYE OF RHINEURA. The eye of Rhineura Jiorldana is not visible externally, nor is there any indica- tion where it formerly came to the surface. The side of the head is continuously covered with plates. There are 4 labials (i, 2, 3, and 4, of fig. 17), the posterior of which is comparatively large. Above the labials from in front backward lie a single nasal (5), a single loreal (6), a single preorbital (7), and a group of temporals (8). Above this series of plates lie a supranasal (9), joined to its fellow of the other side, a prefrontal (n),and 2 supraciliaries (12, 13). In heads cleared with xylol the black eye can be seen to lie underneath the angle between the 2 supraciliaries and the preorbital. The dermis and epidermis over the eye are not different from these structures over neighboring regions except that in one instance (plate 3 c, df) a solid column of cells 32 /A thick extends from Harder 's gland to near the epidermis, without, how- ever, fusing with the latter. Fisher found that in Trogonophis the epidermis is reduced to half its thickness and free from pigment over the eye. In Amphisbcena strauchi and A. darwini the skin is not thinner and the pigment is little or not at ah1 less over the eye. A conjunctival sac has been described for various Amphis- baenians. No such structure is present in Rhineura. Harder's gland (plate 3, figs. B, c, H.gl.} is out of all proportion to the size of the eye. In a horizontal section it measures about 4 times as long as the eye (medio-laterally) and 3 times as wide (antero-posteriorly). Duvernoy found that in Typhlops Harder's gland is 10 times as great as the eye. It is divided into 2 distinct lobes, that over the anterior face of the eye is histologically quite different from that over the posterior face. In vertical section the gland is seen to entirely surround the eye except sometimes at its lower posterior quarter. The large size of Harder's gland has given rise (Duvernoy) to the conclusion that its function is not connected with the eye. Its secretion is poured directly into the tear duct and through it into the nasal cavity. The distance of the eye beneath the outer surface of the epidermis measures between 320 and 560 p. in specimens between 280 and 310 mm. long. It is sur- rounded by 2 layers of connective tissue. These are thin over the distal half of the eye. Over the proximal narrow end of the eye they become thick ; and since they are prolonged beyond the eye, stain a different tint, and readily become separated, they are easily distinguishable. They probably represent the sclera and choroid. If so, the choroid is practically free from pigment except possibly in rare instances where a few pigment granules were detected in cells closely applied to the eye. There is no indication of any differentiation into a cornea or capsule of any sort. The fibrous sheaths are at the proximal end drawn out into a cone. A supposed scleral cartilage has been found in one individual. Here a bar about 20 p- thick extends from over the center of the distal face of the eye for 160 p. around its pos- terior face. It stains and has the structure of bone rather than of cartilage. No traces of any muscles have been found connected with the eye. The eye is directed outward and forward. Its axis is horizontal and makes an angle of about 60° with the sagittal plane of the body. It does not occupy a defi- nitely fixed position on its axis, for in the eye of one side the choroid fissure was found directed caudad, in the other eye ventrad. It is irregularly pear-shaped, with its anterior face convex, its posterior face flat or even concave. The eyes in 3 speci- mens give the following measurements in microns : 50 BLIND VERTEBRATES AND THEIR EYES. Measurements (in microns} of eyes of Rhineura. \li 1 in LATEHAL DIAMETER. ANTERO-POSTERIOR DIAMETER. DISTANCE FROM SURFACE. \ : SPECIMEN. Led Kinht eye. Lefl eye. Risht eye. Left eye. Right eye. mm. 275 32° 320 128 176 4>So 336 280 312 298 1 60 iSi 320 368 31° 320 320 216 176 560 560 MINUTE ANATOMY OF THE EYE OF RHINEURA. All the structures vary greatly in different eyes so that the terms "sometimes," "usually," "frequently," etc., have to be used much more than is desirable. This can not be avoided unless each eye is given a distinct description. c/ir.f. «*— 1 ">ii through Middle of Left Eye of Rhineura, about wo mm. Ion?. B tion through Distal Part of Eye of Rhineura, showing Lens with Capsul -I Individual, a?s mm. long. Horizontal Section. I i-ii Lena of Same Individual. RHINEURA. 51 (a) The Iris. — In the structure of the irideal region the eye of this species is unique among the degenerate vertebrate eyes so far described. In all other eyes, with the possible exception of Troglichthys, elements of an iris are distinctly recog- nizable. In Rhincura the fold of double epithelium between the pigmented and un- pigmented part of the retina whose margin is the margin of the pupil has been obliterated and the pupillary edge forms the extreme outer edge of the blunt end of the pear (p, fig. 19 a). The pigmented layer of the retina in other words merges directly into the unpigmented layers of the retina. The entire thickness of the retina is thus exposed at the distal face of the eye. chr. I L *<=* FIG. 19. (a) Horizontal Section of Left Eye of Specimen, 280 mm. long. (6) Another Section through Same Eye, showing Exit of Optic Nerve, the Pigmentless Condition over Anterior Face of Eye, ami Invaginatcd Pigment at End of Pear. (c) Outline of Pigment in Proximal End of Right Eye of same Individual, showing Invagination of Pigment to form a Cyst. 52 BLIND VERTEBRATES AND THEIR EYES. (b) The Vitreous Body. --The vitreous cavity is represented by a vertical slit extending from the axis of the eye downward to the edge. The choroid fissure (fig. 18, chr.f.) thus remains permanently open in so far as the edges of the opposite sides of the fissure are not united. A space a few microns wide was found in one eye. In other cases there is no real cavity and no vitreous body. The hyaloid membrane (fig. 18 and fig. 19, hd) is represented by a few cells with elongated nuclei. Blood-vessels were not found in it.1 (c) The Lens. — In two specimens no traces of a lens were found, but in two other specimens a lens was present. There being no pupil and no vitreous cavity, the lens is situated in a little depression in the distal face of the retina (figs. 1 8 b, c, d). The lenses differ greatly from each other. In the better developed instances (fig. 18 6) it is composed of a spherical mass of cells. The nuclei are granular and are surrounded by a hyaline cell body. These little cap- sules are closely packed in a slightly darker matrix. The whole lens is surrounded by a fibrous capsule containing elongated nuclei. Both eyes of one individual are provided with lenses as described. In another individual the 2 lenses differ ma- terially not only from those described, but from each other both in structure and size. The left lens consists of a lenticular nodule containing about 6 dense nuclei (fig. 1 8 d). On the right side (fig. 18 c) the lens is much larger. It consists of 2 large nucleated capsules surrounded by a matrix containing a few dense elon- gated nuclei similar to those of the capsule surrounding it (figs. 18 b, c, and d, are drawn to the same scale). The difference exclusive of size between the 2 lenses c and d and the lens represented in figure 18 b, may be due to differences in the method of preparation. (d) The Retina. - - The numbers in the following paragraphs are not consecu- tive, but are those used to designate the corresponding layers in the figures. (i) The pigment epithelium forms a complete outer layer of the eye exclusive of its distal face and a narrow strip along the choroid fissure. The extent to which this epithelium is pigmented differs greatly in different eyes. A region along either side of the choroid fissure is free from pigment, occasionally parts of the anterior face of the eye are free from pigment (fig. 19 b), and very frequently the cells of this layer around the distal margin of the eye are free from pigment. Over the anterior face of the eye this layer is usually composed of a regular layer of cells whether these are free from pigment or not (figs. 19 a, b). On the posterior face the series of cells is not nearly so regular. The pigmented epithelium is here invaginated I folded upon itself in various ways. The infoldings are sometimes solid masses of pigment cells, but sometimes they form hollow spheres which contain a mass of concentrically arranged unpigmented material, probably of choroidal origin (pl;il<- 3, i . ;md le\t-lig. 19 c.) What the significance of these cysts may be I can not conjecture. Indications of similar structures were seen in the eyes of Amblyopsis. The narrow stalk of the pear-shaped eye is usually filled with an irregular jumble of pigment cells. In favorable sections it is seen that these are also the ult of an imagination of the pigment epithelium from the pointed end of the eye (fig. 19 b). The pigment epithelium has not been reduced at the same rate as the rest o! the retina; as a consequence it is infolded in various ways. Small iwn with camera lucida from sections ni.nniinl in balsam; 2 mm. objective and 4 eye- tnade from above down and arc so drawn that the anterior face of the figure is toward the lop of tli'- p RHINEURA. 53 pigment cells are sometimes found in the inner layers of the retina among the gan- glionic cells and along the optic nerve within the eye. Pigment cells were also found in the eyes of Typhlomolgc (figs. 5 a and 7 a, z). There are rarely any pigment cells over the distal face of the eye. (i a) X, nuclei. — In the eyes of Rhiiieura, Typhlichthys, and Troglichthys a few cells with elongated, tangentially placed nuclei are present between the pig- mented epithelium and the outer nuclear layer. They are distinctly outside of the outer limiting membrane (figs. 19 a, b; plate 3, fig. E, x). The origin of these nuclei is difficult to explain. Possibly they are derived from the pigment epithe- lium which in some of the unpigmented regions (fig. 19 b, x) is more than one layer deep. If the outer layer should become pigmented, the inner nuclei, if they remained unpigmented, might give rise to these longitudinal cells. (2) Rods and cones are not present. There is in some cases a distinct space between the pigment epithelium and the outer nuclear layer. This space when present is partially filled with filmy, hazy structures, but nothing suggesting defi- nitely either a rod or cone was detected (fig. 19 a and plate 3, fig. E). (3) The outer nuclear layer consists of about 2 series of elliptical nuclei. They form a compact and distinct layer a few microns from the outer limiting mem- brane (figs. 18 a, 19 a, b, and plate 3, fig. E). (4) The outer reticular layer is represented by a series of distinct but irregular gaps between the outer nuclei and the inner nuclei. Horizontal cells are not pres- ent (figs. 19 a, b, c, and plate 3, fig. E). (6) The inner nuclei are smaller, rounded, and less granular than the outer nuclei. They do not form as compact a layer as the outer nuclei. It is impossible to distinguish between bipolar and spongioblastic cells (6 in figs. 18, 19, and plate 3). (8) The inner reticular layers, as is usual in degenerate eyes, are well developed in the eyes of Rhineura. They are frequently crossed by Miillerian fibers. (9) The ganglionic layer is represented by a number of nuclei loosely grouped about the vitreous slit. The individual nuclei are distinctly larger than those of the inner nuclear layer and less oval than those of the outer nuclear layer (9 in the figures). (10) A distinct optic fiber layer is not present and the optic nerve is nowhere within the eye a compact strand of fibers. A loose flocculent strand of fibers passes through the proximal part of the retina. Its path through the pigmented layer is difficult to trace. Beyond the eye the optic nerve can be followed by means of the fibrous sheaths and pigment cells associated with it rather than by the presence of any fibers with a distinctly nervous structure. The optic nerve leaves the eye, not at the proximal end or the narrow end of the pear, but anterior to the pigment mass in the narrow part of the pear (fig. 19 b, n.op.}. 54 BLIND VERTEBRATES AND THEIR EYES. TYPHLOPS LUMBRICALIS.1 Typhlops linnbricalis (Linnaeus), a blind snake, is generally distributed in the West Indies and Guiana. The specimens examined were obtained in the neigh- borhood of Cafias, Province Pinar del Rio, Cuba. It is a burrowing form that lives just beneath the surface, being thrown out even by the plow. The snakes were first placed in formalin and after a few days were transferred to alcohol. Only one young specimen was obtained, and it was preserved in Zenker's fluid. For decalcification, the heads of some were placed for at least 3 days in 10 per cent nitric acid and others in Perenyi's fluid from i to 2 weeks. One series was stained by the iron haematoxylin process, the others with haemalum and eosin. It was very difficult to obtain satisfactory sections and especially complete series from the specimens, since no method was found to decalcify properly and to get the integument in condition for sectioning. The lengths of the individuals examined were 10, 20, 21, and 21.5 cm. The color is brown above, on the ventral side it is yellowish white. The body is cov- ered with scales of uniform size, while those of the head are somewhat larger. The surface of the entire body is very smooth and shining and rather hard. The tail, which is about one-twentieth of the body's length, ends in a short, sharp spine. The mouth is small and lies on the ventral side some distance back from the tip of the snout. GENERAL ACCOUNT OF EYES IN SNAKE. Snakes differ from other animals in having the edges of the two eyelids entirely grown together. A disk-shaped, conjunctival sac is thus formed and the layers over the eye between this sac and the exterior form the "brille." Six weakly developed muscles are present. The 4 straight ones arise in the neighborhood of the foramen opticus, while the 2 oblique ones arise from the surface of the prefrontal which is turned toward the eye socket. Closely connected with the eye is Harder's gland, whose function is doubtful. Leading from this gland is a single duct, which either empties into the duct from Jacobson's gland or directly into the mouth cavity. The secretions of the gland are thus not functional in connection with the eye. The sclera consists of closely woven fibers. Ciliary muscles are not found, but in -\t In the iris is a great bundle of equatorial muscle fibers running obliquely, .vliich seem to be a continuation of the iris musculature. The ciliary processes are weakly developed. The retina consists of the usual layers. The nerve-fiber layer is very thin (0.003 to 0.004 nim.). The ganglion-cell layer consists of a single, rarely two, layer of small cells, each with a very large- nucleus (0.012 to 0.013 mm.). The inner reticular layer contains, al apparently regular intervals, elongated, oval nuclei (0.042 to 0.045 mm.). The inner nuclear layer consists of two kinds of cells (0.052 to 0.054 mm.). The outer relicular layer is very thin (0.004 to 0.005 rnm.). 1 By Effa Funk Muhsc. Si-c Hint. Bull. vi. p. 261, 1903. EIGENMANN po.s i'/i. Eye of Typhlops lumbricalis. A. Horizontal section, from specimen 20 cm. long. A and B two-thirds objective, 2 inch eyepiece. B. Transverse section, from specimen 2 1 cm. long. (Scales not shown.) C. Diagram of eye of adult. D. Diagram of eye of young. EYES OF TYPHLOPS. 55 The sensory epithelium consists of the outer nuclear layer and the cone layer which is made up of single and twin cones. There are no rods. A single cone consists of two sections, an outer extremely small section, 5 to 6 /A in length and an inner much larger section, almost completely filled with a larger, pear-shaped, strongly refractive body, the ellipsoid, 14 to 16 /A in length and 8 to 9 p. across its widest part, which is turned toward the limiting membrane. The twin cone con- sists of two parts, one similar to a simple cone, the other cylindrical and very slender, its structure being otherwise like that of a simple cone. It is probable that the two parts of the twin cone are connected with but one nucleus. The nuclei of the cones vary greatly in form, and leading from these into the inner layers of the retina are relatively very large fibers or processes. Passing between the limiting membranes are the radial supporting Miillerian fibers. THE EYES OF TYPHLOPS VERM1CULARIS. The work thus far on blind snakes has been done by Kohl on Typhlops vermi- cular is, a species found in Greece and the southwestern part of Asia, and on Typh- lops braminus, a species found in the islands of the Indian Ocean and in Africa south of the equator, accounts of which are given in his "Rudimentare Wirbelthier- augen." 1 He found that in depth the eye of Typhlops vcrmkularis is equal to about one-sixth that of Tropidonotus. The brille is thicker in Typhlops than in Tropidonotits and compared with the axial diameter of the respective eyes it is seven times thicker. In Typhlops the brille is equal in thickness to about half that of the ordinary skin of the head. In Tropidonotus it is equal to one-fourth. The cornea of Typhlops measures 0.0052 mm., and compared with the relative sizes of the eyes is equal to about half that of Tropidonotus, which measures 0.064 mm. The conjunctiva is thickened at the edge of the disk-shaped sac and consists here of gland cells, the fornix conjunctiva. The supporting membranes of the eyeball, choroid, and sclera are relatively equal to about half those of Tropi- donotus. Harder's gland in Typhlops is many times larger than the eyeball. The six muscles are present. The lens is elliptical, while that of Tropidonotus is almost globular. The ratio of the lens volume of Typhlops to the eye volume is i to 14.04, while in Tropidonotus it is i to 3.6. The lens epithelium of the former is relatively 6 times greater than that of Tropidonotus. The retina at the back of the eye of Typhlops, and the retina of Tropidonotus bear the actual ratio of 8 to 13, while compared with the eye axis in each case the Typhlops retina is 4 times greater. The fovea centralis and area are absent. The fiber layer has its greatest thickness near the exit of the nerve and gradually becomes thinner until, near the iris, scarcely a fiber is found. The globular gan- glion cells are arranged in a single layer except occasionally for short distances, when they lie in a double row. The inner nuclear layer seems to be subdivided into four layers. There are no twin cones. Each cone consists of a cone cell, stalk, middle and end members. The cone nuclei lie in two series, but the stalks vary in length so that the distal ends of the cone members reach nearly the same level. 1 Kohl, Dr. C., Rudimentare Wirbelthieraugen, Erster Theil, Heft 13, Bibliotheca Zoologica. Vi^lag von Theodor Fischer, 1892, Cassel. 56 BLIND VERTEBRATES AND THEIR EYES. THE EYES OF TYPHLOPS LUM BRICALIS. The eye shows through the large ocular scale, which entirely covers it. It appears as a black spot surrounded by an unpigmented circle. The preocular, also a large scale, overlaps the ocular and reaches just to the edge of the eye (figs. 20 a, b). Compared with one of the garter ^ as pas. os. snakes and in proportion to the size 11S ..-isJrw&-j °f trie head, the eye of Typhlops lum- .po.s. /c5(*/*rP bricalis is situated farther from the ra"vx)\^7 surface and occupies far less space, while Harder's gland, associated with the eye in both, is relatively much larger in Typhlops. In a specimen of FIG. 20. W Dorsal View of Head of Typhlops, 21 cm. long TvpklohS 1 lllllbric(llis 21 CITl. in length, (6) Lateral View of Head of same Specimen. . ,~ the eye measured 0.306 mm. in width, and 0.387 mm. in depth. The greatest width of the gland of the same was 0.711 mm. and the length was 1.067 mm. The gland completely surrounds the eye up to the edges of the conjunctival sac (plate 4, figs. A, B). In proportion to the size of the eyes, the gland of the garter snake is much smaller than that of Typhlops liimbricalis, but compared with Rhincnra floridana the gland of Typhlops litinhricalis is but little more than half as large. The eye is covered by layers of epidermis and dermis that differ from these same layers on neighboring parts by being thinner, more compact, and free from pigment and glands. The ocular scale, however, which covers the eye region, does not differ in thickness from the other scales of the head (plate 4, fig. A). A conjunctival sac is present with a diameter at least as great as the greatest width of the eye bulb. The conjunctiva, which forms this sac, is very thin over the cornea and next to the brille, where it measures 0.003 mm. At the edge of the sac it is differentiated into glands, the fornix conjunctiva, and measures 0.016 mm. (plate 4, figs. B and c, F. cj.). In horizontal section, the eye axis is seen to be turned forward about 30° away from a line at right angle to the horizontal axis of the body. Eye muscles are present, but from the sections used, the exact number could not be determined. Choroid and Sdcra. - The dense pigmentation makes it impossible to dis- tinguish between the different coats at every point. Beyond the retina with its pigment layer is an open vascular space, and this is followed by another dark layer, the two together representing the choroid. The choroidal pigmentary layer seems to consist of long fibers circularly arranged. The sclera can be followed by start- ing with the outer covering of the optic nerve and tracing its continuation about the eye. -Here again the pigmentation makes it difficult to determine the structure. Both iris and ciliary processes are present, for the black layer extends over the anterior surface of the lens, leaving a pupil equal in diameter t<> about one fourth of the circumference of the lens. At points near the equator of tlie lens this dark layer is enlarged into the ciliary processes and in connection with the capsule helps to hold the lens in place. EYES OF TYPHLOPS. 57 Cornea. - - This structure is present and can be traced to the region of the ciliary processes. Lens. — A large lens is present, its depth being equal to about two-fifths of the eye depth. From the sections little could be determined about its structure. A well-developed capsule surrounds it (plate 4, fig. c). Retina. --The same layers are present that are found in snakes in general, but the comparative thickness of the various layers is different. In the garter snakes, for instance, the retina is of a uniformly even thickness even to the ciliary process, a single layer of cells continues on over the surface of the processes and iris, but in Typhlops lumbricalis the retina at the back of the eye is very thick and gradually becomes thinner till it ends a short distance from the ciliary processes (plate 4, fig. c). At this point the arrangement could not be definitely determined in the sections. At the back the retina, exclusive of the pigment layer, measures 0.0725 mm. e.m. m.m. S.P.I. FIG. 21. (a) Section of the Retina of an Adult Specimen, 21 cm. long. (b) Section of the Retina of a Specimen, 10 cm. long. Ends of fibers were seen projecting inward from the ganglion-cell layer, but no definite fiber layer could be distinguished (10 in fig. 21 b). The ganglion-cell layer (9 in the figures) consists of a single row of large nucleated cells, somewhat irregularly arranged (0.008 mm.). The inner reticular layer (8) consists of a mass of fibers interwoven in a close network. This layer . measures, at the back of the eye, 0.015 mm. The inner nuclear layer (6) consists of at least 3 layers of cells, loosely arranged. The course of some of the fibers can be followed among these cells. This layer measures 0.016 mm. The miter reticular layer (4) is very thin and consists of a few fibers so arranged as to leave a great number of spaces between the two nuclear layers. The distance between the nuclear layers is about 0.005 mm- Cones. — The sensory epithelium shows two distinct parts, an inner layer of nuclei (3) and an outer row of cones (2). In the sections these two were so separated 5.S BLIND VERTEBRATES AND THEIR EYES. that a loose tissue was visible, consisting probably of the limiting membrane and ends of the Mullcrian fibers. The outer nuclear layer in the adult consists of a single row of nuclei, with a mass of quite homogeneous material about them. This part of the sensory epithelium measures 0.018 mm. The cones are pear-shaped bodies with the smaller end pointing outward, and at intervals of every four or five a shorter one occurs. Each element is differentiated into two parts. By the iron hcEmatoxylin process of staining, the outer small end is densely stained, while the body of the element is a light granular mass (fig. 21 a). The pigment layer (i) is a continuous layer of even thickness, similar in every respect to that of the garter snake. One young specimen, 10 cm. in length, was examined. The eye as a whole, as well as the lens, is nearly spherical. The eye measures in width 0.290 mm. and 0.322 mm. in depth. All parts are so developed that the vitreous cavity is relatively much smaller than that of the adult. The coats are thicker, the ciliary processes better developed, the lens capsule thicker, and the retina at the back actually measures one and two-thirds the depth of the adult retina. The ele- ments of each layer are much more numerous than in the adult, and they are packed much more closely together (fig. 21 />). The ganglion nuclei are apparently arranged one against the other. In the inner reticular layer occur the "interpolated cells." These were not found in the sections of the adult eye that were examined. The cells of the inner nuclear layer are smaller and arranged in five or six rows. There is a well-developed outer reticular layer similar in its make-up to the inner reticular. Instead of a single row of cone nuclei with its surrounding homogeneous mass, as in the adult, this layer in the young consists of five or six rows of small closely arranged cells. The cones likewise are smaller and more numerous (fig. 21 b). Comparative Measurement of Retinal Layers in millimeters. Fiber layer. Ganglion- cell layer. Inner reticular layer. Inner nuclear layer. Outer reticular layer. Sensory epithelium. Total depth. O Os2 • : 1 1 jfi O I 731 oo 1 8 0081 01 zs, .0221 .0022 .0 324 .0821 Tvphlops lumbricalis (adult) ... . .008 .01 s .Ol6 .OO^ .030 .072^ Tvphlops lumbricalis (young, 10 cm.) .005 .010 .024 .032 .008 .040 .I2O& Kflulive Proportions of Eve Parts. Tropidonotus n.itri\. Typhlops vermicularis. Typlilnps lumbricalis (adult). Kvr ilrplh. 2.^41 mm. 0.4399 mm. 0.40.^2 mm. B ille: ixis:: : 77. j : 10.77 : 12.5 CotlM a ; : 59.9 :84.6 = 85 I .I'll ilrjitll : Eyes : : 1.^6 : 3-°3 : =5 ("oats: Kvr a\i^:: : 2i.'s : 38-58 :=5-4 Retina at bark : : : 19.19 : 5.36 : 5-5 CONCLUSIONS. 59 CONCLUSIONS AS TO THE EYES OF BLIND REPTILES. AMPHISB^ENA. (1) The eye muscles have entirely disappeared. (2) Only the uveal parts of the iris remain. (3) The lens has retained its shape and position, but its structure has been greatly changed. No capsule is present. (4) Harder's gland is many times larger than the eye and pours its secretion into the conjunctival cavity and thence into the mouth. (5) The sclera, scleral cartilages, cornea, vitreous body, and pigment epithe- lium have undergone but little change unless it be in the reduction in size. (6) The cuticle passes over the eye unchanged. (7) The aqueous cavity is no longer present. (8) All the layers of the retina are still present. As shown in figure 6, the great reductions in the depth of the layers, in comparison with those of Anolis, have taken place in the nerve fiber, ganglion cell, inner reticular and inner nuclear layers. (9) If the eye has been reduced from an eye of the average size, all parts have certainly undergone considerable change, and this change has been approximately equal among the several parts. (10) The retina does not show such a profound change as either the iris, muscles, or lens. However, it has been greatly changed, as it extends only 50.7 per cent of the distance around the eye. (u) The eye of Amphisbccna shows that the more active parts of the eye are the ones to degenerate first. They are the parts which have been most affected. RHINEURA. (1) The eye of Rhineura has reached its present stage as the result of a process of degeneration that probably began in the early Miocene. (2) The dermis and epidermis pass over the eye without any modifications. The conjunctival pocket has vanished. (3) Harder's gland is many times as large as the eye and pours its secretion into the tear duct and thus into the nasal cavity. (4) The eye muscles have disappeared. (5) A cornea is not differentiated. (6) The lens is absent in half the eyes examined and varies greatly in those in which it is present. (7) The vitreous body has practically disappeared. (8) The pigment epithelium is variously pigmented. It is of greater extent than is sufficient to cover the retina and has been variously invaginated or puckered over the proximal and posterior faces of the eye. (9) An uveal part of the iris is not present. (10) The eye of Rhineura does not represent a phylogenetically primitive stage ; it is an end product of evolution as truly as the most highly developed eye. (n) The adult eye shows few indications that there has been a cessation of development at any definite ontogenetic stage. It does not resemble as a whole any ontogenetic stage. 60 BLIND VERTEBRATES AND THEIR EYES. (12) An arrest in the ontogenetic development has taken place in so far as the number of cell multiplications concerned in forming the anlage of the various parts of the eye have decreased in number, and in the lack of union of the lips of the choroid fissure. (13) It is possible that the absence of cones or rods is due to an arrest in the histogenesis of the retina, but since these structures are normally formed in the young of Typhlotriton, and disappear with age, it is possible that their absence in the adult eye of Rhincura is also due to ontogenetic degeneration. (14) The irregularity in the structure and existence of the lens and the great reduction of the vitreous body offer evidence in favor of the idea of the ontogenet- ically and the phylogenetically earlier disappearance of the ontogenetically and phylogenetically newer structures. (15) Horizontal nuclei found between the pigment epithelium and the outer limiting membrane are probably derived from the proximal layer of the optic cup. (16) The different layers of the retina have reached a degree of differentiation out of proportion to the great reduction of the dioptric apparatus and general structure of the eye. TYPHLOPS. (1) The dermis and epidermis over the eye differ from the same over neigh- boring parts, by being thinner, more compact, and free from pigment and glands. (2) The conjunctiva! sac is present and has a width at least as great as the eye. (3) Harder's gland surrounds all but the distal part covered by the conjunc- tival sac. (4) Eye muscles present, but their number and structure could not be made out. (5) A large lens with capsule is present. (6) The various layers of the normal snake retina are present but the com- parative thickness is different. EYES OF THE POLISTOTREMA STOUTI. 61 EYES OF THE CYCLOSTOME POLISTOTREMA STOITI. The eyes of this myxinoicl of the Pacific coast were examined by Allen and by Stockard. Allen found that they show a very primitive structure, which is in reality the result of a complex process of degeneration. The eyeball is found em- bedded in a mass of fat about three times its size. In one case, the eye was found to lie some distance beneath the outer surface of the mass of fat. Normally, how- ever, the corneal surface lies on a level with the surface of the fat and is often flattened to form a rather extensive free surface. No eye muscles nor traces of such were discovered. No oculomotor nerves were found. No traces of them are discoverable in embryonic life (Kupffer). The choroid and sclerotic coats are represented by a very thin layer of unpigmented, non-vascular connective tissue without any appreciable distinction between corneal and sclerotic portions. The retina remains in the early condition of an optic cup, the outer layer (pigment layer) not being fused with the remaining layers. All specimens showed the layer in question to be widely separated from the bulk of the retina. This pigment layer is composed of a single layer of cubical cells devoid of pigment as far as could be ascertained. A layer corresponding to that of the rods and cones in higher vertebrates is clearly present. The nuclei of these structures (outer nuclear layer) are strikingly well developed and regularly arranged. Certain characteristic cells of the inner nuclear layer could be readily made out. The ganglionic layer is represented by cells scattered irregularly throughout the inner reticular layer. Fibers from these last-named cells can be traced in a more or less direct course to the optic nerve. The outer rim of the optic cup is in many cases differentiated in such a manner as to suggest a rudimentary iris. A structure unmistakably like an iris was found in one specimen examined. The cellular structure of this rudi- mentary iris is almost identical with that of the pigment layer. No indications of muscle fibers or pigment are to be seen. Certain deeply staining coagula within the optic cup give evidence of a vitreous body. Some large, clearly-marked cells, probably those of the vitreous body, are found attached to the surface of the retina. Evidences of a choroid fissure are to be seen in the fact that the ventral part of the retina is thinner than the dorsal in almost all specimens. In one case the choroid fissure was found to persist. The most striking feature, however, is the extreme variation. The optic nerve enters the eye at various angles. Variation occurs in all parts of the eye and is especially notable in the measurements of the thickness of the retina and the dimensions of the eye as a whole. Stockard found that the lens-bud results from a contact of only a portion of the optic cup with the ectoderm. This structure continues to develop for a time until, in an embryo considerably more advanced and measuring 15 mm. in length, one sees the lens-bud with a slight indication of a constriction about the periphery of its area of union with the ectoderm, as if it were preparing to pinch off. Here the progressive development of the lens ceases and degeneration begins. It soon disappears entirely. He considers the cessation of development in the lens due to the absence of a durable contact with the optic cup upon which lens formation is directly dependent. G2 BLIND VERTEBRATES AND THEIR EYES. THE FISHES. GENERAL REMARKS ON THE EYES OF FISHES. It is not the intention to review the literature on the normal eyes of fishes. A list of papers dealing with their macroscopic aspect has been furnished by Ziegen- hagen in 1895, while those dealing with minute structure have been enumerated by Krause in 1886 and Cajal in 1894. The current literature is discussed periodi- cally by Virchow in "Die Ergebnisse der Anatomie und Entwickelungsgeschichte." The topographical relationship of the cells of the retina obtained an entirely new light by the application of the methylene-blue method chiefly on the part of Dogiel, and the Golgi method principally through Ramon y Cajal. The layers of the retina of fishes as made out by Ramon y Cajal are as follows, beginning at the periphery and going toward the center of the eye : 1. Epithelial-pigment layer. 5. Horizontal cells. 8. Inner molecular layer. 2. Rods and cones. 6. Bipolar cells. 9. Ganglionic layer. 3. Outer nuclear layer. 7. Spongioblasts. 10. Optic fiber layer. 4. Outer molecular layer. Throughout this work the layers are designated on the figures and frequently in the text by these numbers. The literature bearing on the eyes of the blind species will be given under the different species. ^ie h°rizontal relations, especially the mosaic of the single and twin cones in the retinas of fishes, has been dealt with by •• Hanover, Miiller, Krause, Friis, Ryder, Beer, Eigenmann, and Shafer. It was found that in many fishes the single and twin cones form a regular mo- saic. The number of parts entering into each unit of the retinal mosaic is remark- ably constant for any species, but differs considerably in different species of fishes. The "shape" of the unit differs in different parts of the retina. The pattern may be made up of twin cones only.1 The axes or lines joining the centers of the components of each twin if continued may be at right angles to each other and form a square (fig. 22 a), or they may be approximately parallel (Scbastodes, c), or they may be FIO. a. Types of sinde.and i),,ui,i,- cones in Retin.is variously inclined to each other and form of Various Fishes. i 1 /o T\ rhombs (ScorpcBna, o). In other genera (Perca, Micropterus, Etheostoma, and Pimephales) a single cone is pliHi'd in UK- center of each of the units of 4 twin cones (d). In still others (Blcnniiis, <•) a single coin- is added at each angle of the unit, and in still others 1 Kiause found only single cones in the eel. GENERAL REMARKS ON THE EYES OF FISHES. 63 (Salmo, Coregonus, /), a single cone is found both at each angle and in the center of the unit. The most complicated unit (Esox, g) is composed of 5 twins, 4 form- ing the sides and i a diagonal, and of 4 single cones, i in each corner. These patterns are all regular, but not mathematically so. In some families (Silurida- and Catostomidse) no regularity could be made out. In general the number of rods is inversely proportional to the number of single cones. FIG. 23. i to 6 show Section of Eye of Bass 6 cm. long. The Eye measured 3.8 mm. in Diameter from Cornea to Back, and 4.7 mm. from Anterior to Posterior Edges. 7, 8, and 9 show Sections of Eye of Bass 33.5 cm. long. The Eye was 10 mm. in Diameter from Cornea to Back, and 13 mm. from Anterior to Posterior Edges. All figures drawn to the same magnification. C, part turned toward cornea; B, part pointing from cornea. In the black bass, the only species in which the pattern was examined over the entire eye, the number of components in each unit of the mosaic is the same, but the shape of the pattern varies regularly from a rectus at the anterior and posterior faces of the eye, to a rhomb above and below. The elements of the unit and the entire unit increase in size with the growth of the eye. New elements are not added after the pattern has been established. 64 BLIND VERTEBRATES AND THEIR EYES. 2- 3 - 4-- •< 6 - r THE EYES OF ZYGONECTES NOTATUS. Of the eyes of a number of species of normal fishes, namely those of Cyma- logaster aggregates, Carassius aurates, Ameiurus sp., Coregonus sp., and Zygonectes notatus examined, I shall briefly describe the eyes of but one. Zv^oiHTtcs notatus (Rafinesque) was selected for comparison, since it is a member of the Cyprinodontidae, a family closely related to the Amblyopsidas. I am not aware that this species has any advantage over other species of the family. It has large, well-developed eyes, that we may assume to be fully and normally devel- oped. The material examined was alcoholic. It had been preserved by simply placing in alcohol without any intention of future histological examination, but the structures were all well preserved for making out the horizontal relations of the single and twin cones. The protoplasmic and nervous processes of the cells were of course not brought out as with Golgi's method. A specimen 38 mm. long had the eye 2.24 mm. in length, 2 mm. in vertical diameter, 1.12 mm. from axis of optic nerve to front of iris, 1.6 mm. from axis of optic nerve to front of cornea; lens 0.96 mm. in diameter; pigment layer measures 56 p ; outer nuclear layer, 36 ju ; outer reticu- lar, 4 p; tangential cell layer, 9 /A; inner nuclear, 40 /A; inner reticular, 52 p.; ganglionic layer, 12 /A; optic-fiber layer, 28 /*; total thickness of retina, 237 /A. The regularity of arrangement of single and twin cones is very striking. The basal part of the single cones con- tains refractive granules increasing in size outward where the series ends in a lenticular vacuolated body separating the granular from the distal part of the rod. The twin cones are all without granulation. This marked difference between the two enables one to distinguish between them at a glance in tangential sections. The twin cones are arranged in series in such a manner that the axes joining the cones in any neighboring series are at right angles to each other, while in every alternate series they extend in approximately the same or parallel directions. The single cones alternate in all directions with twin cones (fig. 24 b). The outer nuclei are irregular, compressed, and elongate, two distinct layers. The outer molecular layer has an irregular outer boundary produced by the process extend- ing toward the outer (ills. The inner nuclear layer is divided into an outer layer of small bipolar cells and an inner layer of larger, more coarsely granulated >poiigiobla -ti< cells. When any breaks occur in the retina, owing to mechanical or chemical causes, they usually occur between these outer bi-polar and inner spongiost- i ells of the inner nuclear layer. ID through Retina THE BLIND TYPHLOGOBIUS OF CALIFORNIA. TYPHLOGOBIUS: THE POINT LOMA BLIND FISH AND ITS RELATIVES. 65 San Diego Bay is in part surrounded by mud flats which are covered by water at high tide. Sand beaches take the place of the mud flats where the channel approaches the shores. On the ocean shores a sandy beach stretches several miles to the southeast from the mouth of the bay, while on the west rises the point of land called Point Loma. The entire ocean beach at the base of this promontory is rocky. In many places all the earth has been removed by the action of the waves, leaving the bare rock ; in other places, and more especially between the outer point and Ballast Point, large bowlders lie embedded in the sand (frontispiece). These are all covered at high tide, while but a few small pools remain about the FIG. 25. (\v \valer mark, and doubtless form an important item of the food of ihr larger tithes. They spawn in the early part of May. The young rise to the surface- at night, and arc then sometimes taken in the surface dredge. They can, however, lie procured more abundantly in the latter part of May in the pools left at low tide about the piles of wharves. The most remarkable of the gobies is undoubtedly the blind one inhabiting the crab holes under rocks at Point Loma. In its pink color and general appear- ance it much resembles the blind fishes inhabiting the caves of southern Indiana. Its pet uliarities are doubtless due to its habits. The entire bay region is inhabited by a rarideoid crustacean which burrows in the mud, which, like the blind fish, is pink in color. Its holes in the bay are frequented by Clevdandia, etc., while at THE BLIND TYPHLOGOBIUS OF CALIFORNIA. 67 the base of Point Loma, where the waves sometimes dash with great force, the blind fish is its associate. On rough clays few fishes are seen, though ever so many stones are overturned. On mild days, on the contrary, at very low tides quantities are found almost invari- ably in company with one of the crustaceans mentioned above. Sometimes the fishes live quite out of water on the damp gravel and sand under a rock, but more frequently small pools of water fill all the depressions under the rocks, and the fishes swim rapidly away to hide in the crab holes, several of which always branch from the cavity in which the rock has lain. Very rarely are the fishes found swimming in rocky tide pools. In the bay the gobies habitually live outside of the holes, descending into them only when frightened ; but at Point Loma they rarely leave their subterranean abodes, and to this fact we must attribute their present condition. How long these fishes have lived after their present fashion it would be hard to conjecture. The period which would produce such decided structural changes can not be a brief one. The scales have entirely disappeared, the color has been reduced, the spinous dorsal has been greatly reduced, the eyes have become stunted, and the whole frontal region of the skull and the optic nerves have been profoundly changed. The skin, especially that of the head, has become highly sensitized. The skin of the snout is variously folded and puckered. The nares are situated at the end of a fleshy protuberance which projects well forward, just over the mouth. At the chin are various short tentacles, and a row of papillae (which probably bear sensory hairs) extends along each ramus of the lower jaw and along the margin of the lower limb of the preopercle. The eye is, however, the part most seriously affected. It is quite evident and apparently functional in the young (fig. 26 ft). Objects thrust in front of the fish are always perceived, but the field of vision is quite limited. With age the skin over the eyes thickens and they are scarcely evident externally. As far as I could determine they do not see at this time, and certainly detect their food chiefly, if not altogether, by the sense of touch. A hungry individual will swim over meats, a fish, or a mussel, etc., intended for its food without perceiving it by sight or smell, but as it comes in contact with any part of the skin, especially that of the head region, the sluggish movements are instantaneously transformed, and a stroke of the fins brings the mouth immediately in position for operations. Ritter's experiments showed that it would not choose between light and dark, but, "On the whole, both from these observations on the living fish, and from the structural conditions, ... I am of the opinion that the power of perceiving light is not wrholly lost even in the adult." The optic nerve is very slender and the lens proportionately very large. In the youngest individual caught (fig. 26 b), the membranes of the fins were thin, the color cells well formed and arranged not unlike those of the young Gil- lichthys. The movements were similar to those of the other gobies, and not at all sluggish like those of the adult. Their favorite position is standing or sitting with the broad pectorals extending out at right angles to the body. In this posi- tion the fish can, with a sudden stroke of its pectorals, move quickly and rapidly. In the old fish the fins are thick and smaller in proportion, and all the vivacity seems to have disappeared. The color has degenerated, or at least not developed in proportion to the growth of the fish. 68 BLIND VERTEBRATES AND THEIR EYES. All these gobies are tenacious of life, especially the blind ones. Several of the latter have been kept in a half-gallon jar of water for several weeks without change of water, and others have been kept several months in confinement in my laboratory. When the water becomes somewhat stale, they frequently rise to the surface and use the water as a plane to which they attach themselves by means of their ventrals. The earliest date at which I procured young was October 25. The smallest caught at that time is represented in figure 26 b. The covering of the ovarian egg consists first of a finely striate membrane, the zona radiata of all teleostean eggs. Exterior to this is a network of threads with the meshes coarsest at the entodermic pole and forming almost a continuous mem- brane at the ectodermic pole. When the eggs are deposited, the meshwork of threads is stripped off the egg and remains attached to the zona radiata around FIG. 27. Larval Typhlogobius in its membrane. the micropyle. In the eggs deposited naturally by the females in confinement the threads were wound together to form a cord at the micropylar end of the egg. The cords of many of these eggs were attached to each other, and the eggs thus came to be laid in bunches like those of grapes. In their natural habitat the eggs are fastened by the threads to the lower surfaces of the rocks under which they live, and the membranes are expanded into long club-shaped bags. The yellow of the blind-fish egg is entirely confined to the yolk, which contains many oil globules. The granular protoplasm is opaque. In females with ripe eggs they are frequently to be seen forming a yellow band along the flanks. The eye in the larvae just about to be hatched (fig. 27) is apparently normal. The histology of the adult eye was studied by Ritter, who comes to the follow- ing conclusion : 1. In the smallest examples studied the eyes, though very small, are distinctly visible even in preserved specimens — so distinctly that the lens is plainly seen. In the largest examples, on the other hand, they are so deeply buried in the tissue as to appear even in the living animals as mere black specks, while in preserved ones they are in many cases wholly invisible. 2. Neither In small nor in large specimens does the epidermis over the eye differ in thickness or structure from that of adjacent regions. In the large individuals the much greater thickness of the tissue hire is brought about by an increase in the sub-epidermal connective tissue, the growtli of which can be seen taking place in the embryonal connective-tissue cells that are found here. \ is the i ase with rudimentary organs generally, the eye is subject to great individual varia- tion in size, form, and degree of differentiation. 4- 'I he "»b parts <>f the normal teleostean eye, no traces of which have been found, are the argeniea. the lamina suprachoroidea, the processus falciformis, the cones of the retina, the vitreous body proper, the lens capsule, and in one specimen the lens itself. 5. In the parts present the rudimentary condition of the organ is seen in the very slight develop- ment of the choroid, no cellular elements being present in this excepting in the chorio-capillaris, and here to a quite limited extent, the rest of that layer being composed exclusively of pigment; THE BLIND CAT-FISH. 69 in the fact that the choroid gland is composed entirely of pigment; in the fact that the iris, though of fully the normal thickness, is almost entirely of pigment, there being on its outer surface in some specimens a small amount of cellular material, which probably represents the ligamentum annulare; in the great proportional thickness of the pigment layer of the retina and the entire absence in it of anything excepting pigment; in the incomplete differentiation of the layers of the retina, there being in some individuals scarcely more than a trace of the external reticular layer separating the two nuclear layers, and there being in no specimen studied a retina sufficiently developed to enable one to homologize with certainty the layers marked out; in the minute size of the optic nerve, and the fact that it is ensheathed in a thick layer of pigment for nearly its entire course through the retina; and, finally, in the small size of the motores oculi. 6. The surest evidences of actual degeneration are found, first, in the greatly augmented quantity of pigment in all the parts that are at all pigmented in the normal eye ; and, secondly, in the presence of pigment in regions where none is found in the normal eye, as in the hyaloid membrane. No undoubted instances of degeneration through the breaking down and dissolution of the tissue without the formation of pigment, such as have been described particularly by Looss, have been found, though in a single specimen (the one in which no lens is present) a process of this nature may be taking place. THE EYES OF THE BLIND CAT-FISH, AMEIURUS N1GRILABR1S. All that is known of this fish is contained in the following extract from Cope's paper (Proc. Acad. Nat. Sci., Phila., 1864, p. 231) : For a knowledge of the first genus of blind Sihtrid from our country, I am indebted to my friend Jacob Stauffer, secretary of the Linnasan Society of Lancaster, an ardent explorer of the zoology and botany of southern Pennsylvania, and who has furnished me with many valuable notes and specimens. This fish, of which specimens have been taken in the Conestoga Creek, a tributary of the Susquehanna, is simply a blind representative of the ordinary type of Silurids, characteristic of North America, and is not to be arranged with the exotic groups. * * * The color of the upper surfaces, tail, fins, barbels, and under jaw is black; sides varied with dirty yellow, abdomen and thorax yellowish white. * * * A specimen died in 20 minutes after capture, when put in water, though uninjured; the Amchtri, like other cat-fishes, will live for many hours after complete removal from their element. It is occasionally caught by fishermen, and is supposed to issue from a subterranean stream, said to traverse the Silurian limestone in that part of Lancaster County and discharge into the Conestoga. Two specimens of this fish present an interesting condition of the rudimental eyes. On the left side of both a small perforation exists in the corium, which is closed by the epidermis, representing a rudimental cornea; on the other the corium is complete. Here the eyeball exists as a very small cartilaginous sphere with thick walls, concealed by the muscles and fibrous tissue, and filled by a minute nucleus of pigment. On the other the sphere is larger and thinner walled, the thinnest portion adherent to the corneal spot above mentioned ; there is a lining of pigment. It is scarcely collapsed in one, in the other so closely as to give a tripodal section. Here we have an interesting transitional condition in one and the same animal, with regard to a peculiarity which has at the same time physiological and systematic significance, and is one of the comparatively few cases where the physiological appropriateness of a generic modification can be demonstrated. It is therefore not subject to the difficulty under which the advocates of natural selection labor, when necessitated to explain a structure as being a step in the advance toward, or in the recession from, any unknown modification needful to the existence of the species. In the present case observation on the species in a state of nature may furnish interesting results. In no specimen has a trace of anything rep- resenting the lens been found. 70 BLIND VERTEBRATES AND THEIR EYES. THE AMBLYOPSID^:. The Amblyopsidae are a small family of fishes, first brought to the notice of naturalists by W. T. Craige, who presented a specimen to the Philadelphia Academy in 1842. De Kay, "Natural History of New York" (Reptiles and Fishes, p. 187, 1842), gives a brief description of AnMyopsis spclaus. It was followed at once by articles by Wyman ( 1 843 and later, 1850, 1854 a and ft) and other articles by Thomp- son (1844) and by Telkampf (1844). Renewed interest in the Amblyopsidae was aroused by Agassiz's discovery of an epigean relative, Chologaster cornutus, in the ditches of rice fields in South Carolina. Typhlichthys subterrancus was described by Girard in 1859 from a well near Bowling Green, Kentucky ; Chologaster agassizii, by Putnam from a well at Leba- non, Tennessee, in 1872; Chologaslcr papillifcnis, by Forbes in 1882. In 1898 the present author described Typhlichthys rosa>, and a short time afterwards he demonstrated that this species is generically distinct from Typhlichthys, naming it Troglichthys. More recently (1905) he described Typhlichthys osborni and Typhlichthys wyandotte. RELATIONSHIPS OF THE AMBLYOPSIDAE. The Amblyopsidae are members of the order Haplomi, first characterized by Cope.1 They have recently been defined by Boulenger, as follows : Air-bladder, if present, communicating with the digestive tract by a duct. Opercle well devel- oped. Pectoral arch suspended from the skull; no mesochorochoid. Fins usually without, rarely with a few spines; ventrals abdominal, if present. Anterior vertebrae distinct, without Weberian ii les. The order consists of a number of families of which the Galaxiidae and Aplochi- tonida? are found in the fresh waters and occasionally in the oceans of the south temperate zone; the Scopelidae are found pelagic and abysmal in the ocean, the Kneriida.- in Africa, the Dalliidas in Alaska and Siberia, the Poeciliidse in fresh water and along the shores of the tropical and temperate zones, and the Esocidae in fresh waters of the north temperate zone. The Amblyopsidse are distinguished from the other families by the doubling forward of the alimentary tract, the opening of the oviduct and anus being placed close behind the throat, in front of the pectorals. The genera of the Amblyopsidae may be distinguished by the following char- acters : a. \Vntral lni.|in i 'ill ; pylorii • oiva 2 or 3 . . . . . . . • • Amblyopsis aa. Ventral fins absent "tic cceca 2 i with cartilages Troglichthys cc. Sclcra witli Typhlichthys W>. Eye well <]• ited; pvlnric coeca i ...... Chologaster 1 1'ror. AUNT. Assoc. Adv. Si irmr, Indianapolis, i.Syj, }_>S and 333. EIGENMANN PLATE 5 c Amblyopsis. (A) side, (B) dorsal, and (C) ventral views. DISTRIBUTION OF THE AMBLYOPSID/E. 71 DISTRIBUTION OF THE AMBLYOPSID/E. Amblyopsis spelaeus De Kay. Plate 5. Amlilyopsis spehrus, DE KAY, Nat. Hist. N.V., Reptiles and Fishes, 1842, p. 187, Mammoth Cave, Ky. — WYMAN, Ann. and Mag. Nat. Hist., xii, 1843, p. 298; Amer. Jour. Sci. and Arts, xlv, 1843, pp. 04 to 96, Kentucky. — THOMPSON, Ann. and Mag. Nat. Hist., xiii, 1844, p. 112. — TELKAMPF, Miiller's Arch., 1844, pp. 381 to 394, taf. 9. — WYMAN, Proc. Bost. Soc. Nat. Hist., iii, 1850, pp. 349 to 357. — AGASSIZ, Amer. Jour. Sci. and Arts, xl, 1851, p. 127. — WYMAN, Proc. Bost. Soc. Nat. Hist., iv, 1854, p. 395, v, p. 18; Amer. Jour. Sci. and Arts, xvii, 1854, p. 258. — POEY, Mem. Cuba, ii, 1853, p. 104. — GUNTHER, Cat. Fishes Brit. Mus., vii, 1868, p. 2, Mammoth Cave, Ky. — PUTNAM, 1872, Amer. Nat., p. 30, fig., Lansing, Mich. [p. 20], well near Lost River, Ind. — Cox, Report Geol. Res. of Ind., Rhodes Cave, near Corydon; Gulf of Lost River. — COPE, Report Geol. Res. of Ind., iii and iv, 1871 and 1872 (1872), p. 161, Little Wyandotte Cave, Ind.; Ann. and Mag. Nat. Hist., 1872, Little Wyandotte Cave, Ind. — JORDAN, Rept. Geol. Nat. Res. of Ind., vi, 1874 (1875), p. 218, Mammoth Cave. — COPE, Rept. Geol. Nat. Res. of Ind., viii, ix, x, 1876, 1877, 1878 (1878), p. 483, Little Wyandotte Cave, Ind. — JORDAN AND GILBERT, Synopsis, 1883, p. 324. — PACKARD, Cave Fauna of N. A.. Mem. Nat. Ac. Sci., 1886, p. 14, Hamer's and Donnelson's caves, Lawrence Co., Ind.; Clifty cave; Elrod's cave (p. 127), 4 miles west of Orleans, Ind. ; Mammoth Cave, Ky. — HAY, Rept. Geol. and Nat. Res. of Ind., xix, 1894, p. 234. — JORDAN AND EVERMANN, Fishes N. A., 1896, i, p. 706. — BLATCH- LEY, Rept. Geol. Nat. Hist. Res. of Ind., xxi, 1896, p. 183, Sibert's well cave, a part of Little Wyandotte Cave, and in caves near Mitchell, Ind. — EIGENMANN, Proc. Ind. Ac. Sci., 1897(1898), p. 230; Degeneration of the Eyes of the A mblyopsidtr, its Plans, Processes, and Causes, Proc. Ind. Ac. Sci., 1899, p. 239 (summary). — EIGENMANN AND VODER, Ear and Hearing of the Blind Fishes, Proc. Ind. Ac. Sci., 1898 (1899), p. 242. EIGENMANN, Eyes of the Blind Vertebrates of N. A., Archiv f. Entwickelungsmech., viii, 1899, p. 545; Pop. Sci. Mo., !vi, 1900, p. 485; Marine Biological Lectures, 1900, for 1899, p. 113. — Cox, Report Bureau of Fisheries, 1904, p. 392, issued 1905. Most of the Amblyopsidae are confined to the caves of the Mississippi drainage basin. Amblyopsis spclcciis has the widest distribution. It is recorded from the following places: Mammoth Cave, Kentucky; Rhode's Cave, near Corydon; Lost River and one of its "Gulfs"; Elrod's Cave, Orange County; Little Wyan- dotte, near the southern boundary of Indiana ; Hamer's and Shawnee Caves in Lawrence County, Indiana ; Clifty Caves, near Campellsburg, Washington County. Vague reports of blind fishes have come from near Milford in northern Indiana; from Lansing, Michigan ; and from Hiram, Ohio. None of the alleged specimens from the north had been preserved and none could be secured until recently, when I received a specimen of Amblyopsis from near Hiram, Ohio, with a letter to Prof. H. H. Lane, in substance as follows : HIRAM, OHIO, July 7, 1906. The fish was brought by a student who resided near the place where it was found. The state- ment made was as follows: The township of Shalersville built a roadway of logs and earth across a swamp, known locally as the Podunk Swamp. The next spring the roadway sank out of sight and in its place there was a canal of reddish brown water. This fish was said to have been caught out of this water. The swamp I have occasionally visited, but have never seen any fish in the water. After the sinking of the road referred to the county rebuilt it at considerable expense only to have it sink out of sight again as before. It has not been touched since and the same stretch of water across it is there to-day. The swamp is one of the kind common to the glacial area and is surrounded by morainic hills. It was no doubt originally a lake and has been converted into a swamp by the growth of vegetable matter. This specimen makes the other northern records also probable. The specimens from Milford, Indiana, were reported to have been caught under circumstances identical with those reported for the Hiram specimen. This species is thus known to be distributed east of the Mississippi, both north and south of the Ohio River, which divides the cave region, and also far north in northern caves or even in glacial swamps. It is probable that it has a very wide distribution in the ground water. It has become quite rare in and about Mam- moth Cave. I have visited this cave several times, also Colossal Cavern, Cedar Sinks, and other caves in Kentucky, but so far have not succeeded in capturing or seeing any specimens south of the Ohio River. 1 This cave, plate A, has been variously calk-d Shawnee cave, Donnelson's and Donaldson's cave. 72 BLIND VERTEBRATES AND THEIR EYES. I have visited many caves in the Lost River region of Indiana and others have visited different caves without finding this species. Amblyopsis has been pumped out of a well at Mitchell, Indiana. I have taken it in only three caves; one specimen in Clifty Cave and one in Hamer's Cave. The only place where this species is known to be at all abundant is in the caves of the Donaldson farm of Indiana University. Troglichthys rosae Eigenmann. Plate 6, Figs. A, B, c. Typhlichthys subterraneiis, CARMAN, Bull. Mus. Comp. Zool., xvii, 1889, p. 232, wells and caves, Jasper County, Mo.; not of Girard. — KOHL, Rudimentare Wirbelthieraugen, 1892, p. 59. Tvfi/ilichthvs rosa:, EIGENMANN, Proc. Ind. Acad. Sci., 1897 (1898), p. 231, Sarcoxie, Mo. Troglichthys roscc, EIGENMANN, Science, N. S. is, 1899, p. 280, Day's Cave, Sarcoxie, Mo.; Degeneration in the Eves of the Amblyopsidas, its Plans, Processes and Causes, Proc. Ind. Acad. Sci., 1898 (1899), p. 239 (sum- mary); Eyes of the Blind Vertebrates of N. A., Archiv f. Entwickelungsmech., viii, 1899, p. 573; A Case of.Convergence, Proc. Ind. Acad. Sci., 1898 (1899), p. 247. — Cox, Report U. S. Bureau of Fisheries, 1904, p. 391; issued 1905. This species has thus far been collected by Miss R. Hoppin and by myself at Sarcoxie, Missouri. Miss Hoppin found it in Wilson's Cave, Day's Cave, Center Creek, and wells. Her reports were published in full by Mr. S. Carman. I found the fish in the fall of 1898, in a pool just within the mouth of Day's Cave. Judging from the localities where it is said they occur either in wells or in caves, the species is distributed over an area 300 miles long by 100 miles broad. It has been reported to me as occurring in wells at Cassville, Marionville, and Springfield, Missouri, and somewhere in Arkansas, in a spring in Newtonia, from a cave at Joplin, Missouri, and another near Springfield, Missouri, and from Turnback Cave near Marionville. A specimen from Arkansas is said to be in the United States National Museum. It is said that 7 miles southeast of Lead Hill, in the left hollow off Cane Sugar Orchard Creek, a half mile below an old mill, there is a cave where blind fishes have been found. These were described in such a way as to leave no doubt of the authenticity of the locality. Mr. C. H. Thompson, of the Shaw Botanic Garden in St. Louis, gave the following account of a cave reported to him : Tn a cave about 1 3 or 14 miles north of Frederickstown, St. Francois County, Missouri, there is a stream of water averaging 4 to 6 feet wide and i to 3 or 4 feet deep. In these deeper "pools " by feeling under the rocks one will find fish which are blind. The stream does not flow out at the mouth of the cave, but a few rods down the slope of the hill, directly below the cave entrance, a large spring breaks out. This is probably the same stream as that found in the cave. The spring forms the source of Coldwater Creek. By consulting the map the source of Coldwater Creek, as there indicated, is northeast of KmU-rickstown. Coldwater runs in a northeast direction through St. Genevieve County into the Mississippi. From the map the location of the cave is in all probability the extreme southeast corner of St. Francois County. Typhlichthys Girard. The characters of the three known species of Typhlichthys are purely technical and may be summari/ed as follows: a- \Vnlili "i head more than ii in length to base of caudal; length of head 3!; first anal ray i 'Mai ray than to anus wyandotte aa. Width of head c; in length to base of caudal; length of head 3 103.4; orbital fat-mass elongate, inconspicuous in life, not projecting; cheeks little swollen; eye on an average 0.16 mm. in di.imeter, the smallest o.T4 mm sublerranetts • Width of" ' of caudal; length of head 3$; orbital fat-mass round and very conspicuous in life, |>il the town is a sink or depression with vertical walls which was prob- ably can-ed by the falling of the roof of a large cavern. At one end of the sink an inclined plane leads into the underground stream, which supplies the city with water. The -tn-am also furnishes the power to light the city. A dam across the cave liirm'shes the head for the power, but so modifies the conditions above it as to make collecting practically impossible. A convenient break in the dam made DISTRIBUTION OF THE AMBLYOPSID^E. 75 it possible, on one of three visits, to ascend the stream to a pile of fallen rocks from under which the water flows and which makes further progress impossible. This stretch is not great. It was noted for the abundance of blind crawfish ; no blind fishes were found here. On the right side of this stream, near the entrance, an older, dry channel leads off. At the end of the gallery a small rivulet runs to the left through a series of small pools separated by thin vertical partitions ; to the right it expands into a broad stream, quite shallow, but with such a depth of soft mud at the bot- tom that progress was impossible without a boat. In this expanse Typhlichthys osborni was very abundant. In the fall of 1907 this cave was visited again, but no fishes were found where previously they had been abundant. Below the dam in the main cave the stream is swift and the floor so rock-strewn that progress is difficult and dangerous and fishing unprofitable. Typhlichthys wyandotte Eigenmann. Typhlichthys sublerraiieiis, EIGENMANN, Proc. Ind. Acad, Sci. 1897 (l898)> P- 23C- Corydon, Ind.; not of Girard. Typhlichthys wyandotte, EIGENMANN, Biol. Bull., vni, Jan., 1905, p. 63. Typhlichthys -wyandottc is known from a single specimen from Corydon, Indi- ana, sent in 1886 by Superintendent Funk of the schools of Corydon to Indiana University. This is the only record of the genus north of the Ohio River. Repeated efforts to secure additional specimens have failed. Key to the Chologasters. a. Eye large, contained 5.5 times in the length of the head. b. Eye over i mm. in diameter; tactile papillae very small; sides with 3 well-defined longitu- dinal lines ............... connitiis bb. Eye less than i mm. in diameter; tactile papillae large ...... papilliferus aa. Eye contained 10 times in the length of the head; color very faint agassizii Chologaster cornutus Agassiz. Chologaster contains, AGASSIZ, Amer. Jour. Sci. and Arts, xvi, 1853, p. 135, Ditches of rice fields at Waccama in S. C. — GUNTHER, Cat. Fishes Brit. Mus., vii, 1868, p. 2. — PUTNAM, Amer. Nat., vi, 1872, p. 30. — JORDAN AND GILBERT, Synopsis Fishes of N. A., 1883, p. 325. — GILBERT, Bull. U. S. Fish Comm., viii, iSSS, p. 22, Okefinokee Swamp, Millen, Ga. — JORDAN AND EVERMANN, Fishes North and Mid. Amer., i, 1896, p. 703. — EIGENMANN, Degeneration of Eyes of Amblyopsidse, its Plans, Processes, and Causes, Proc. Ind. Acad. Sci., 1898, p. 239 (summary); Eyes of the Blind Vertebrates of N. Amer., Archiv f. Entwickelungsmech., viii, 1889, p. 543; Marine Biological Lectures, 1899 (1900), p. 113. Chologaster avitus, JORDAN AND JENKINS, in Jordan Proc. U. S. Nat. Mus., viii, 1888, p. 356, pi. 44, fig. 8, Outlet of Lake Drummond, Dismal Swamp, near Suffolk, Va. — Cox, Report Bureau of Fisheries for 1904, p. 386 (issued 1905). The Chologasters have a wide and discontinuous distribution. Chologaster cornutus Agassiz has been found in the ditches of rice fields in South Carolina ; in the Okefinokee Swamp at Millen, Georgia; and in the Jericho Canal near Suffolk, Virginia, in an outlet of Lake Drummond. Its range is entirely east of the Alle- ghany Mountains, and it is found in lowland streams only. I visited the locality near Suffolk, but found no specimens. Chologaster papilliferus Forbes. Plate i, Fig. A. Chologaster papilliferus, FORBES, Amer. Nat., March, iSSi, and Jan., 1882, Cave spring in southern Illinois. — JORDAN AND GILBERT, Synopsis Fishes N. A., 1883, pp. 325, 890. — JORDAN AND EVERMANN, Fishes North and Mid. Amer., i, 1896, p. 704. — EIGENMANN, Proc. Ind. Acad. Sci., 1897 (^QS), p. 231; Degeneration in the Eyes of the Amblyopsidae, its Plans, Processes, and Causes, Proc. Ind. Acad. Sci., 1898, p. 239 (summary); Eyes of the Blind Vertebrates of N. A., Archiv f. Entwickelungsmech., 1899, p. 545; Marine Biological Lectures, 1899 (1900), p. 113. Chologaster papilliferus Forbes is known only from cave springs in Clinton and Jackson Counties, Illinois. Most of the specimens have come from a spring in Jackson County, Illinois. 76 BLIND VERTEBRATES AND THEIR EYES. Chologaster agassizii Putnam. Plate 6. Clwtogaster agassizii, PUTNAM, Amer. Nat., VI, 1872, p. 22, well at Lebanon, Tenn. ; Mammoth Cave, Ky. — JORDAN, Kept. Geol. Nat. Res. of Ind., VI, 1874(1875), p. 218 (reference to Putnam's specimens). — HAY, Geol. and Nat. Res. of Ind., xix, 1894, p. 234. — JORDAN AND EVERMANN, Fishes North and Mid. Amer., 1896, I, p. 704. — EIGENMANN, Proc. Ind. Acad. Sci., 1897 (1898), p. 230; Eyes of the Blind Vertebrates of N. A., Archiv f. Entwickelungsmech., VIII, 1899, p. 546; Proc. Ind. Acad. Sci., 1898 (1899), pp. 239, 251; Marine Biological Lectures, 1899 (1900), p. 113. Chologasler agassizii Putnam is known only from Lebanon, Tennessee, and caves about Mammoth Cave. I have taken it in the Styx in Mammoth Cave and in Cedar Sinks, near Mammoth. I found the Chologaster in only one locality in Mammoth Cave. A short distance after descending the corkscrew the Styx appears on the right. On one visit Chologaster was abundant in and around the remains of an old boat, but I secured only a few small specimens on account of their agility and the easily roiled water. They were much more alert than the blind members of the family and made quickly for the lower edge of the wall of the cave, below which many of them escaped. On a subsequent visit the locality had been quite modified, and I secured even fewer specimens than before. Cedar Sinks, the other locality from which I secured Chologaster agassizii, is a highly interesting region. It lies several miles from Mammoth Cave and is reached over a rough road leading, without modifications from the engineer, up and down the steep slopes of the interminable sink holes of the region. Cedar Sinks was formed by the caving in of the roof of an enormous cave room. The vertical walls of the room are still standing. I have been told the bottom of the sink embraces about 4 acres. In the bottom are 2 funnel-shaped depressions holding water. The walls of the funnels are so steep that it is just possible to climb out if one has been foolish enough to slide down. At the base of the highest rock bound- ing the sink are two openings. One leads to an extensive underground stream which can be followed a very restricted distance ; the other, to a stream and cave which must be quite extensive, judging from the inflow of air at the time of one of my visits. Small pools or streams in one of the entrance galleries yielded a few specimens of Chologaster. THE COLOR OF THE AM BLYOPSID/E. The three species of Chologaster are colored with varying intensity from C . contains, which is darkest, to C. agassizii in Mammoth Cave, in which the r»!or is faintest. The color cells arc in all cases arranged in a definite pattern. This is determined by the underlying muscles. The pattern consists of three lon- gitudinal bands on the sides following the line where the muscle segments are angularly bent and cross stripes along the line separating successive segments (plate 6, upper figures). Tin- lower side of the head and the abdomen of Chologaster papilliferm are sparingly pigmented and translucent. The underlying liver and gills give the parts a rosy tinge. On the sides and top of the head pigment is abundant. There is a more densely pigmenled area extending along the middle of the back, begin- ning as a narrow stripe at the nape and widening to the dorsal fin behind, where it occupies the entire back. On the sides are 3 narrow stripes, which, owing to the accumulation of pigment in 2 layers, arc quite dark. Each stripe has a lighter central band, widest at the middle of the sides. A light band, without the con- spicuous bordering dark stripes, runs along the middle of the belly. The sides are thickly covered with a layer of pigment, leaving usually colorless lines where con- COLOR OF THE AMBLYOPSID^. 77 nective tissue separates successive myotomes. On the sides of the tail the pigment is dense on either side of these colorless lines. A dark band extends along the sides of the head through the eye. The top of the head is dark (plate i, fig. A). The pattern of Chologaster cornutus agrees with that of C. papillifcrus. The longitudinal bands are much darker and wider and without the light central streak. The middle band is much wider than the others and is continued forward to the tip of the snout. The amount of color present varies very greatly with the locality from which the specimens come. The general color of C. agassizii is light gray (plate 6, upper figures). The scales are lighter than the area surrounding them. The color pattern is more striking than in the other species of the genus. Each somite is bordered by a dark line. The lines of successive somites are separated by an almost imperceptible colorless line. A broad, not sharply defined, band extends along the sides. The middle of this is lighter than the margin. Another one extends between the somites and the ven- tral musculature, another from the nape between the lateral somites and the dorsal muscles, and a diverging one from near the nape to either side of the dorsal fin. Dark areas are caused by the accumulation of pigment along the borders of the small muscles of the fins. Still another dark area is found about the caudal. The ventral surface is white, except the accumulation of pigment along the lines sepa- rating the muscles. The fins are uniformly light gray. A light area is found on both the upper and lower part of the caudal peduncle, just within the first short rays of the caudal. The general color of Typhlichthys is cream and pink. It is abundantly pig- mented. In younger specimens the pigment is arranged in more definite areas about the head. In the old it is more uniformly distributed, being, however, spe- cially abundant about the brain. The pigment pattern of the body is precisely as in Chologaster except that the individual pigment cells are minute and their aggre- gate not evident except under the lens. The retention of the color pattern of Chologaster in Typhlichthys is not less interesting than the retention of similar habits. It is perhaps due to different causes. The color pattern in Chologaster is determined by the underlying mus- cular structure and the retention of a similar pattern in Typhlichthys is due to the same underlying structure rather than to the direct hereditary repetition of the color pattern. In Amblyopsis the color is much less marked than in Typhlichthys. Amblyopsis is flesh -colored, ranging to purple in the gill-region, where the blood of the gills can be seen through the overlying structures, and over the liver, which can be seen through the translucent sides and ventral wall. About the head and bases of the fins the color is yellowish, resembling diluted blood. The surface of the body is slightly iridescent and that of the head has a velvety, peach-bloom appearance. The general pink color of Amblyopsis is due to the blood, not to any abnormal development of blood-vessels in the dermis. In the fins where the blood-vessels are near the surface, the general effect is a yellowish color. The surface vessels of the dermis also appear yellowish. It is only on account of the translucent condi- tion of all the tissues, permitting the deeper vessels to show through a certain thick- ness, that the pink effect is produced. Amblyopsis has always been spoken of as white. The term "white aquatic ghosts" of Cope is very apt, for they do appear white in the caves and their gliding motion has an uncanny effect. All alcoholic specimens are white. 78 BLIND VERTEBRATES AND THEIR EYES. The chromatophores in Amblyopsis are differentiated and contain color before the yolk is absorbed. The black chromatophores are minute granules, few (15 or thereabout) to the segment. In an older larva the pigment was much more abundant. The eyes are pigmented early, shortly before hatching, and, owing to their pigment, they soon become conspicuous and remain so till the fish has reached 50 mm. in length, when the overlying tissues have become thick. The pigment of the body is lost, or, what amounts to the same thing, does not increase much with age. There is an abundance of pigment cells in the adult, but they are very poor in pigment, and, being in the dermis and covered by the thick layer of epidermis rich in glands, are not apparent. Pigment cells are also abundant in deeper tis- sues in the adult, so that, while no pigment is visible on the surface, an abundance of chromatophores is present in deeper tissues. The pigment cells can not be made to show themselves, i.e. become greatly pigmented, even by a prolonged stay in the light. The old, if kept in the light, will not become darker; and a young one reared in the light until ten months old not only showed no increase in the pigmentation, but lost its pigment, taking on ihr exact pigmentless coloration of the adult. Pigment cells appear late in Amblyopsis. When the young are two months old pigment is abundant. This pigmented condition is evidently a hereditarily transmitted condition. It disappears with age. In the first instance this disappearance was probably individual. But as in the flounder, the depigmentation has also become hereditarily transmitted, for even those individuals reared in the light lose the color. Numerous facts and experiments show that, while pigment may be and is devel- oped in total darkness, the amount of color in an individual animal depends, other things equal, directly on the amount of light to which it is habitually exposed. A number of apparently contradictory observations may be noted : (a) The absence of pigment in pelagic animals or their larvoe, which depend on their colorless condition for their existence, is evidently due to causes entirely dif- ferent from those preventing the formation of pigment in cave animals. Natural selection has, in pelagic animals, eliminated the color. (6) The migration of pigment granules due to temperature or light and the expanding of chromatophores, when an animal is over a dark background or in the dark, and the contracting over a light background, which may take place at once or at the expiration of several days, is evidently also a different question. The i ibservations of Cunningham, Agassiz, and Semper along this line are of interest. (c) Fisch.e] (A. M. Anat, vol. XLVII, pp. 719-734, plate xxxvi, 1893) has noticed that larva,- of salamanders reared in water at 6° to 7° are dark, while others kept in water from 15° to 58° are light. (d) Flemming (A. M. Anat., vol. XLVIII, pp. 369-374, 1896) found that with uniform temperature in two vessels side by side, the one dark, the other light, the salamander larva; in the dark vessel develop pigment cells rich in color granules; the larva- in the white vessels become pale, although the number and character of the pigmenl cells is not otherwise changed. The difference is entirely due to the character of the vessels, for if the larvae are taken from the dark to the light vessel, they become light colored in a few days. (c) Semper ("Animal Life," p. 89) records that " * * * in the tadpoles of our common toads and frogs the pigment is equally well developed in yellow, blue, or COLOR OF THE AMBLYOPSID^E. 79 red light, and in absolute darkness." This was to be expected, for even in the young of cave animals pigment is, as a rule, well developed. (/) Pouchet (Arch, de Physiol. ct d'Anat, 1876, and Rev. Sclent., vol. xin, 1897) has demonstrated that change in color cells, such as are mentioned under (b) and (\ springing upward. After a few weeks they responded much less often, and after several months they paid very little attention to jarring of any kind. HABITS OF CHOLOGASTER. 85 It was mentioned before that one fish, in the course of 4 hours, took n pieces of cotton from the end of a thread and after that refused to take it again, although the fish was kept for several months. In this case, then, it learned to discriminate within a very short time, and remembered the difference between the cotton and the meat. It took the meat, if brought in contact with the lips, after it refused the cotton. Another fish was tested by dropping water on the surface of the aquarium. The fish came to the surface and grabbed at the drop. I tested the fish once everyday for 12 days, and on the twelfth day it refused to grab, but came up near the surface, poised as if ready to grab, and then sank slowly toward the bottom. The thirteenth day it responded, but not very readily. For the next 8 days I tested it every day and got no attempt at grabbing, although it came near the surface every day. I did not test it again for 3 days, when it again snapped at the drop. It came up to the sur- face at the first few drops, but sank gradually toward the bottom. Upon continuation of the drop- ping, it came up again and grabbed. I then left it undisturbed for 5 days before testing and again it grabbed. This was the twenty-ninth day of the experiment. I then started with an interval of i day and increased it by i day each time, thus making the intervals i, 2, 3, 4, and 5 days. It did not snap at the drop until after the interval of 5 days. This was the forty-fourth day of the experiment. I again waited 5 days before testing the fish and got no response further than that the fish came near the surface. On account of the lack of time the experiments were discontinued. Whether the fish would eventually have learned not to snap at the drop, I can not say, but that memory plays some part in its reactions is evident from my observations. The conclusions reached are as follows: (1) Sight is as a matter of course excluded from food seeking. (2) The olfactory sense, if any, plays a very minor part in detecting food. (3) The sense of taste enables them to discriminate between things in contact with the snout. (4) The tactile sense is the one by which they find and precisely locate their food. THE HABITS OF CHOLOGASTER. The following extract, from a letter from Mr. E. B. Forbes, is of interest: Doubtless you have received the little Chologaster which I sent you yesterday. The spring in which they are found is in an almost inaccessible part of Jackson County and I drove 17 miles from Cobden, Illinois, in a wagon to this place. The spring is a very large one, flowing from the bottom of a 250-foot cliff of flint and limestone. The little fishes were found under stones at the edges of the spring, very close to the bluffs, and when disturbed they swam back under the cliff. After the rough drive home they were still alive and seemed vigorous when handed over to the expressman. I found this species in other springs than the large one mentioned and have no doubt that it is rather widely distributed. None were found at any considerable distance from the face of the cliff. I found that Chologaster agassizii acts similarly in the River Styx in Mammoth Cave. As soon as my net touched the water they darted in under the ledge of rock at the side of the little pool in which I found them. The Chologaster in general make-up is like Amblyopsis, but somewhat more elongate. It sits with its pectorals extended. When it moves horizontally for some distance the pectorals are usually pressed to the sides, the propelling being done largely by the tail, very much after the manner of a salamander, which it resembles. In swimming toward the surface it uses its pectoral fins chiefly, and the fish usually sinks to the bottom as soon as its efforts to raise itself are stopped. Individuals kept in aquaria with one end darkened either collected in the dark- ened area, floating about, or under leaves or sticks in any part of the aquarium. They are frequently found under a floating board where they float with the tops of their heads in contact with the board, their bodies slanting downward. Typhlichthys, living in total darkness, has retained the habit of staying under floating boards and sticks and under stones. Miss Hoppin noticed that Trog- Uchthys swims with its back to the aquarium, and I have repeatedly noted the same in the young of Amblyopsis up to 50 mm., and the still younger Amblyopsis frequently hides under rocks. 86 BLIND VERTEBRATES AND THEIR EYES. Chologaster papilliferus detects its food entirely by the sense of touch. Two which were kept in an aquarium for over a year were starved for a few days. They became very nervous, continually swimming along the sides of the aquarium. Some individuals' of Ascllits were introduced. These, though quite near, produced no effect if moving in front of Chologaster. The moment one came in close proximity to a fish from any direction, by a flashlike motion it was seized. None of them were swallowed. The fishes became very alert after the introduction of the sowbugs and when swimming forward would strike at a part of a leaf if it came in contact with the head of a fish. It seemed evident that the eye gave no information of the character of the object. As the Asellns was not altogether to their taste, Gam- ma rus was introduced. One of these, swimming rapidly toward the chin of the Chologaster from behind and below, was instantly seized when it came in contact with the fish. The eye could not have located the Gammarus at all. The action is in very strong contrast to the action of such a fish as Le pom-is, which detects its food by sight. It is undoubtedly this peculiar method of locating and securing food which has enabled the Amblyopsidae to establish themselves in caves. On March 20 the eyes were removed from 7 living specimens of Chologaster papilliferus with the following results: Within half an hour after removing the eyes, examples of Asdhis were intro- duced into the aquarium, which were readily detected and captured. In captur- ing them the chologasters were not as accurate as fishes might be expected to be that do not ordinarily depend on their eyes to help in locating prey. It may be borne in mind, however, that the eyes were removed from the surface and that in addition to the removal of the eyes some of the tactile organs were probably dis- turbed or destroyed. A rod held in the hand was readily perceived by the blinded fishes, who avoided it with as much dexterity as an Amblyopsis would, except that their actions in avoiding the rod were very much quicker than the action of an Amblyopsis. The latter, if approached from in front, will back water with its pectorals and then, if the rod comes nearer, it will turn to one side or another, frequently with lazy delib- eration. Chologasters, on the other hand, would turn tail with a flashlike motion when the stick was approaching them. They could be approached from the back more readily lhan from other regions. The action of the blinded fishes was in this respect precisely like that of an unblinded one in the same aquarium. Removing the eyes makes no appreciable difference in the appearance of the fish, and a number of colleagues were asked whether the fishes were detecting the rod by sight (with the eyes) or by tactile sensation. Not knowing that the eyes had been removed, the verdict, in the major- ity of cases, was in favor of the eyes; in the other cases it was doubtful. There was no general disturbance of the fishes in the aquarium when the rod was intro- duced. Only the ones immediately concerned responded. On April 4 I was able to touch each of 5 blinded chologasters on the snout with a glass rod before it made any attempt to get away. The same is true of some which had not been blinded. The blinded chologasters readily swim about in the aquarium, regardless of protection or of contact with the sides of the aquarium. They not infrequently lie at the bottom, but the general tendency is to swim about freely. One of them lived for 2 years after the operation. REACTIONS TO LIGHT. N7 At 10 a. m. of one day the blinded fishes were removed from the large aquarium and replaced by a number with eyes. These at first remained at the bottom, but on the following morning they were swimming about as the blinded ones had been. The general conclusion from these experiments is that the Chologastcr papilliferus with comparatively well-developed eyes can get along without them just as well as with them. REACTIONS TO LIGHT.1 A long series of observations and experiments was made to determine the reaction of Chologastcr and Amblyopsis to white and monochromatic light. Incidentally other characteristics were brought out. Some previous experiments on blind or blinded vertebrates may be recalled. Dubois (Compt Rend., t. ex, pp. 358-360) and Semper (p. 79) record that Proteus, the blind salamander of Europe, is sensitive to diffuse light. Graber records that blinded salamanders prefer dark chambers to light ones. Korang (Centralblatt f. Physiol. vi, pp. 3-6) notes that concentrated light deprived of heat rays thrown upon the leg of a frog whose brain had been laid bare and covered with extract of beef, caused it to respond each time with reflex movements. That Amblyopsis avoids the light, even the diffuse daylight of a room, is without question. An aquarium was divided in the center by a black partition; one end of the aquarium was covered and the sides painted black, and a small opening was left in one of the lower corners of the partition to enable the fishes to move readily from one chamber to the other. The fishes had no difficulty in find- ing this opening, and at the beginning of the experiment, before the fishes had quieted dovvn from the excitement incident to moving them, they swam back and forth from one chamber to the other as rapidly as it was possible to note the changes. The following are some of the results obtained at separate times: Experiment I: Observation on 6 individuals placed in the above aquarium, May 12, 1906, gave, between 9.43 a. m. and 10.20 a. m., 104 events in the dark, and 220 in the light. This would indicate that the fishes have a preference for the diffuse daylight of the room over that of the dark chamber. But these specimens had been in the light several days, so the light-perceiving or light-reacting organs may have been fatigued. Subsequent events and tables indicate the opposite in such a striking way that the evidence is conclusive. A rapid moving of different individuals from one chamber to another was due to the excitement caused by preparing the aquarium, and the preference shown for one or the other conditions of illumina- tion was entirely overcome by the excitement produced. Experiment II: Conditions as in the first experiment with the same 6 individuals in the afternoon of the same day, the aquarium placed so that sunlight entered the lighted end of the aquarium. Result, 114 events in the light, 204 in the dark. The second experiment shows that there is an inclination to seek the dark rather than sunlight. That the fishes had not gotten into a normal condition is evidenced by the rapid changes of different individuals from light to dark and vice versa. Toward evening as the direct light was excluded the fishes began to go over to the lighted compartment. 1 For further studies see Payne, Biol. Bull, xin, pp. 317-323. 88 BLIND VERTEBRATES AND THEIR EYES. Experiment III: On May 13 the same 6 specimens were used under the same conditions as in experiment I. The aquarium had been quiet since 5 p. m. the evening before. Time. In the ,l.irk. In the light. Time. In the dark. In the light. Time. In 111, dark. In the light. Time. In the dark. In the light. A.M. A. M. A. M. A.M. h. m. s. I], m. s. h. m. s. h. m. s. 921 o .1 3 9 27 50 4 2 9 35 °° 5 I 9 45 3 6 o 921 20 t 2 9 28 50 3 3 9 3(> '5 4 2 9 45 15 5 I 9 -'1 3° 5 I 9 29 30 4 2 9 37 ° 5 I 9 46 15 4 2 9 21 35 4 2 9 29 50 5 I 9 38 o 4 2 9 48 o 5 I 1 () 22 2O 4 2 9 30 o 4 2 9 42 o 5 I 9 48 40 4 2 "9 22 30 5 I 9 32 20 3 3 9 42 30 4 2 9 49 ° 5 I 9 24 20 4 2 9 32 5° 4 2 9 43 3° 5 I '9 5° 15 4 2 '9 25 30 4 2 9 33 ° 5 I 1 9 43 5° 5 I 9 5° 3° 5 I 3 9 26 10 4 2 9 33 5° 4 2 9 44 3° 4 2 '9 51 ° 5 I 9 26 50 3 3 '9 34 1° 4 2 9 45 ° 5 I 169 ,65 ' Two exchanged. 3 One other came out, went back. 3 Two exchanged, the one last out returning, « One came out, but went back at once. Experiment IV : On May 13, from 2 to 3 p. m., during the period correspond- ing to the time when records were made the day before, the fishes stayed in the dark chamber except occasionally when one would come into the light only to quickly turn and swim back into the dark. Experiment V : On May 15 the fishes remained in their dark chamber nearly all day except during the excitement caused by changing the water, when they swam freely into the light. It is evident that the incessant changing during the first observations recorded was due to excitement caused by the change of water and aquarium. A small opening was made in the front of the dark chamber, through which observations were made. A few individuals on this occasion came out. Experiment VI: On May 17 no blind fishes were in the light chamber between 8.30 a. m. and 9.20 a. m. Through an opening in the top of the dark chamber several were observed to come to the opening between the two chambers but quickly to withdraw. The sides of the light chamber were painted with a wedge-shaped dark area the better to protect the dark chamber from oblique rays. Effect of jarring. - - The aquarium was moved slightly in order to note the effect of jarring. While no fishes had been in the light chamber during the morn- ing, 4 were now out in a few moments; these returned and during 7 to 10 minutes the changing to and from the dark chamber was kept up. At 91' 30'" 17 approach opening of dark chamber without going out. 16 approach corner above the opening. At 9'' ;;"' dtrw off 2.5 inches of water to 0.5 inch from level of top of opening. At 91' 45'" one came to opening and relumed; 7 went through opening. Evidently still some disturbance. LI ft I IK (il, ei \ations at 9'' 48'". After the fishes had become quiet it was seen that while they were constantly moving past the opening it was rare that one passed out into the light chamber, and thru they invariably showed signs of uneasiness, frequently turning sharply round ami reenlering the dark chamber, at other times making a complete circuit; this at a time when there was no direct sunlight. At 12 m. a dark tunnel was constructed by leaning a black pane of glass against the dark partition, leaving an opening at the side of the aquarium opposite to that in the opening of the first partition. For some time after this was done the fishes stayed in the light chamber in which they had been put, without being able appar- REACTIONS TO LIGHT. 89 ently to find their way out. After a day, however, all had collected in the dark chamber and it was rare that any of them came out into the light chamber. They remained in the dark chamber for days without coming out, except occasionally at night. On May 24 the blind fishes remained in the dark compartment until night, when all collected in the light compartment, only to be found back again in the dark the next morning. Everything indicates that they readily perceive light, even the diffuse light of a room, and that they individually react negatively to light. Four Amblyopsis which had been kept for a day in a vessel painted black and covered to exclude the light were experimented upon as follows : a ray of light from a microscope mirror about 2 inches in diameter was thrown on each success- ively. After from i to 5 seconds the fishes became uneasy, the uneasiness giving place to discomfort, the fishes making vigorous efforts to get out of the ray. Another jar, not painted, containing both Amblyopsis and blind Cambarus, was placed where light could be reflected upon them from the mirror of a micro- scope. The Cambarus, if in motion, came suddenly to a halt ; if quiet, it backed or moved off at once. The fishes also responded to the light but it took several times as long for them to do so. Bright sunlight appears to be irritating; if exposed to it, the fishes swim about uneasily. A shadow passed suddenly across them when in the diffuse light of a room does not affect them, nor do they, when swimming, seem disturbed by a ray of light entering the dark chamber through a small hole in the paint made for the experiment. Two examples kept in a pail in my cellarwere quietly floating, but when a lighted match was held above them the fishes at once darted to the bottom and sides of the pail. The heat could not have been a factor in this case; the reaction to the light of the match was quick and violent. A similar observation was made on 40 individuals in two aquaria. They were captured one morning, and the observation made the second night after. They had been kept in the dark during most of the intervening time. A lighted match, held near the aquaria, produced a very general and active movement among all the individuals. Even more striking than this was the action of a colony of Amblyopsis in an open pool. During the bright part of the day the fishes remained under the rocks at the bottom. Occasionally a nose could be seen poking out from under a rock ; perhaps one of the fishes came out at times during the day. In the morning and evening and at night, they could be seen swimming in various parts of the pool. The following experiments make it evident that the direction of the light does not influence the actions of these fishes, but that their behavior is due to a per- ception of difference in the intensity of light. A large box, covered at its southern end, was sunk into the ground where the water of a spring flowed through it. Throughout the lighter parts of the day the fishes stayed in the shade of the south- ern part of the aquarium. It was only in the evening, in the morning, and at night that the fishes ventured forth. A similar box 2x4x8 feet, divided in the middle by a partition running to near the bottom, had lids hinged so that either or both compartments could be covered and darkened. Within a short time after one of the compartments was darkened all of the individuals would be found in the dark- ened compartment, irrespective of the direction of the sun's rays. tJQ BLIND VERTEBRATES AND THEIR. EYES. Mr. F. Payne has made further studies and found that their negative heliotro- pism is sufficient to overcome their positive geotropism if an 800 candle-power arc lamp is used 16 inches from the aquarium. He also found that the young fish to an inch in length react more strongly to light than older ones, even if their eyes are destroyed, and that one part of the body is as sensitive as another to a pencil of strong light. The 7 blinded chologasters mentioned previously were placed at 9 a.m. in an aquarium which was dark at one end and light at the other, but with no partition between. In the bottom of this aquarium, extending from the lighted into the darkened area, was placed a plate of glass propped up at one edge so as to enable the fishes to get under it. The conditions in the two parts of the aquarium were as nearly alike as possible except as to light. The blinded chologasters collected in the darkened half of the aquarium and remained there. The reaction was quite positive. No sunlight entered the aquarium — only the diffuse light of the room. The same reaction took place when sunlight entered the aquarium. Later, the pane of glass was taken from the bottom of the aquarium and placed against its sides, and the fishes collected behind it in the dark end. A number of normal chologasters in another aquarium had the same habit of squeezing them- selves in between the sides of the box in which they were and the small glass aquarium placed in it. It is evident that Chologaster is also negatively helio- tropic and positively stereotropic. A series of observations was made to determine to what rays, if any, Amblyopsis reacts most vigorously. For this experiment a glass jar 3 feet long and 8 inches in diameter was divided into 6 compartments by 5 partitions. Each partition had a ver- tical slit extending half-way up from the bottom to enable the fishes to swim freely from one compartment to another. The compartments were thus all connected. A cap was screwed tightly over the end of the jar, which was placed horizontally in a window-sill where each compartment would have an equal amount of light.1 The jar was surrounded with bands of tissue paper in several layers of violet, blue, green, orange, and pink so that each compartment was lighted by one series of rays, Three Amblyopsis were used for these observations; they were selected for their size and named, A, the smallest, B, the middle-sized, C, the largest. These fishes had been in confinement some time, but had been transferred from the cave, with as little exposure to light as possible, to a dark room where they were very seldom exposed to the light. Observations were made as opportunity presented itself. It was found that some compartments were visited by a certain fish without any definite regard for color. During January, for instance, fish C moved out of the pink and orange compartments but once; fish A remained almost exclusively in yellow, visiting pink once, orange once, and green 4 times. Fish B, on the other hand, remained mostly in the violet, visiting blue 7 time's and green 3 times. From this \ve must conclude either that different individuals react differently or that one color does not produce a stronger reaction than another, and the latter seems the more reasonable conclusion. (Sec table on page 91.) To determine whether the apparatus had anything to do with the distribution and also whether widely separated elements of the spectrum would cause the fishes ' For over a month these fishes were sealed in this jar without change of wuUr. REACTIONS TO LIGHT. 91 Series of Observations to determine to which Rays Amblyopsis react most Vigorously. Date. Time. Violet. blue. Green. Yellow. Orange. Pink. Remarks. 1896 h. m. Dec. 16 9 3° A, B C . 10 B, C A . 2 A C B i? 93° A B, C 12 30 B A, C 18 lO^O A, B C 21 9 A B, C 12 B A, C I B C A 22 10 B A, C 23 2 A B C clear 2(3 I B A C cloudy 1897 Jan. 4 10 B A C cloudy 5 IO B A C snowing 12 I! A c snowing 4 B A c snowing 6 4 B A c 7 4 B A c clear 8 4 B A c clear 9 12 B A c clear ii IO B A c broken 4 B A c clear 12 IO B A c clear 12 B A c cloudy 4 B A c cloudy 13 IO B A c cloudy 14 12 B A c cloudy 4 B .. A c cloudy IS IO B A c cloudy 4 B A c cloudy 18 IO A, B c clear 12 B A c clear 4 B A, C clear 21 I B A C cloudy 26 8 14 24 12 18 Fish No. c I I 2 6 9 iS B 2O 7 6 I A 5 o 6 18 3 2 Total 26 8 U 24 12 18 to react positively or negatively, they were put into a rectangular aquarium im- pervious to light, except at the ends, and divided by a median partition. The ends were covered with translucent celluloid film, care being taken, of course, to have each end equally light. Random observations taken through 20 days show: A, once in the blue compartment and 34 times in the red ; B, 6 times in blue and 39 times in red; c, 27 times in blue and 18 times in red; a total of 34 times in the blue and 91 times in the red. If only A and B had been used, we would have been justified in concluding that Amblyopsis is positively tropic toward the red end of the spectrum as against the blue. If only c had been used, we would have been justified to draw the opposite conclusion. The fishes in the red compartment had become nervous and were swimming near the red window, that is, on the side opposite the opening between the compartments. Their proneness to remain in the same compartment may have been partly due to this nervousness, the cause for which was not apparent. Four specimens of Chologaster were placed in the apparatus having 6 different colored compartments. Between January 26 and February 4 rather irregular observations were made. The number of specimens for each compartment on a purely chance distribu- tion would have been 12.6, leaving out of consideration the element that the end 92 BLIND VERTEBRATES AND THEIR EYES. compartments contained but one opening, only one compartment bordering each. A strong positive reaction toward violet is indicated, and a strong negative reaction toward pink and blue. The totals were: violet, 25 ; blue, 6; green, n ; yellow, 13 ; orange, 14; pink, 7. To test these results, the second aquarium, with but two compartments and three specimens, was used. The specimens are marked A smallest, B medium sized, c largest. In series I, 20 out of 24 events occurred in the red. The windows were interchanged, transposing colors, when, in series II, out of 24 events 13 occurred in the red. This indicates a decided positive reaction toward the red. In series III, 16 out of 20 events occurred in the red. A new aquarium was substituted, with the windows side by side, looking toward a west window. Out of 17 events (series IV) 13 occurred in the red. The colors were then interchanged, so the fishes would be compelled to change compartments in order to be in the same light. In this series 27 out of 29 events occurred in the red. These series give conclusive evi- dence that the affinity of these fishes is strongly in favor of the red. It may also be noted that the smallest specimen was most frequently found in the blue. BREEDING HABITS OF AMBLYOPSIS. The eggs are laid by the female, to the number of about 70, into her gill cham- ber. Here they remain for perhaps 2 months, till the yolk is nearly all absorbed and the young fish has attained a length of about 10 mm. If at any time a female with young in her gill pouches is handled, some of them are sure to escape. This was observed and gave rise to the idea that this species is viviparous. We owe the first observations on the breeding habits of Amblyopsis to Thomp- son, who states that a fish "was put in water as soon as captured, where it gave birth to nearly 20 young, which swam about for some time, but soon died * * * they were each 4 lines in length." It is unfortunate that the highly interesting suppo- sition of Thompson that they were viviparous has gained common currency. Putnam adds to the above, judging from some data in his possession, that the young are born in September and October, and further along remarks that they are "undoubtedly" viviparous. The first young I obtained were secured on May 9, 1896. The little fishes could move actively for a few moments, but as they were encumbered with much yolk, they soon settled to the bottom and remained quiet. A large number of old ones were in the water in which the young were found, and the mother of this lot was not identified with certainty. Another lot of young obtained on September 5 of the same year were much farther along in their development. Some were pre- served and others placed in various aquaria, where one lived to be 10 months old. As before, the parent was not with certainty determined, simply because it was taken for granted that they were viviparous and the ovaries only were examined. Two other lots of young were obtained on June 5, 1897. One of these lots was in the stage of the first lot obtained, with a large amount of yolk still present, while in the other lot the yolk had almost entirely disappeared. These had been carried in the gill cavity of the mother, and it became evident either that the fishes were not viviparous at all, or that their viviparity was not nearly of the pronounced character hitherto supposed. On March IT, 1898, 29 individuals were captured. Four were females with eggs in their gill cavities. The youngest stage among these was at the end of segmen- EIGENMANN D Views of Amblyopsis, in the early stages. A and B. Embryos on egg, (A) younger stage, (B) older stage. C. Larva at time of hatching. D. Older larva. RIVALRY OF MALES AND SECONDARY SEXUAL DIFFERENCES. 93 tation, the oldest was a gastrula covering but one-third of the yolk. The eggs had not been developing more than 5 days, probably not more than 2 at the utmost, and decided beyond a doubt that these fishes are oviparous and not viviparous. In one individual 61 eggs were found, in another 70. The exact number in the other two, I can not give, but the number does not differ greatly from the above. From one side of one I took 35 eggs, from another individual an uncertain number. The remaining eggs were left in the gills to develop, but all that were not subse- quently preserved finally died. The female with eggs can readily be distinguished by her distended gills, and since dead eggs become opaque, such can readily be distinguished through the translucent operclcs and branchiostegal membrane. Dead eggs are retained in the gill cavity till they disintegrate. I have never secured as many young from any female as the eggs enumerated above. This may have been either on account of the dying of many eggs or the liberation of the young during the struggle of capture. Emphasis need be laid on the fact that A-wblyopsis is not- "viviparous and that its breeding period extends at least from the first of March to November and probably throughout the year. A female with nearly ripe eggs was secured on September 9, and since these would have been carried either as eggs or young for about 2 months longer, November is a safe limit. During March the spawning season is evidently at its beginning, and it is during this month and April and May that the early stages may be looked for with the greatest confidence. No eggs were deposited in the laboratory. Females with eggs in the gill cavities had to be sought for in the caves. To secure embryological material when a female containing favorable stages was captured, she was isolated in a small aquarium and the number of eggs needed freed from the gill cavity by gently raising the edge of the operculum. The rest of the eggs were permitted to remain in their natural surroundings until another lot was wanted. During the early stages of development the edges of the operculum are closely pressed to the neck and there is no danger of freeing more eggs than are wanted unless the fish is roughly handled. During the later stages of development the tension of the oper- culum is relaxed and eggs or larvae can be much more easily removed, but there is a correspondingly greater danger of liberating more young than are wanted. If the female is disturbed or confined during the latest stages of brooding, some or all of the young will escape. The eggs freed from the gill cavity will continue their development uninterruptedly, but the gill cavity of the female offers such a unique and self -regulated hatchery that they were usually left in it. RIVALRY OF MALES AND SECONDARY SEXUAL DIFFERENCES. In an aquarium containing six specimens of Amblyopsis, two took a great antipathy to each other and engaged in vigorous contests whenever they came in contact. Frequently they came to have a position with broadside to broadside, their heads pointing in opposite directions. The fight consists in quick lateral thrusts toward the antagonist to seize him with the mouth. The motion is in- stantly parried by a similar move by the antagonist. This blind punching may be kept up for a few seconds, when, by their vigorous motions, they lose each other and jerk themselves through the water from side to side, apparently hunt- ing for each other. At this time they are very agile and move with precision. 94 BLIND VERTEBRATES AND THEIR EYES. When the belligerents meet, one above the other, the snapping and punching is of a different order. While jerking through the water, just after a round, if one of the belligerents touches one of the neutrals in the aquarium, it frequently gives it a punch, but docs not follow it up, and the unoffending fellow makes haste to get out of the road, the smaller ones most quickly. If, after an interval of a few seconds, a belligerent meets a neutral, they quietly pass each other without paying any further attention ; whereas if the two belligerents meet again, there is an im- mediate response. Whether they recognize each other by touch or by their mutual excitability, I do not know. In another aquarium I saw one belligerent capture the other by the pectorals. After holding on for a short time it let go, and all dif- ferences were forgotten. The thrust is delivered by a single vigorous flip of the tail. These fights were frequently noticed, and, as far as determined, always occurred between males. The absence of secondary sexual differences in the cave fishes is a forcible argument in favor of sexual selection as the factor producing high coloration in the males. The absence of secondary sexual differences in caves opposes the idea of Geddes and Thomson, that the differences are the external expression of maleness and femaleness. THE EGG AND GENERAL DEVELOPMENT OF AMBLYOPSIS. The eggs are large, measuring 2.3 mm. in diameter. The yolk is translucent, of various tints of amber. The yolk measures 2 mm. in diameter and contains a large protruding oil-sphere i to 1.2 mm. in diameter. When the egg is deposited, the yolk is flabby and composed of yolk-spheres of various sizes loosely put together. After the egg has been in water for some time, the yolk forms a tense rounded mass. The egg is heavier than water. The oil-sphere lies uppermost in the egg, and the germinal disk forms at the side of the egg. Attempts at artificial fertilization have not been successful beyond obtaining well-developed germinal disks. The rate of development will probably be found to vary considerably with the temperature of the water. In a series of eggs in which the gastrula covered half the yolk when observations began, the blastopore was reduced to the size of the oil-sphere in 9 hours, when the embryo encircled about a third of the yolk; 16 hours later the blastopore was closing. The rate of development of the series of eggs taken in May was as follows, the mother containing the eggs having been kept in a small aquarium without change of water and at the temperature of an ordinary living room. The temperature of the water in the cave is 12° C., that in the room was 22° C. On May 4, 9 p. m., the gastrula covered approximately half the yolk. It lies eccentric, neither below nor at the side, the germ being evidently heavier, the oil-sphere at the top. May 5, (i a. m., the embryo surrounds about a third of the egg, the blastopore is about as wide as the oil-sphere, 1.2 mm., and the latter seems to fully fill it. At 2.3op. m. the embryo is i .6 mm. long and has 4 protoverlebra. At 6 p. m. the blastopore has narrowed considerably and invariably lies at one side of the oil-sphere, the embryo lying oblique to the vertical axis of the egg. This eccentric position becomes more and more evident as the blastopore closes toward lop. m. The embryo is 1.76 mm. long, with 6 protovertebra-. At 10 p. m. the eyes and brain are shaped like the ace of spades, the eye lobes evidently not yet narrowly separated from the brain by a narrow stalk, the blastopore closing, the embryo 1.92 mm. long, and with 10 protovertebra;. On May 6, at 6 p. m., the embryo lies horizontal around the margin of the yolk ; the cavity of the central nervous system has appeared ; a large piece has been eaten out of the yolk; the lens is just beginning to develop. There are 12 or 13 proto vertebrae. At 8 a. m. the embryo is 2.4 mm. long; at u a.m. no marked change is seen; at 6 p. m. tail is beginning to bud out; embryo, 3 mm. long, encircles half the yolk; 17 proto- vertebra present. THE MIGRATION OF THE ANUS. 9.r> There is a regular change in the position of the embryo with development. The blastoderm is formed at the side of the yolk. When the gastrula covers half the yolk, the egg has rotated so that the gastrula covers more of the lower than of the upper surface of the yolk. Still later, some hours before the closing of the blastopore the latter structure lies to one side of the yolk-sphere, which always occupies the upper pole of the egg ; the embryo extends from this region obliquely over the yolk. After the formation of the tail the embryo is always found coiled about the upper half of the yolk. The period spent in the egg lasts about a month. In the laboratory some embryos hatched in about 28 days, but in the cold cave streams this period would probably be several days longer. The yolk has been but little affected at the time of hatching, measuring 1.8 mm., the oil-sphere about i mm. ; and since the yolk is all absorbed before the young are freed from the gill membrane, probably another month is spent under the gill membrane. FIG. 30. (a) Internal Anatomy of Aiublyupsis speltzus. i, anus; 2, opening of oviduct; 3, oviduct; 4, ovary, which is single; 5, liver; 6. duodenum; 7, gall sac; 8, pectoral fin; 9, one of pyloric Cieca; 10, Ciecum; n, stomach; 12, spleen; 13, air bladder; 14 and 16, intestine; is, pancreas; /., liver. (b) Alimentary Canal of Chalogaster comutus, pc., pyloric c.-eca; s., stomach; -.-..vent. (c) Alimentary Canal of Chvlogasler papillifcrHs. (d) Alimentary Canal of Cttologastcr agassizii. (e) Alimentary Canal of Typhlichlhys sublerranfUS. The young, on hatching, are about 5 mm. long and lie on their sides. The motion of the tail produces no effect other than to cause them to spin around with the yolk for a pivot. The metamorphosis of the larva into the definitive fish is completed before it leaves the gill cavity of the mother. The longest individ- uals I have secured from the gill cavity measure about 10 mm. THE MIGRATION OF THE ANUS. Certain structures gain an entirely new significance in the light of the breed- ing habits. These are the enlarged gill cavities with the small gills, the closely applied branchiostegal membrane, and the position of the anus and sexual orifices. The anus in all of the species has undergone a curious translocation. The primary cause of the transposition probably lies in the ovary and oviduct, and not 96 BLIND VERTEBRATES AND THEIR EYES. in the alimentary canal. The opening of the oviduct has moved forward until it lies in front of the pectorals and it has carried the anus forward with it. In newly hatched individuals the anus has its normal position behind the ventrals. When the fish has reached a length of 25 mm., the anus has reached a point in front of the ventrals, hut it is still nearer the ventrals than the pectorals; with a length of 35 mm. the anus has moved forward to just below the insertion of the pectorals. In mature specimens it lies considerably in advance of the pec- torals (plate 5, fig. c). The forward movement of the sexual orifice takes place in both sexes. Nothing is definitely known of the advantages of the location of the opening of the oviduct. They can be inferred from the habit of Amblyopsis in carrying its eggs in the gill cavity. Located as it is, the oviduct may be covered by the gill membranes of the 2 sides alternately, or, if the fish takes an oblique position in the water with the head down, the eggs may flow directly into the gill cavities, being carried downward by gravity and held in the groove in front of the anus by adhesion. It is difficult to imagine even a formal explanation of the origin of the position of the sexual orifice in the Amblyopsidae. The anus was probably carried forward as the result of the forward movement of the sexual orifice, and it is this that demands explanation. Very probably the habit of carrying the young in the gill pouches antedates the present position of the anus. The eggs may have been allowed to flow into the gill openings, the female occupying a position with head downward during oviposition. If this were the case, then, while the individual skill would count for much in transferring the ova, a variation or mutation which lessens the distance between the sexual orifice and the gills would be of distinct advantage and would probably be transmitted by natural selection. The actual transfer of the ova into the gill cavity has not been observed. THE TACTILE ORGANS. The tactile organs are among the most important in the consideration of the blind forms. Their minute structure will form the basis of a separate paper. The prominent tactile organs about the head of Amblyopsis have been mentioned by nearly every writer, and they have been figured by Putnam-Wyman and Leidig ; but the figures of the distribution of the ridges are worthless. The description by Professor Forbes of Chologaster papilli- fi-rus is the only systematic enumeration of the ridges that has appeared. The accom- panying figures (32 and 33), drawn by me with the camera lucida, verified and copied i Organ in Il,;,y LViT. U. U. LXXX, give me exact CAieiH in weak osmir acid which brought out outlines i ••• r ,1 •! * A 7/^.rtA^-*,, 4 ocular. and position of the ndges in Amblyopsis, Typhlichthys, and Chologaster papillifcrus. It will be seen that in the number and distribution of the tactile area the three forms agree very closely, the eyed form having the same number and dis- tribution of ridges or rows that the blind forms have. In C. papillifcrus most of the ridges are much less prominent than in the blind species, being sunk into the skin. About the nose and chin, however, the ridges are as prominent THE TACTILE ORGANS. 97 as in the other species. In Chologaster cornutus there are no distinct ridges at all, the tactile organs being arranged as in other species of fishes. In specimens of the same size the papillae are not more prominent in papillifcnis than in cornutus. It is only in the oldest of papillifcnis that the papillae become prominent. The number of individual papilla; in each tactile ridge differs considerably with age (size), so that an exact comparison between the large Amblyopsis and the much smaller species of Chologaster and Typhlickthys can not be made. From a num- ber of counts, Professor Cox found that ridge No. 6 contains in Chologaster papilli- ferus, 6 organs; in Typhlichthys, n ; in two specimens of Amblyopsis, respectively 3.33 and 4.25 inches long, 12 and 20. The tactile ridges in the head of Amblyopsis FIG. 32. (a, 6, c) Distribution of Tactile Ridges in Amblyopsis; lateral, dorsal, and ventral views. (c/, e) Distribution of Tactile Ridges in Typhlichthys subterranevs; dorsal and side views. are shown in plate 8, figures A and B. The outermost layer of skin has been re- moved from a small area over the right eye of A, showing the numerous taste buds. Figures c and D show head of Chologaster papillifcnis under slightly greater mag- nification. Figure D shows especially the tactile organs about the mouth. The skin passes over the eye without a free orbital rim, and the eye does not show well. Aside from the tactile organs in ridges there are many solitary ones not evi- dent from the surface in Amblyopsis. When the epidermis is removed by macera- tion, the dermal panillae on which they rest give the whole head a velvety appearance. In the young, at least, of Amblyopsis, each of the tactile organs of the ridges is provided with a club-shaped filament abruptly pointed at the end (fig. 31). They wave about with the slightest motion in water and are so numerous as to give the whole head a woolly appearance. 98 BLIND VERTEBRATES AND THEIR EYES. Tellkampf has remarked : The blind fish is found solitary and is very difficult to be caught, since it requires the greatest caution to bring the net beneath them without driving them away. At the slightest motion of the water they dart off a short distance and usually stop. * * * During my stay at Mammoth Cave I observed that the AmUyopsis * * * remained motionless while I moved a burning lamp around them, but they were disturbed by a slight motion of the water, proving that the light made no impression upon their optic nerve, while their sense of touch was acute. FIG. 33. (a, b, c) Distribution of Tactile Ridges in Troglichthys. Side view of entire fish, dorsal and ventral views of head. (<2t«,/) Distribution of Tactile Ridges in Chologaster papilliferus. Side view of entire tish, dorsal and ventral views oi anterior part of body. Dr. John Sloan in Packard, 1887, wrote : We carried our lighted candles within a few inches of them when near the surface, but they seemed wholly insensible to their existence; but if a drop of tallow fell in the water near them, they would swim rapidly away. I brought home 12, as many as could live in my bucket. Of these 12 caught in September none died until next June, when the water became warmed to near 70°, when several of them died with tetanic convulsions ( ?). I put the remainder in my cellar, where the temperature ranged from 45° to 60°, where one, "Blind Tom," lived n months, making 20 months of existence without having taken any visible food. While in my aquarium they manifested total indifference to light and sound. * * * They manifest great sensibility on the back and sides to any approaching body, but do not notice an attack from below. It is not possible to capture one by a side sweep of the net, but by passing it under him a considerable distance below and bringing it up slowly there is no difficulty in taking them. In their native pools and in the aquarium when disturbed they do not strike the bottom or sides of their surroundings, but seem to have a sense of resistance (if the term is pardonable) which protects them. Miss Hoppin in Garman remarked: I am very sure they frray-fishes], as well as the white-fish [Troglichthys] have the tactile sense developed in an unusual degree. At the least touch upon the water they dart away. * * * Nu- merous tests convince me that it is through the sense of touch, and not through hearing, that the fish is disturbed. * * * If I strike the vessel so that the water is set in motion, he darts away from that side through the mass of water, instead of around in his usual way. If I stir the water or touch the fish, no matter how lightly, his actions are the same. EIGENMANN PLATE 8 - ,, ••, • B D Photographs of the tactile organs of Amblyopsis and Chologaster. A. Head of Amblyopsis from above, showing tactile ridges. B. Same head from side. Tactile organs especially numerous about mouth. C. Head of Chologaster papilliferus, from above, under slightly greater magnification than A. D. Same head, from side, especially showing tactile organs about mouth. THE TACTILE ORGANS. 99 Blatchley states: * * * the least movement of the water frightened them, and they darted rapidly away, usually at right angles to the course they were pursuing. The sense of touch, rather than that of hearing, is, in my opinion, the one which has been intensified by long residence in the dark and silent recesses of the caves. I have not found the slightest difficulty in capturing Amblyopsis with a small dip net, either from a boat or while wading through the subterranean stream, and I have caught one in the hollow of my hand. At such a time any amount of noise I was capable of making did not affect the fishes found swimming in the water. Frequently they were taken in the dip net without apparently taking any note of the vibrations produced in the water until they were lifted out of it ; very rarely a fish became noticeably scared. Such a one would dart off a few feet or a few inches and remain on the qni rive. If not pursued, it soon swam off quietly; if pursued, it not infrequently escaped by rapidly darting this way and that; when jumping out of the water, often an abrupt turn in the opposite direction from which it started would land it in the net, showing that their sense of direction was not very acute. At other times, if disturbed by the waves produced by wading, one or another individual would follow a ledge of rock to the bottom of the stream, where it would hide in a crevice. But very frequently, much more frequently than not, no attention was paid either to the commotion produced by the wading or by the boat and dip net. In general it may be said that the fishes in their natural habitat are oblivious to disturbances of the water until frightened by some very unusual jar or motion, probably a touch with the net, when they become tensely alert. The fact that they are not easily frightened suggests the absence of many enemies, while their frantic behavior if once scared gives evidence that occasional enemies are present and that they are very dangerous, or that the transmission of the instinct of fear is as tenacious as the transmission of physical characters. Contrary to Sloan's observation, that they detect the presence of a solid sub- stance in their path, I have never noticed that the fishes in confinement became aware of the proximity of the walls of the aquarium when swimming toward them. Instead, they constantly use the padded, projecting lower jaw as bumpers. Even an extremely rapid dart through the water seems to be stopped by the projecting jaw without serious inconvenience. Sticks, straws, etc. are never avoided by the fishes, even when the fishes had not been disturbed for hours. By this I mean that they are never seen to avoid such an object when it is in their path. They swim against it and then turn. An object falling through the water does not disturb them even if it falls on them. Gently moving a pencil in front of them does not disturb them much, but if the pencil is held firmly in the hand it is always perceived and the fish comes to a dead halt half an inch before it comes in contact with it. On the other hand, they may be touched on the back or tail before they start away. They glide by each other, leisurely and dignified, and if they collide, as they sometimes do, they usually display no more emotion than when they run against a stick. But this in- difference is not always displayed, as was noted under the head of breeding habits. A number kept in an aquarium having a median partition in which there was a small opening were readily able to perceive the opening, swimming directly for it when opposite it. This observation is in direct contrast to their inability to perceive solid substances in their path. A sharp tap on the sides of an aquarium 100 BLIND VERTEBRATES AND THEIR EYES. in which 6 blind fishes were swimming, where they had been for a number of days undisturbed in a dark room, caused nearly all of them to dart rapidly forward. A second tap produced a less unanimous reaction. This repeated on successive days always brought responses from some of the inmates of the aquarium. Those responding were not necessarily the nearest to the center of disturbance, but some- times at the opposite side of the aquarium or variously distributed through it. After a few days the fishes took no notice of the tapping by any action observable in the artificially lighted room. Such tapping on a well-lighted aquarium containing both Chologaster and Amblyopsis was always perceived by the Amblyopsis, but the only response from these imperturbable philosophers was a slight motion of the pectorals, a motion that suggested that their balance had been disturbed and that the motion was a rebalancing. The Chologaster, on the other hand, invariably darted about in a frantic manner. One individual of Amblyopsis floating on the water was repeat- edly pushed down by the finger without being disturbed; but if touched on the side, they always rapidly dart away. From everything observed it is quite evident that Amblyopsis is not keener in perceiving objects or vibrations than other fishes, and ordinarily pays much less attention to them. Mr. Payne's observation on the feeding habits leads one to conclude that they possess greater power of discrimination between vibrations. Some observations on young Amblyopsis are of interest in this connection. The young with a large amount of yolk still attached show a well-developed sense of direction. A needle thrust into the water near their heads and in front of them causes a quick reaction, the young fishes turning and swimming in the opposite direction. They will do this two or three times, then, becoming exhausted, will remain at rest. Sometimes an individual will not move until it is actually touched by the needle. The needle must come within about an eighth of an inch of the fish before it is noticed. Then, if the needle produces any result, it causes the fish to quickly turn and swim some distance, when the fish falls to the bottom again and remains at rest. If the needle be placed behind the fish, it will swim directly forward ; if at the side or about the middle, it swims directly forward or turns and swims in the direction opposite the origin of the disturbance. Younger specimens have no power over the direction of their progress — the wiggling of the tail simply produces a gyration, with the yolk as pivot. A young blind fish, 6 months old, swims about in a jerky manner, chiefly by use of its pectoral fins. It keeps close to the side of the vessel, usually with its back to the glass. (The aquarium was a cylindrical jar 300 mm. in diameter and 300 mm. high.) From whatever direction it may be approached it perceives a stick thrust toward it as readily as a seeing fish can, and will invariably dart away a short distance, sometimes making sharp turns to avoid the stick and always successfully. Ii can lie approached from the top nearer than from the sides or from in front. It does not avoid the sides of the aquarium, which it frequently strikes. THE EAR OF AMBLYOPSIS. Anatomically considered, the ear of Amblyopsis is normal. Numbers of ears together with the brains have been dissected out. These were treated with Flem- ming's strong solution or with Hermann's fluid, either of which stained the nerve matter black. In the first place, the three semicircular canals are present and THE EAR OF AMBLYOPSIS. 101 each has its ampulla fully developed. The three ampullae and the sinus utriculus superior communicate with the utriculus in front, behind, and above. Below, the utriculus communicates with the sacculus, which terminates posteriorly in an appendage, the lagena. The three ear bones are present, one in the recessus utriculi, one (the largest) in the sacculus, and the other in the lagena. The auditory nerve divides into two branches, the ramus anterior and the ramus posterior. The ramus anterior divides into three branches — the ramulus ampullae anterioris, which extends to the anterior ampulla ; the ramulus ampullae externae, which extends to the external ampulla; the ramulus recessus utriculi, which extends to the recessus utriculi. The ramus posterior gives off a heavy branch, the ramulus sacculi, which extends to the sacculus. The rest of the ramus posterior divides into the ramulus lagenae, which extends to the lagena; and the ramulus ampulla posterioris, which extends to the posterior ampulla. Another branch, the ramulus neglectus, which is normally given off where the ramus pos- terior divides into the ramulus ampulla posterioris and ramulus lagenae, has not been identified. The normal fish ear has seven auditory spots — the macula acusticus recessus utriculi, three cristae acusticus ampullarum, macula acusticus sacculi, papilla acusticus, and the macula acusticus neglecta. InAmblyopsis all of these auditory spots are present. 102 BLIND VERTEBRATES AND THEIR EYES. DOES AMBLYOPSIS "HEAR"? (BY FERNANDUS PAYNE.) Until the time of Bateson and Kreidl, it was generally taken for granted that fishes could hear because they had ears. Bateson concluded from his observations on congers, flatfishes, pouting, etc., that fishes perceive the sound of sudden shocks, but do not seem to hear the sounds of bodies moving in the water. Kreidl was the first to make experiments to test the hearing of fishes. He experimented on the gold-fish (Carassius auratus) and concluded that gold-fishes do not hear with the ear, but that they do react to sound waves by means of sense-organs in the skin. Lee's observations supported KreidPs results, and he further concluded that the sole function of the ear in fishes is equilibration. Parker was the first to get positive evidence against the conclusions of Kreidl and Lee. His experiments were based on Fundulus hetcroditus. He used three classes of fishes; first, nor- mal, that is, unmaimed, ones ; second, fishes with the auditory nerves cut ; and third, fishes with the skin rendered non-sensitive but with the ears intact. His apparatus consisted of a heavy aquarium with a slate bottom, two glass sides, and two slate ends, one of which he replaced by a piece of deal board to serve as a sounding board. To the middle of one edge of the sounding board he attached a stout beam of wood so that it stood out horizontally about i m. in the plane of that end. He stretched a bass-viol string from the free end of the beam over a bridge in the center of the sounding board to its opposite side. When the string was plucked or bowed, it produced about 40 vibrations per second. The fishes to be experimented upon were placed in a small cage suspended from a cord attached at its ends to the walls of the room. The end toward the sound- ing board was covered with coarse cloth. He subjected 10 normal fishes each to 10 tests, and from the 100 tests he got 96 pectoral-fin responses. Fishes with auditory nerves cut responded only 18 times in a total of 100 trials, and Parker thought these 18 times were in part acci- dental occurrences and in part due to the slight movements of the aquarium caused by the vibrating string. Instead of the vibrating string he substituted an electric tuning-fork which vibrated 128 times per second. With the tuning-fork, where the vibrations of the aquarium could be eliminated, he got no responses with the earless fishes. Fishes in which the skin was made insensitive, but with the ears intact, responded to sound 96 times in a total of 100. These fishes reacted almost exactly as the normal ones did. From these results Parker concludes that the killifish hears. Although his conclusion, that a fish hears, is contrary to Kreidl and Lee, he does not say that the observations of these men are entirely wrong, for the cars in different fishes may function differently. In fact, Parker found no evidence of hearing in the smooth dog-fish (Mustelus cams') when he subjected it to the same experiments as the killifish. Bigelow used Parker's methods of experi- menting and re-examined the gold-fish. He concludes that the gold-fish hears.1 1 Since writing tin aln>ve Km TUT in Lucae's Festschrift, 1905, reviewed the evidence advanced to show that fishes can hear, and con< hide . that while they react to rapidly repeated tone-vibrations such as are produced by a tuning-fork or an electric bell, it is not proven that they perceive this with their ears. He used 25 species of fishes and found that in no case did any of these 25 species react in any way to a single sharp click. He con- cludes from these experiments that fishes do not hear. EXPERIMENTS ON HEARING. 103 From the evidence at hand it is very probable that some fishes hear and that others do not. The following experiments have been made on the blind fish Amblyopsis spelaus. Various opinions have been expressed about the hearing of this fish. Wyman states: It is said that the blind fishes are acutely sensitive to sounds as well as to undulations produced by other causes in the water. In the only instance in which I have dissected the organ of hearing (which I believe has not before been noticed), all its parts were largely developed. The following words of Professor Cope are frequently quoted : If these Amblyopsis be not alarmed, they come to the surface to feed and swim in full sight, like white, aquatic ghosts. They are then easily taken by the hand or net, if perfect silence is pre- served, for they are unconscious of the presence of an enemy except through the medium of hearing. This sense is, however, evidently very acute; at any noise they turn suddenly downward and hide beneath stones, etc., on the bottom. Subsequent writers have generally disagreed with Cope. Dr. Sloan (in Packard, 1884) wrote: I tested their hearing by hallooing, clapping my hands, and striking my tin bucket when they were in easy reach and near the surface. In no instance did they change their course or notice the sound. Miss Hoppin (Garman) failed to get any response from Troglichthys as long as noises only were resorted to. She says: I may scream or strike metal bodies together over him, as near as possible, yet he seems to take no notice whatever. Blatchley states that noises do not attract them. Eigenmann's observations (Proc. Brit. Ass. A. Science, Toronto Meeting) on Amblyopsis confirm those of Miss Hoppin on Troglichthys. No ordinary noises produced had any effect on Amblyopsis. Whistles, tuning-forks, clapping of hands, shouting in the reverberating caves, were alike disregarded. Amblyopsis, since it is blind, does not require precautionary methods to exclude sight as a possible disturbing element. If there are sounds in the water of the caves that concern the blind fishes and the ears are sound-perceiving organs, we might expect the ear to be better de- veloped along with the tactile organs as a compensation for the loss of sight. But if there are no sounds, we might expect them to degenerate along with the eye unless the function is something else than sound perception. Amblyopsis has few, if any, enemies in the caves. There are certainly none that make sounds, so the ears of the fishes would not be kept on the alert for them. There is less variety of sounds in the air of the caves than on the outside. This may make but little difference, as sound generated in the air does not penetrate readily into the water. Rippling of the water is certainly perceived more readily by the tactile organs than b/the ear. Besides, the fishes are confined to the quiet pools. My methods of experimenting were practically the same as those of Parker and Bigelow. I used a heavy slate-bottomed aquarium, 24 inches long, 14.5 inches high, and 12.5 inches wide. I removed the glass from one end and substi- tuted a board 2 inches thick. This served as a sounding board. The fishes were confined in a smaller aquarium (4x5x8 inch) suspended in the larger. The end of the smaller aquarium was covered with cheese-cloth toward the sounding board. 104 BLIND VERTEBRATES AND THEIR EYES. The large aquarium rested on a masonry pedestal, which eliminated all vibra- tions of ""the floor. The small one was suspended by cords running from one side of the room to the other. After various trials with small tuning-forks which produced vibrations between 100 and 512 per second, which gave negative results, I used a large fork 12.5 inches in length vibrating 100 times per second and which produced a large volume of sound. I used (a} unmaimed blind fishes and (b) others whose auditory nerves had been cut. I also attempted work with fishes whose lateral line nerves and nerves to the skin had been cut, but the specimens either died or did not revive suffi- ciently to get normal reactions. (a) Unmaimed blind fishes when placed in the aquarium invariably dart to the bottom and remain there for a short time, after which they begin to swim about rather rapidly. They soon become more quiet if nothing further disturbs them, but continue swimming about in a leisurely way, stopping now and then for a few seconds at a time. After they have been in the aquarium for 12 or 24 hours, these stops are more frequent and longer. The fishes strike various attitudes during these stops, but they seldom rest upon the bottom. Instead they are usually poised as if ready to dart away. The body seems so well balanced that they have no trouble in maintaining any position they may happen to take. During these stops the tail always projects straight backward and the pectoral fins stand at right angles to the body. If at this time the sounding board is caused to vibrate, the fish responds either with a quick movement of both the tail and pectoral fins or by the pectoral fins alone. Twenty fishes were each subjected to 5 tests, and out of the 100 trials there were 97 responses and 3 failures. (6) Fundulus, with the auditory nerves cut, acts as normal blind fishes do in swimming slowly or in resting, but when stimulated, loses entire control of its equilibrium. Parker suggests that in resting or swimming slowly the fish depends upon the eye for orientation, but in quick movements the ear comes into play. The reactions of Amblyopsis seem to support this suggestion, for with both auditory nerves cut they have no control of their orientation. When resting, they lie on the side or back, either at the surface or on the bottom. In swimming slowly they sometimes move forward in irregular lines, but when they attempt rapid locomotion, they move in irregular spirals about the long axis of the body and make no progress one way or the other. With only one auditory nerve cut the movements are quite different. The fish is able to move forward, but it goes in a corkscrew-like path, turning over on its axis as it swims along. The same result was obtained by Kiuynmann by thrusting a pin into one of the auditory organs. The operation of eliminating the ear is a comparatively easy one to perform. Of those operated on, more than half recovered. They generally lived for 2 or 3 weeks, and some even longer. The observations were made from i to 2 days after the operation. With these fishes three kinds of responses were obtained. If they were perfectly quiet when the sounding board was caused to vibrate, they either responded by a slight movement of the pectoral fins or by a movement of both caudal and pectoral fins. If, on the other hand, they lay with the body quiet and with the pectoral tins moving slowly when the sounding board was caused to vibrate, they responded by stopping the fin movements. Ten fishes were each subjected to 10 tests, and out of 100 tests there were 96 responses. This result EXPERIMENTS ON HEARING- 105 differs very little from the reaction of fishes not operated upon. Since the ears have been eliminated, there is only one conclusion to reach and that is, that blind fishes detect vibrations with a frequency of 100 per second by means of sense- organs in the skin. As stated, I have not been able to eliminate the skin and lateral-line organs, and so can not say definitely whether or not the ears play any part in the reactions of normal blind fishes. Since the reactions are the same, ear or no ear, the part the ear plays in sound-wave perception, if any, is certainly small. Using the word "hearing" in the sense in which Kreidl and Parker used it, that is, if we define hearing to be the sensation received through the ear and caused by vibrations either in the air or water, the experiments cited do not enable one to conclude definitely whether the blind fishes hear or not. If they do hear, their power in this direction is very limited. The rssults show conclusively that they detect waves of 100 vibrations per second by means of sense-organs in the skin. 106 BLIND VERTEBRATES AND THEIR EYES. THE BRAIN OF AMBLYOPSIS. (By E. E. RAMSEY.) A comparison of the microscopic appearances of the brain of a normal fish and that of the blind fish, Amblyopsis spelceus De Kay, discloses a number of inter- esting conditions. The optic lobes and the optic tracts are measurably degenerate. The hemispheres are larger in Amblyopsis than in the average of normal brains. The brains of Campostoma anomalum, Percina caprodes, Eupomotis gibbosus, and Amblyopsis were measured with regard to the comparative widths of the optic lobes and the hemispheres. Five fishes of the same length were taken of each species. The averages obtained are as follows : Species. Optic Lobes. Hemispheres. Comparative widths Campostoma anomalum . . . mm. 5 5 mm. 2.8 } 7 p. it. 56 74 Percina caprodes 6.4 ?.<; ^4 i2<; It is thus seen that the hemispheres are relatively larger in the blind fish than in the more normal forms, and that the optic lobes are relatively much smaller in the former. There is no noticeable variation in the cerebellum. In length there is a marked shrinkage, chiefly in the optic lobes, as shown by the position of the cerebellum which lies directly on the lobes. In the normal brain the cerebellum is situated well back, hardly reaching the lobes. The following table gives an idea of the length of the brain, as compared with the length of the fish. The brain length is measured from the tip of the olfactory lobes to the posterior part of the cerebellum : Amblyopsis. Campostoma. No. Ix-nKth of body. Length of brain. Per cent. Length of body. Length of brain. Per cent. I mm. 92 mm. 5-5 6 I mm. 88 mm. 8.5 Q.6 2 80 5-3 6.6 2 I03 9 8.7 3 90 5-5 6 3 72 7-5 10 l 88 S-8 6.6 4 68 7 IO 5 80 5-2 6-5 5 58 6-3 10 i, IOO 6 9.8 av. ii,; :iv The result shows the brain of Amblyopsis to be only two-thirds as long as that of Camposloma. This shrinkage in width and length is great enough to show itself in the extent to which the cranial cavity is filled. A great depth of fatty tissue cov- ers the dorsal surface of the brain. The only other external modification of any note is the absence of either optic nerves or optic chiasma. 'I 'he optic lobes are normally composed of 7 layers, which from outside to inside are as follows : (1) A peripheral zone. (2) An optic fiber layer from the optic nerve. OPTIC LOBES OF AMBLYOPSIS. 107 (3) An optic cell layer. (4) A deep cell layer. According to Krause this layer contains in its outer part the cells which serve as terminal stations for the optic nerve, and in its inner sublayer the end stations for the fifth layer (Marklager). (5) A deep fiber layer. (6) A granular layer. (7) The ependyma and its epithelium, which lies next to the ventricle of the lobes. The optic lobes of Awiblyopsis show a marked degeneration. The dorsal walls are not more than half or two-thirds as thick as those in the normal brain. Its contour is so flattened that the ventricle is almost obliterated (16 in fig. 34 b). The torus longitudinalis, which in the normal brain is suspended in the ventricle in the median line entirely below the layers of the lobes, is between the lobes and on nearly the same level with them. The torus thus forms a commissure connecting the lobes. The band of fibers connecting them dips downward in the normal brain and crosses to the opposite side through the torus; in the degenerate lobe they cross from one side to the other in almost a straight line (15 in fig. 34 b). The shrinkage in length is shown in the fact that the hypophysis is crowded forward to the anterior level of the lobes- Ti- FlG. 34. (a) Cross-section of Brain. Amblyopsis speliEus near Anterior part of Uptic Lobes. Specimen 77 mm. long. (b) Cross-section through Middle of Optic Lobes of A mblyop sis speltztts. Specimen 77 mm. long. r, first layer of optic lobe; 2, degenerate optic liber layer; 3, optic cell layer; 4, deep cell layer; 5, deep fiber layer; 5 a, diagonal fibers of deep fiber layer ; 6, granulated layer; 7 a, optic tract region; 13, ependyma; 15, torus longitudinalis; 16, ventricle. The optic nerve of the normal brain is derived from the second and fourth layers of the lobes. The fibers of the second layer pass downward on both sides of the lobes, and the inner ones cross over at the ventral surface, where they join the fibers of the same layer from the other side. They then continue forward and downward to the optic chiasma as the optic tracts. The fifth layer is composed of diagonal fibers and descending fibers. These latter nerves pass downward and become a part of the optic tract. As has been said, the wall of the optic lobes of Amblyopsis has undergone con- siderable shrinkage in thickness. The outer layer is not changed. The second layer, which is derived from the optic nerve, is entirely wanting. The optic nerve is represented by a small bundle of tissue, which is probably the remnant of the neurilemma. In the brain where the second layer should be, there is a narrow space containing practically no tissue. The third layer is unchanged. The fourth 108 BLIND VERTEBRATES AND THEIR EYES. layer consists normally of two sublayers ; the outer one has both nerve fibers and nerve cells — the latter according to Krause being the terminal stations of the optic nerve — and the inner sublayer has the terminal stations of the fifth layer in it. The outer sublayer is entirely atrophied in the lobes of the blind fish ; and the inner one, if at all present, is indistinguishable from the third layer (3 and 4 in fig. 34 b). The fifth layer is reduced to diagonal fibers. The descending fibers which join tin- optic tracts are atrophied. The diagonal fibers are more apparent than in the normal brain. These fibers form a broad commissure in the torus longitudinalis, which runs laterally to the outer edge of the lobes, where it turns back into the substance of the brain just beneath the ventricle and becomes diagonal. Cross- sections of fibers arising from various levels of the lobes are shown (5 in fig. 34 b~). The sixth layer is a granular layer. Its thickness is less than in the normal brain. No other change is noticeable. The thickness of the seventh layer, epen- dyma, is not more than half that of a normal brain. The cells show some shrinkage. The differences in the lobes thus appear to be: first, in the atrophy of the second layer; second, the outer sublayer of the fourth layer is entirely gone; third, the descending fibers of the fifth layer are wholly wanting ; fourth, the granu- lar layer is not so thick and the ependyma is not only thinner but reduced in the numebr of its cells. The optic tracts, that part of the nervous tissue which lies between the optic lobes and the optic chiasma, are entirely wanting. The space occupied by these tracts in the normal brain is in this brain partially occupied by tissue in which I have not been able to make out any structure. All the stains that have been tried have failed to reveal any cells. These tracts do not take the stains with the same readiness and in the same degree that those in normal brains do when subjected to exactly the same treatment. Three fishes, Amblyopsis, Campostoma, and Eupomotis, were killed and the heads placed in Fohl's mixture for the same duration of time. The brains were removed from the skull as soon as they were sufficiently hardened and were placed in the same bottle in order that the conditions might be alike. The three were embedded in the same block and sectioned side by side. The tissue of the tracts of the brains of Campostoma and Eupomotis differentiated very well — but the degenerate brain showed no structure. In the dissections of the head of the blind fish, I have been unable to find any indications of optic nerves leaving the lobes. In both the dissections and the sections which have been made of the entire head and brain, there seems to be no break in the enveloping membranes on the anterior ventral surface of the lobes \\here the optic nerves originate. The vestiges of the optic nerve can be followed backward from the eye for a short distance. The only tracts leading away from the lobes are those which connect them with cerebral hemispheres and cerebellum. Those which pass forward to the hemispheres are from the diagonal fibers of the fifth layer. These pass laterally, but before reaching the lateral aspect of the lobes, turn downward through the granular and epithelial layers, and then course forward toward the ventral surface of the hemispheres. CONCLUSIONS. 109 CONCLUSIONS ON THE AMBLYOPSID£. 1. Amblyopsis speldus is found from Mammoth Cave north to Michigan. It is the only blind species occurring on both sides of the Ohio. 2. No direct comparison of specimens from south and north of the Ohio has been made. 3. There are 3 species of T vphlifli 111 ys occurring in 3 different localities, one of them north of the Ohio. 4. Troglichthvs is confined to the caves of southwestern Missouri. 5. The 3 species of Chologasler are found in 3 disconnected areas. 6. The color pattern of Chologaster is controlled by the underlying musculature. 7. Amblyopsis has been permanently bleached so that even individuals reared in the light do not acquire color. Its colorless condition is due to the transmission of the environ- mental adaptation in past generations of cave-dwellers. 8. Respiration is probably in part carried on through the skin. 9. Amblyopsis is a bottom and pelagic (ubiquitous) feeder on living, moving animals. 10. Chologasler does not depend upon its eyes for detecting and securing prey, or for avoiding a rod held in the hand. 11. Amblyopsis is negatively phototactic. It seeks the dark regardless of the direction or wave length of the rays of light. 12. In well-lighted, open pools Amblyopsis hides under rocks during daylight. 13. Chologaster when deprived of its eyes is negatively phototactic, and positively stereotropic. They are positively tropic to red as against other rays of the spectrum. 14. Amblyopsis probably breeds during the entire year, but more individuals carry developing eggs between March and May. 15. Amblyopsis is not viviparous, but the eggs to the number of about 70 are carried in the gill chamber of the female from fertilization till the larvae are about 10 mm. long. The eggs hatch in about a month, having a length of about 5 mm. 16. There are few, if any, secondary sexual characters which argues in favor of the origin of these through sexual selection as against Geddes and Thompson's explanation that they are the result of maleness. 17. In newly hatched Amblyopsis the anus is in the normal position, behind the ventrals. When the fish reaches a length of 25 mm., the anus has reached a point in front of the ventrals; when 10 mm. longer, the anus has moved forward to between the bases of the pectorals. In mature specimens it lies anterior to this point. 18. The heads of the Amblyopsida; are provided with tactile ridges, rows of tactile organs regularly and definitely arranged. 19. These fishes are not keener in perceiving vibrations than other fishes. They may have greater power of discrimination between vibrations. 20. The ear of the Amblyopsis is normally developed. These fishes do not " hear" in the ordinary sense of the word. 21. The external peculiarities of the brain of Amblyopsis are the absence of optic nerve and chiasma; the hemispheres are relatively larger than in other fishes and the optic lobes are much smaller. 22. The dorsal walls of the optic lobes have only half the normal thickness, the differences being due to (a) the atrophy of the second layer; (b) the outer part of the fourth layer has disappeared; (c) the descending fibers of the fifth layer are wholly wanting; ( «^V FIG. 35. (a) Section through Lower Left Half of Iris of Ciwlogaster papilliferus, seen from in front. t, iris; c, cornea; ep, epidermis; d, dermis; sub. o., suborbital. (6) Section of Right Half of Head of Ciwlogaster papiltiferus. (O Section through Retina at Entrance of Optic Nerve. (rf) Inner Surface of Retina nearly tangential at Entrance of Optic Nerve, (ej Vertical Section of Pigment Cells of Retina, depigmented with Chromic Acid. (_/") Tangential Section through Pigment Cells. Upper part of figure passes through nucleated part of cells, middle through processes of cells, and lower through cones only. 112 BLIND VERTEBRATES AND THEIR EYES. The 6 normal eye muscles are present in Chologaster. The 4 rectus muscles arise near a common point just behind the point of exit of the optic nerve from the skull. The M. rectus superior passes from this point outward, upward, and for- ward. The M. rectus inferior passes nearly horizontally outward and forward. The M. rectus externus passes nearly straight out at right angles to the axis of the body to the posterior face of the bulb. The M. rectus internus is probably the longest, passing outward and forward to the anterior face of the eye. The two oblique muscles originate near a common point well in front of the exit of the optic nerve and are inserted near the insertion of the M. rectus superior and inferior. There is nothing remarkable about any of these muscles and they are mentioned solely as a basis of comparison with the condition found in Ambly- opsis. The space from the wall of the brain case outward about the eye muscles and eye is bounded by a connective tissue capsule. Within this capsule, the space between the muscles and the posterior part of the optic pit and the eye is filled with fat. Above this capsule lies another mass of fat and below it still another (fig. 35 b). The- supraorbital does not help to protect the eye, which lies entirely lateral from it and extends above it. The suborbital bones are thin, hollowed sheets of bone backing the suborbital mucous canal. Their number, etc., has not been deter- mined, but their location is of importance in view of a statement made by Kohl concerning their absence in Troglichthys. The sclera is represented by a thin fibrous capsule which is sometimes widely separated from the eye by reagents. In the largest specimen it is but 4 ^ thick. It is continued over the front of the eye in contact with the dermis as a thin cornea (fig. 35 a). This is much more compact than the rest of the sclera. It readily separates from the dermis. The sclera is never at any place cartilaginous. I was at some trouble to demonstrate the absence of cartilage, even in the largest specimen, in order to detect if possible the homologues of the cartilages in Ambly- opsis and Troglichthys rosa, and can state positively that no cartilage is found associated with the eye of Chologaster papilliferus or in fact with the eye of any of the species of Chologaster. The choroid is very thin. Just within the sclera is a homogeneous, sometimes excessively thin, layer containing a few nuclei, the suprachoroidal lymph space. If the eye contracts through reagents, the choroid which clings to the eyeball is separated from the sclera by the widening of this space. Pigment is not abundant except over the iris and below and at the sides of the entrance of the optic nerve. About the entrance of the optic nerve a mass of pigment is prominent, being espe- cially conspicuous in the largest specimen (fig. 35 c). A mass of pigment which may be homologous with this has been described by Ritter in Typhlogobiiis, who found no cellular structure in the pigmented mass in Typhlogobiiis and identified tliis | lamented mass as the choroid gland. A choroid gland or the rete mirabile is not found. A processus falciformis is not present. Blood-vessels are not numer- ous and it was impossible to separate a distinct vascular layer of the choroid. In the largest specimen the choroid is much richer in blood-vessels ventral of the pigmented mass at the entrance of the optic nerve than elsewhere. The capillary l;iyer reaches here a total of Q /^ in thickness. A layer of excessively thin pigment cells lies close to the pigmented layer of the retina. It is so thin and so closely applied to the pigmented layer of the retina that it is only in a few tangential sections that this part of the choroid becomes evident. EYES OF CHOLOGASTER PAPILLIFERUS. 11.'! The optic nerve enters the retina as a single strand. It spreads out in all direc- tions as soon as it has passed the pigmented part of the retina (fig. 35 c). Some of the fibers pass behind the ganglionic cells just within the entrance of the optic nerve, a condition of importance in the interpretation of the distribution of the optic nerve in the blind members of the family. The diameter of the nervous opticus at the entrance into the pigment layer is 32 /x in the largest specimen. The nerve is not spread out over the ganglionic layer, but is distributed in well-defined tracts between the nuclei. There is no nerve fiber layer proper (figs. 35 c, d). These strands of fiber not only entirely displace the ganglionic cells along their track, but also plow into the granular layer. The pigment layer of the retina is very thick, as compared with the other layers, a condition recalling that described by Ritter for Typhlogobius and usually to be found in degenerate eyes. For a comparative statement of the thickness of the various layers of the retina, see table on page 120. The pigmentary layer is half the total thickness of the retina in the smaller specimen, while in the largest it is still thicker, measuring 104 of the 168 p, of the retinal thickness. About one-eighth of the outer part of this layer usually appears as a solid mass of pigment where the margins of the cells touch. Just within this is a region where the cells are contracted, there being large, open, pigmentless spaces; at the inner- most part there is again an accumulation of granular or rod-shaped pigment granules which obscure almost everything else in the ordinary sections. (See fig. 35art of Iris of Zygoncctfs /MM/H\. EYES OF CHOLOGASTER PAPILLIFERUS. 115 cells here form a distinct row in section, while the rest for the most part are irregularly placed. The difference in size is especially noticeable near the entrance of the optic nerve. The nuclei are mostly spherical. A few nuclei are found more elongate and with their longer axis at right angles to the retina (Miiller's fiber nuclei). The largest spherical nuclei measure 5 ju. in diameter. The inner granular layer varies in thickness and contains few cells. The ganglionic layer consists of a single layer of nuclei, rather irregularly placed. The nuclei measure 6 [>. in diameter. For reasons explained in a previous para- graph a distinct nerve-fiber layer is not present. A thin nucleate membrane, the hyaloid membrane, containing the blood-vessels, lies directly on the ganglionic layer (fig- 35 c). It is quite evident from the foregoing that the retina is very much simplified as compared with that of Zygonectes. The point of greatest degeneration lies between the outer nuclear and inner reticular layers. The horizontal nuclei arc all but entirely eliminated. The bipolar cells are, in the adult, reduced to two layers of nuclei, and the spongioblasts are reduced to a single layer of cells. Even this dis- tinction and differentiation is only seen in the largest individuals. Twin cones are abundant and apparently not lacking in number and structure, but are arranged in a different manner. Rods are much fewer in number than in either Corcgonus or Zygonectes. The chief difference between the youngest and oldest specimens of papilliferus examined lies in the thickness of the pigmented layer and the outer nucleated and the outer granular layers. The relative thickness of the pigmented layer increases very much with age. The irideal region needs a few words since its structure helps to explain certain conditions in the blind fishes. The epithelial part is composed of two layers of cubical cells, of which the outer are the larger. The outer cells are normally filled with pigment to such O^ an extent that their outlines can not be made out, the 0 Q 0 g inner cells are free from pigment. The outer layer ..(TOO O passes directly over into the pigmented layer of the ^ C\ ^n retina. Where the inner layer of the iris merges into the & inner layers of the retina it is composed of a group of o, Qy ^r\ PO cells with elongated nuclei (fig. 36/, nl. /.). The uveal rg gg £ part of the iris is composed of a thin layer of cells A with irregular nuclei, and the pigment cells of this KIG. 37. \udciuf Epochal Lay,* of , . .,,.,. Lens of C/K-/ w. r. layer are much thinner than the epithelial pigment. The ligamentum ciliary does not contain many muscle fibers, but is abundantly supplied with granular nuclei. The things of greatest importance are these granu- lar nuclei, the epithelial pigment and the oval nuclei at the ora serrata. As com- pared with the same region in other fishes the shortness of the section of the iris is at once striking (fig. 36 g). The absence of ciliary muscles and the insignificance of Decemet's membrane are also notable. The lens offers no peculiarities. The shape of its epithelial nuclei may be seen in figure 37. 116 BMND VERTEBRATES AND THEIR EYES. CHOLOCASTER ACASSIZII. Only a single specimen of this species appears to have been put on record. Putnam described it from Lebanon, Tenn. The present account is based on five specimens secured by me in the river Styx in Mammoth Cave and in Cedar Sinks. The eye of Cholo^islcr agassizii Putnam is much smaller than that of C. ptipillifcnts. In a specimen 41 mm. long it is placed 2.08 mm. from tip of the snout, the eye measuring 0.72 mm. in diameter. The distance from eye to eye is 2.72 mm. It is elliptical in outline, with the lateral face depressed. It is directed outward. The optic nerve, which, at its origin, is surrounded by pigment for a distance of 2.4111111., extends almost straight inward. The dermis over the eye is essentially as in pupillifcnis. The epidermis is less simplified. It is thinner than in the surrounding tissue, but goblet cells are found in it, although they are much I1 ii.. sS. ( ~/;/>/i'i:./i/,T ago (n) Cross Mtii, m ol Part of Head. IM Vim, ..I Sri!i(tn through Retina of a Specimen 38 mm. long. ir ) Vertical Set linn through Retina of :t spr, mi. n t,2 mm. long. Rods, Cones, and Pigment Layers omitted. (it) Lower part of Iris of the Same Specimen, 02 mm. long. i Her and much less numerous than elsewhere. The sclera and choroid are as in fHipillifiriis, including a pigment mass below the exit of the optic nerve just within (lie sclera. The optic nerve measures 24 fj. at its point of entrance into the pigment layer of the retina, and it is thus one-fourth smaller in diameter than in papilliferus. The proportionate thickness of the retinal layers as compared with the layers of /; illifrnis is seen in the table. The maximum thickness in the largest specimen is but rp /i as compared with 166 p in />ii/>il/ifcnts, leaving a difference o| IMII (i ft in the other layers. The pigmented layer is, on an average, much thinner EYES OF CHOLOGASTER CORNUTUS. 117 than in papillifcnts. Yet the per cent of the total thickness of the retina in pig- ment is larger than in normal fishes. The nuclei of the pigmented epithelium are irregular in outline. The part of the pigment layer about the nuclei forms a mass of pigment in which cell boundaries can not always be made out. The pigment about the nucleus is in granules; farther in, about the cone bodies, it is in prisms. I have not been able to make out rods. The cones are irregularly elongate so that the cone bodies are at various heights. The pattern of the twin cones has, there- fore, not been made out. The outer nuclear layer consists of nuclei conical in shape, partly outside the outer limiting membrane as in papillifcnts, and a number of oval nuclei form- ing a double series within these in the younger, a single series in the older specimens. The outer reticular layer is distinct to the iris. Horizontal cells could not, with certainty, be identified. Some of the cells lie without the inner nuclear layer in the outer reticular layer and may be fulcrum cells. The inner nuclear layer is three to four series of cells deep. Mullerian nuclei are present. If artificial splitting should take place, the innermost series of nuclei separates from the outer layers ; these probably correspond to the spongioblast cells of other retinas. The inner reticular layer is well defined and contains very few cells. The ganglionic layer consists of a single series of nuclei. A distinct optic fiber layer is not present. The iris is much as in Chologaster papillifcnts, much shorter in section than in Chologastcr corniilitx. The inner cells of the retinal part are pigmented around the margins of the pupil, while in papillifcnts only the outer cells carry pigment. CHOLOGASTER CORNUTUS. The eye of Chologaster cornutus Agassiz is much larger than that of the other species of the genus. The retina on the other hand is simpler. The details of the measurements are given at the end of the account of this eye. But two specimens were available for examination ; they were preserved in alcohol and respectively 27 and 43 mm. long. The very remarkable retina deserves much fuller treatment than is possible with the limited material available. Leaving out of consideration the accessory structures of the eye as choroid, sclera, muscles, etc., which are scarcely if at all different from the same structures in papilliferus, the retinal characters may be briefly described. The pigment layer is very thick as compared with the rest of the retina, form- ing over 60 per cent of the total thickness. The pigment cells form a sheath com- mon to any pair of the twin cones. Connections between the cones and the outer nuclei could not be made out. There are apparently fewer cones than nuclei. For the relation of the cones to the underlying cells and of the latter to the nuclei of the inner nuclear layer, see figures 39 c and (/. The outer nuclear layer consists of a series of nuclei closely packed together with their longer axes vertical. Occasionally a fainter staining nucleus is found among the bases of these cells with its longer axis horizontal (figs. 39 a and 40 6). us BLIND VERTEBRATES AND THEIR EYES. The outer reticular layer is well developed. Its boundary is irregular on the side of the inner nuclear layer, but more regular on the side of the outer nuclear layer. Horizontal cells are very few and widely separated, if, indeed, this layer is repre- sented at all. A few cells horizontally placed are present on the inner face of the outer reticular layer (fig. 39 a). The inner nuclear layer is represented in the smaller specimen by two series of small rounded nuclei (fig. 39 a, 5-7). In the larger specimen a single irregular series represents this layer (figs. 40 b, c, 5-7). Besides the rounded nuclei there are a few irregular-shaped ones and other elongated ones. Some of the latter lie in the plane of this layer, others at right angles to it. The latter are probably Mul- lerian nuclei. O FIG. 39. Cliohgaster cornvtus from a Specimen 27 mm. long. (a) Entrance to Optic Nerve and Part of Retina, 2 mm. and 6. (6) Oblique Section through Pigment Layer to near Outer Nuclear Layer, 2 mm. and 4. (c) Bases of Cones and Underlying Nuclei of Outer Layer. Nuclei, in black, are in deeper focus, 2 mm. and 8. (d) Nuclei of Outer Nuclear Layer and Deeper-lying Nuclei of Inner Nuclear Layer, 2 mm. and 8. The inner reticular layer is well developed and contains a few round nuclei, as in pH/iillifcrn.s. In addition, it contains some vertically elongated nuclei at times reaching through half the thickness of the layer. These are also evidently Miil- lerian nuclei. Some of them extend from the ganglionic layer outward, others from the inner nuclear layer inward (fig. 40 b). The ganglionic layer is very imperfect, being represented by scattered nuclei embedded in the inner layer of the reticular layer. In this layer we have a decided degeneration by a reduction of the number of elements (fig. 40 a, 9). A nerve-fiber layer is not evident in cross-section. The pigmented layer has not been decreased nor have the reticular layers degenerated materially beyond Chologaster papilliferus. The nuclear layers, on llu- other hand, have been very materially affected. The outer layer has been much reduced. But this need not necessarily imply degeneration. The inner EYES OF CHOLOGASTER CORNUTUS. 119 nuclear layer has been reduced one-third and more from the lowest point in papilli- fenis. There is no longer any definite difference between the inner spongiose and outer bipolar cells of this layer, a difference that is usually well marked and is still evident in papillifcnis. An equally marked change has unquestionably occurred in the ganglionic layer where a layer of cells, continuous but for the strands of the n. opticus passing between them, has dwindled to irregularly scattered cells. ' o /' -' ^ ^ •' • •'» „ •* • * *; • • : • • * 3T)0°0 o I FIG. 40. (a) Section Tangential to Ganglionic Layer, showing Distribution of Ganglionic Nuclei, 9. On Left, 4-7, Row of Nuclei of Inner Nuclear Layer, 2 mm. and 4. (6) Section of Retina through Old Individual (47 mm, long). Pigmented Layer left Blank. All Nuclei as seen in one Focus except Vertical Miillerian Nucleus, which is from Another Section. 2 mm, and 4. (e) Fragment of Same Retina at Another Point. (d) Cells of Lens Epithelium, Surface and Tangential, 2 mm. and 4. (e) Cells and Blood Cells from Hyaloid Membrane. The position of the Miillerian fiber nuclei is also unique in this retina. The eye is in some respects more degenerate than that of Typhlichthys sub- terranens. The dioptric arrangements in this eye and the cones are better developed and the layers in general are better differentiated than in T. subterraneus, but the nuclear layers are in the latter species composed of more series of cells. A section of the iris is much longer than in either of the other species of this genus. Since the differences in the eye and retina of the species of Chologaster are largely a matter of measurements, the following tables are added : 120 BLIND \T,RTEBRATES AND THEIR EVES. Measurements of the Eyes of Cliologaster in Groups. [Treated alike.] \\-rtital diameter. Longitudinal diameter. Mcdiu-lateral diameter with the lens. Pupil. Lens. a. Eye dissected out and measured directly : M M M 7C2 ^ M r i j 688 S?2 888 816 320 }OO 7^0 800 (720 without lens) ^60 4*6 l>. Ht-ad mounted in bal.-am, tin- eyes mr.i ured from aljo\, . C. | lapilliferus Forbes, 35 mm. long C. agassizii Putnam, 41 mm. long i. Heads sectioned in paraffine: 880 720 Medio-lateral without the lens 640 (612 without lens) (688 with cornea) 486 (576 with cornea) Medio-lateral 672 48O 80 ^ c?6 738 about 520 with cornea Measurements of the Retina of Species of Chologaster. [Only averages from two to nine measurements are given in each case.] C. cornutus. C. papilliferus. C. agassizii. 27 mm. 43 mm. 29-39 mm. 55 mm. 38 mm. 62 mm. M 47 4 2 6 9 ;. M 52.5 4-5 1-5 7-5 13 4-5 M 64.5 13-5 3 18 IS 8 M IO2 r3 2 19.5 '7 9 M 48.5 14 2 2O 14.5 8 M 74 10 3 22 16 S 73 83.5 122 162.5 107 130 It is seen that the retina of agassizii differs from that of papiUiferus almost alto- gether in the decrease of the thickness of the pigment epithelium. The retina of cvrnitlits differs from that of agassizii in the reduction of the layers inside of the pigment epithelium. TYPHLICHTHYS SU BTERRANEUS. The eye of Typhlichthys sublen-nnci/s has not heretofore been made the sub- jci t <>l study. The following account is based on 3 specimens, 20, 25, and 45 mm. long respectively, from a small cave in the town of Glasgow, Kentucky, and a number <>l specimens of various sizes, the largest 54 mm., from Mammoth Cave, Ki-ntuck These were all collected by myself in the early part of September, 1897. The rye ol this species is in general less degenerate than that of Amblyopsis. The accessory structures are, on the other hand, much more degenerate than in Amblyopsis. The eye can not be seen from the surface. The region of the eye is, however, more conspicuously apparent than in Aniblvopxis on account of the thinner ti— ues of this smaller species through which the orbital fat-mass can be seen. The eye can not be seen even in heads cleared with oil on account of the almost total absence of pigment about the eye and its total absence in the eve itself. TYPHLICHTHYS SUBTERRANEUS. 1-1 The eye is surrounded by a large mass of fat through which connective tissue cells are scattered. A distinct separation of the orbital fat from the other fatty tissues in this neighborhood by connective tissue membranes such as are found in Ainblvopsis is not noticeable in this species. A few pigment cells are found scattered through the fat-mass. They are nowhere massed together so as to become evi- dent to the naked eye. In one eye not a single pigment cell is found about its surface, in another three are found on the surface of the connective tissue sur- rounding the eye. In no case is the pigment about the eye of any significance, for it is as abundantly found throughout the fatty tissue surrounding it. No trace of eye muscles are present. Scleral cartilages arc entirely absent, a condition in striking contrast to that found in Troglichthys roscc, with which this species has been confounded. Sclera and Choro'ul. - - The sclera and choroid coats are not separable in this species. In specimens up to 40 mm. in length the eye is surrounded by a very thin membrane containing here and there a nucleus, and in the region of the choroid fissure and near the exit of the optic nerve a few capillaries. In the oldest speci- men, 54 mm. long, the tissues about the eye are distinctly more fibrous, but even here I have not been able to separate the layers. From the front of the eye a strand of tissue similar to that surrounding the eye extends outward. A blood- vessel reaches the eye with the optic nerve, and a few capillaries are found on the surface of the eye and in the hyaloid membrane, but the details of their distribu- tion I have not made out. This primitive condition of the outer layers of the eye is not so striking as at first appears when the conditions in Chologaster are taken into consideration, for even in Clwlogaxtcr the choroid and sclera are insignificant. The Eyeball. ~ - The eye is on an average i .68 mm. in diameter and has reached this size when the individual has reached 25 mm. in length. In specimens of this length the cells of the retina are still undergoing division. In a specimen 20 mm. long it has a diameter of 1.42 mm. Its maximum differentiation is not reached at the time it first reaches its maximum diameter. The eye is probably potentially functional throughout life as a light-perceiving organ. A minute vitreal cavity, remnants of the hyaloid with its blood-vessels, outer and inner nuclear as well as inner, and usually also the outer reticular layers are well differentiated, and the optic nerve is certainly still connected with the brain at a time when the fish has reached a length of 40 mm. The position of the eye is not fixed, so that in different series of sections, pre- sumably cutting the head in the same planes, the choroid fissure occupies various positions and the eyes are cut in various directions. With this general sketch the various layers may be taken up in detail. Pigment Layer (i in figs. 41 a, 43 c). — No pigment granules are present in the eye, a condition in great contrast to that in either Ainb/yopsis or Chologaster, where the pigment is least affected by the degeneration processes. The absence of pigment in this eye is indeed unique among vertebrates. Whether pigment is developed in earlier stages and disappears I have not been able to determine. In the specimens 40 mm. and less in length the pigment layer consists of a series of cells, but little separated from the underlying outer nucleated layer. The sepa- ration between the layers is greatest near the exit of the nerve and at the iris. In older individuals a considerable space is formed between the pigment layer and 122 BLIND VERTEBRATES AND THEIR EYES. the outer nucleated layer on the dorsal and proximal parts of the eye, but since in all of the cases under consideration a good share of this space is attributable to reagents, a more detailed description is useless. However, in these regions delicate protoplasmic processes extend inward to the nucleated layer. The nuclei of the pigmented layer stain much more faintly than those of the rest of the retina with Biondi-Ehrlich, but just as deeply as the others with haemalum. The cells of the pigment layer are in one series, but occasionally a cell is found below the level of the rest. A few cells very elongate in section may be mentioned here. They were found (fig. 41 a) on the inner face of the pigment layer. These are important in the interpretation of the structure of the eye of Troglichthys roses, where they are also found. Their origin and significance are not known.1 FIG. 41. (a) Sagittal Section through Right Eye of Typhlichthys stibtcrraneus, 25 mm. long. (6) Mullerian Nuclei (?) from Retina of Individual 25 mm. long. (c) Horizontal Section of Eye of Individual 40 mm. long. iind Cones with their Nuclei. — While the outer nuclear layer is very well ) \ i i In .il Sr( tiun through Left Eye of Specimen 40 nini. long. Inner Layer of CelK ut Uvca shows well as a Series of Elongated Nmli-i, »/. /. Section, while passing through Ganglionic Layer, does not pass through Pupil. ol the ovum to the lips of the fused choroidal fissure, the outer follicular cells to the imcle;ir layers, and the interior cavity of the follicle to the inner reticular layer ol" the eye. O/>lif Nerve. - The optic nerve is not as distinct at its exit from the ganglionic layer as in Amblyopsis, hut in specimens even 40 mm. long there is no difficulty in tracing it to the brain. In specimens of the latter size it has a diameter of 9 p.. Il i "Mains many elongated nuclei, some of which are also seen with the optic fibers within the eye (fig. 42 b). The covering of the optic nerve partakes of the same THE EYES OF TYPHLICHTHYS. 125 indefinite nature as that of the eye itself, with which it is continuous. No pigment accompanies the nerve as a distinct layer, but here and there, as in the covering of the eye, a pigment cell may be seen, while about its entrance into the brain cavity some pigment cells are also found. Epithelial Part of the Iris. --The pigment cells, as in Amblyopsis, decrease in height toward the irideal portion of the retina, where they become a series of pave- ment cells with rounded nuclei directly continuous with a layer of cells with elon- gate elliptically nucleated cells forming the inner layer of the iris. The homologues of the elliptically nucleated cells are found in the iris of Chologaster in the region of the ora serrata. At the junction of the outer and inner layers of the iris the cells are sometimes heaped up, making the irideal margin quite thick (fig. 43 b). There is in some cases a distinct free pupil (fig. 43,) while frequently the opening is directly continuous with the choroid fissure which may remain open in this region (fig. 41 c). FIG. 43. (a) Iris of Eye shown in 42 a. (b) Section through Iris and Lens of Right Eye of Tvphlichtkys 42 mm. long. (c) Median Vertical Section of Left Eye of Same Individual. Lens. --The lens was not found in all eyes; when present it is situated at the anterior end of the choroid fissure or behind the iris. It consists of but very few cells. These cells are undifferentiated. No fibers or other signs of differentiation are at all evident. The lens cells are not distinguishable from the neighboring cells, and only the faint lines seen to surround the group serve to distinguish them. Vitreous Body and Hyaloid. - - The choroid fissure is distinctly evident in speci- mens at least 42 mm. long, not as a distinct fissure, except in front, but as a line along which the various nucleated layers of the retina are merged. In the distal part of the retina the fissure is not entirely closed, and it here leaves an opening into the vitreous cavity which is more distinct and larger in the large specimens than in the smaller ones (fig. 41 r). The vitreous cavity, when present at all, is confined to a very narrow region just behind the lens. Here a few oval nuclei and an abundant supply of blood-vessels are to be found (figs. 41 c, 43 a, b}, the latter communicating with the exterior through the open part of the choroid fissure. The vitrcal body or cavity does not extend far into the eye, and in the core of ganglionic nuclei, where the vitreal cavity does not extend, the hyaloid mem- 120 BLIND VERTEBRATES AND THEIR EYES. brane is represented by blood corpuscles and by a few cells with elongated nuclei whose longer diameters are parallel with the optic nerve. of the Eves oj Typhliehthvs sublerrancus. Length of fish. Diameter of eye, axial. Diameter of eye, vertical. Pigment layer. Nuclear layer. Reticular layer. Ganglionic layer. mm. M M M M M M 2O 142 120 24 28 22 7 25 1 60 142 .. .. 32 25 i So 1 60 40 36 20 40 i So 144 12 36 18 12 •42 1 60 1 2O 2O 28 16 8 42 1 60 142 16 32 12 28 \\i-rages 162.66 128 22.4 32 2O 13-75 TROGLICHTHYS ROS/E. In December, 1889, Carman published an account of cave animals collected by Miss Ruth Hoppin in Jasper County, Missouri. Among them were a num- ber of what were supposed to be Typhtiehthys sitbtcrraneus Girard. A compari- son of the eyes of two of the specimens collected by Miss Hoppin with the eyes of specimens of Typhlichthys subterrancus from Mammoth Cave showed that the western specimens represented a distinct species, and that Kohl must have based his account of the eye of Typlilichtliys on specimens from Missouri. In the spring of 1897, I visited the caves examined by Miss Hoppin, at Sarcoxie, Missouri, but as my stay was limited and the caves were full of water I did not succeed in getting any additional material. In September, 1898, through a grant from the Elizabeth Thompson Science Fund I was enabled to make another and this time successful effort to secure this highly interesting material. Kohl described the eyes of Typhlichthys, basing his account on two specimens respectively 36 and 38 mm. long. Dr. Mark informed me that at least one of these specimens came from Missouri, and Kohl's account was certainly drawn from Missouri specimens only. He found that the bulbus is nearly spherical, with a diameter of 0.04 mm. The orbit is a very flat cavity that offers little protection to the eye. Suborbitals are totally wanting and in their place is a cartilaginous protecting capsule, placed over the bulbus clorsally and laterally, and made up of several cartilaginous plates 0.02 mm. thick. Between the plates the connective tissue frequently contains thick and large nuclei which are sometimes united into groups. One such mass he thinks has been taken for the lens by Wyman (Putnam, fig. 5). It lies 0.195 mm. from the outer surface of the epidermis. All tissues covering the eye show absolutely no difference from neighboring parts. Eye muscles are not found, but sometimes there are stiff connective tissue strands connecting the cartilaginous bands with the tissues immediately surrounding the eye. The eye in the speci- mens examined he considers in the stage of the formation of the secondary eye vesicle. There is still a large cavity present representing the primitive eye cavity which is only being encroached upon by the invaginating outer cells, which in part are precociously ganglionic, sending each a process to the optic stalk. The optic stalk no longer shows a cavity, which he assumes became obliterated by the direct ingrowth of nerve fibrils and not in the usual way. The invagination of THE EYES OF TROGLICHTHYS. 127 the inner layer may have progressed farther in one eye than in the other, but there is always a considerable space still left between the inner and the outer layers of the primitive eye vesicle. The elements of the inner layer, the ganglionic cells, he found to send their processes directly inward. They must have gradually re- volved, since in the normal eye the nerve processes are directed outward. Some of the fibers cross each other on their way to the outlet for the nerve. Not all of the invaginated cells send processes. Among those that do there are smaller, round cells without a trace of fibers. From these the rest of the nervous parts of the retina, including of course other ganglionic cells, would probably have arisen. The outer layer of the secondary eye vesicle is also single-layered. The cells are elongate, with oval nuclei, and without a definite arrangement. They are con- nected with the few cells of the optic stalk that still remain. Connective tissue cells are found in the nervus opticus. They are probably mechanically active in de- generation by separating the elements. He found no sheath to the optic nerve, as described by Wyman. The lens he found to be a spherical cell heap o.oi mm. in diameter in the distal pole of the eye. It lies just within the sclera and the cup of invagination. The sclera is made up of several layers of very fine fibrilke. Nuclei are not found in it, but nuclei are found on its outer surface. No vessels are found in the choroid, which consists of connective tissue cells more numerous on the dorsal than on the ventral surface. The Typhlichthys eye is "absolut pigmentlos." The surrounding tissues are rich in pigment, which, however, is not related to the eye. There are pigment masses found here and there, but especially between the bulb and the cartilaginous capsule. It is hard to arrive at the proper explanation of the structure of this highly degenerate eye even with an abundance of material, and it is probably not to be wondered at that Kohl in the work outlined above did not see the eye muscles, mistook the sclera for suborbitals, parts of the retina for the choroid, interpreted the pigmented epithelium of the eye as an extra optic pigment mass, mistook the inner reticular layer for the primary optic cavity, the nuclear layers for the pig- ment epithelium, etc., and arrived at a thoroughly erroneous idea of the general structure of the eye and based his theories on the degeneration of eyes in general on his conception of the structure of this eye. The invaginating cells of the primary optic vesicle are supposed to have been directly converted into the gang- lionic cells, which are usually among the very last products of the histogenesis of the retina.1 By supposing that the eye was arrested at the beginning of the invagination, and that the invaginating cells rotated on their axes and were converted directly into ganglionic cells, Kohl derived the nucleated layers from the outer pigment- producing layer of the primary vesicle, at the same time ruling the pigment layer out of the eye. The eye is very small and situated so deep that it is impossible to see it from the surface (fig. 44 a). In the upper half of a head cleared in xylol it is just evi- dent to the naked eye as a minute black dot (figs. 44 b, c}. As in Typhlichthys and in Amblyopsis, it is surrounded by a fat-mass filling the orbit. It is not at all 1 The mistakes of Kohl, especially as far as they are the result of criticising work done on Amblyopsis while he was working on another species, seem to me to point a moral. A certain species must not be too readily taken as an exponent of a family, order, or class, and a knowledge of related species and geographical distribution is not altogether to be neglected. 12S BLIND VERTEBRATES AND THEIR EYES. uniform in shape in different individuals or even the two sides of the same indi- vidual. It can be located and seen in cleared heads solely on account of the pig- ment which is always abundant over the distal face of the eye. It is located so far beneath the surface as to occasionally lie in contact with the brain case nearly opposite the posterior end of the olfactory lobe. It has thus been withdrawn much farther than in the other blind species. It is very much smaller than the eye of either T. snbterraneus or Amblyopsis. Its size is, however, quite variable, measuring 40, 49, 56, 64, 54 by 96, 56 by 120 p. in different instances, exclusive of choroid and sclcra. bruin eye I n:. j.j. (.j) Cross section of Part of Head of Trvgliflllliys, 25 mm. long, showing Position and lions of Eye. M Hr.nl .,( / ri:flii:hth\s from above, showing Relative Positions of Tactile Organs and Eyes. Partol Same Head, showing Eyes with thi-ir Peculiar Pigmentation and Distribution of Pigment Cells in Surrounding Tissues. Tlie muscles of the eye were in no case normal. I have not found more than two rectus or more than one oblique muscle belonging to any one eye. They can ' be made out from horizontal sections. In cross-sections it is very difficult to identify or follow them. The besl developed rectus was found in a specimen 35 mm. long. It is com- poseil of a number of normal fibers forming a bundle 20/1 in thickness, and from it- origin to its insertion it is 2^6 ju, long. The remarkable peculiarity of this muscle is that i oo /A of this is a tendon 4 /LI in thick ness (fig. 4(1 h,msc.r.}. The tendon spreads into a cone -Imped mass of fibers attached to the proximal face of the eye. Traces nl" two mu-( les were made out connected with the ricjlit eve of another individual. THE EYES OF TROGLICHTHYS. 129 The oblique muscle is attached by a tendon to the face of the eye opposite that of the attachment of the rectus (fig. 46 a, msc.). In the best-developed condi- tion it was found to be but g p. in diameter, taking its origin at a point on the level of the lower surface of the olfactory nerve where the latter pierces the ethmoid and 160 p laterad from it. The muscle itself is in this instance about 200 fj. in length and is attached to the eye by a tendon of equal length. The rectus in the same individual is 208 /u. long. In all the cases enumerated above the muscles of the opposite side were not nearly so well developed. In the one with the well-developed rectus the oblique was indistinct, while in the one with the well-developed oblique the rectus is also well developed, but the striations are not distinct. The scleral cartilages form one of the striking features of this eye. They are quite variable, forming a more or less complete covering for the eye. In some they are several times as long as the eye and in such cases extend much beyond the eye. In one eye 49 fj. in diameter the length of one of the cartilages reaches 160 p. (fig. 45 a). They have not kept pace in their reduction with the reduction of the '/. C. FIG. 45. (a and 6) Two Cross-sections of Eye of Specimen preserved in Alcohol. 38 mm. long. Sec- lions show Variable Extent of Pigment, Choroidal (ch.) Pigment, and Scleral Cartilages. Extent of latter represented by dotted lines in figure fl. eye in size. As a consequence individual cartilages either extend beyond the eye or are bent at acute angles in their endeavor to apply themselves to the shrunken eye (fig. 46 a, sd.c.}. These cartilages were mistaken for the suborbital bones by Kohl. There is absolutely no ground for this supposition. The suborbitals are present (fig. 44 a, siibo.} and widely separated from these cartilages. Further, the eye muscles are attached to the cartilages and to similar ones in Amblyopsis. The presence of these large cartilages is the more remarkable when we con- sider that none are found in Typhlichthys siibterraneus, and in the species of Chologaster, which in other respects resemble Typhlichthys in all but the develop- ment of the eye and the color. It is quite evident that Troglickthys and Typh- lichthys are not derived from a common ancestor (except, of course, remotely). Their present superficial resemblances are the result of converging development under similar environments. A species similar to Chologaster agassizii gave rise to Typhlichthys siibterraneus. What the ancestry is of Amblyopsis and of Tro- glichthys is not known. The cartilages are bound together by an abundant fibrous connective tissue containing a few corpuscles. (These I have found nowhere as abundantly as represented by Kohl.) 130 BLIND VERTEBRATES AND THEIR EYES. The choroid, in so far as this layer can be distinguished from the sclera, consists of a dense layer of fibers closely applied to the eye. Over the distal surface it is split into two layers between which there are a greater or smaller number of pig- ment masses (fig. 45 b, ch.). These would prove effective to prevent the performance of the natural function of the eye were it functional. Pigment cells are much more sparingly found in other parts of the choroid. Blood-vessels are very few in number, a condition to be expected in such a minute organ. This layer was mistaken for the sclera by Kohl. The eye proper of Amblyopsis differs very greatly in different individuals, but in general it maintains a certain degree of development from which the many individual variations radiate. The eye of Troglichthys roses, has similarly a general type of structure which is maintained, but with many variations. This type is more degenerate than that of either Amblyopsis or Typhlichthys subterraneus. 3-7 FIG. 46. Two Horizontal Sections through Eye, showing Extent of Scleral (jd.) Cartilages and Tendons of Oblique (a, msc.) and Rectus Muscles (b, msc.r). Fig. a represents section just above Fig. 6, from an individual 34 mm. long. Drawn under magnification of 560 diameters. The eye of Troglichlhys has been derived from an eye like that of Amblyopsis by the disappearance of pigment from the posterior part of the retina and the reduc- tion of the central mass of ganglionic cells to the vanishing point. In the most highly developed eye of T. rosce (9, fig. 47) I found but three of these cells. Both in size and in structure the eye of T. roscz is the most rudimentary of vertebrate eyes so far known, except that of Ipnops which is said to have vanished. The vitreous cavity and the hyaloid membrane have vanished. The eye has collapsed, the margins of the iris have probably fused, and the pigmented and inner layers of the iris separated from each other. With this general sketch the elements of the eye may be taken up in detail. The pigment layer is variously developed (i in figs. 46 and 47) and may be quite different on the two sides of the head. One peculiarity is practically always present and very striking. The layer forms a covering over the distal face of the eye where, a priori, there ought to be no pigment, and is thinnest or absent over the proximal face THE EYES OF TROGLICHTHYS. 131 where it ought to be most highly developed. Kohl has cut the Gordian knot by excluding this pigment from the eye entirely by the choroid (sclera), but there is certainly no such membrane intervening between this pigment and the rest of the eye as Kohl has figured. On the contrary the choroid very clearly surrounds it, and from its own epithelial structure there is no room for doubt as to its nature. As said, its extension over the sides and back part of the eye differs materially in different eyes. In a number of instances no pigment cells are present either on the sides or at the proximal surface; in others the sides are well covered. If by any means the tissues of the eye are separated from each other, the space is always formed between the pigmented layer and the rest of the eye. Processes are at such times seen to extend down from the pigment cells toward the rest of the retina. The cell boundaries and nuclei of the pigment cells are for the most part distinct. The cells are deepest over the distal pole of the eye and from this point they decrease in size to the proximal pole. Toward the upper face, where the pigment epithelium approaches the lens, the densely pigmented cells are transformed into much thinner •«<•/. c FIG. 47. Horizontal Section through only Eye with Central Ganglionic Cells. From an Individual 34 mm. long. pigmentless cells. These are probably the homologues of the pigmentless cells over the distal face of the eye of Amblyopsis, and, if so, are all that is left of the outer layer of the iris. The explanation of the condition of the pigment epithelium in this eye presents more difficulties than any other structure. In the eye of T. snbterraneus no pig- ment is developed, but the pigment epithelium is normally developed. In this eye pigment is formed in the cells that are present, but the epithelium has any- thing but a normal structure. The pigment cells in the proximal face of the eye have either disappeared or been displaced. The only other alternative, that they are present but without pigment and indistinguishable from the cells of the outer nuclear layer, while possible, is scarcely probable, for in many eyes there is but a single layer of cells representing all of these structures, and in other cases even these have vanished. The objection to the idea that the cells have vanished is to be found in the fact that they are so well developed over the distal face. This point can only be settled by a study of the development of the eye, but one other sug- gestion may not be out of place. A comparison of this eye with that of Amblyopsis 132 BLIND VERTEBRATES AND THEIR EYES. will suggest the homology of the anterior cell mass in the latter case, with the pig- ment cells always present between the retina and the irideal pigment layer in the former species. This correspondence is further strengthened by the fact that frequently the pigment in T. rosce over the front of the eye is in more than one layer of cells. Since, however, I was unable to arrive at an entirely satis- factory explanation of the origin of this pigment mass in Amblyopsis, it will not help us much, should the two structures be homologous. Attention may be called here to the fact that both in Amblyopsis and in the pres- ent species the lens — and therefore the lost pupil — are not situated at the distal pole of the eye, but above this point, and that both in regard to the pupil and the eye in general the location of the pigment masses in the two species is the same. The pigment is granular, not prismatic. «•/. e. FIG. 48. Cross-sections through Right and Left Eye of an Individual 25 mm. long. Sections be pass th Fig. a is a Composite from 3 Sections. Fig. b represents one Section, but the '' Lens " is from the Nex Sections be pass through Lens. :t Section. The lens is the only structure of the eye concerning which Kohl has not made any mistake.1 It is a small group of cells closely crowded together and about 10 or 12 fj- in diameter (figs. 48 a' and b, I). There are no signs of fibrilation or the result of any other histogenic process; it appears as an aggregation of indifferent cells. On its surface there are at times cells that are evidently of an epithelial nature, being flattened so that their sections appear much longer than deep. It lies at the upper outer face of the eye at the margin of the pigment mass described in the last section. It is not covered by pigment or other retinal substance. kohl ( onsidered this condition a primary one. The lens, however, does not lie in an incipient secondary optic cavity, the vitreal cavity, as Kohl supposed, but in the remains of such a structure. Under the circumstances it is doubtful whether the uncovered condition is primary. It seems more probable, considering the condition in Amhlyopsis, that the lens was inclosed by the closing of the pupil over (he eye, and that the present naked condition is the result of the subsequent degeneration of the iris over it. That the latter is the phylogenetic origin of its present condition there is no doubt. '<• history of the lens in Amblyopsis, I am not sure now whether Kohl was or was not mistaken about these *• -- - -r . , • A to G, Photographs of the eyes of Amblyopsis ; H, eye of Troglichthys. A. Horizontal section of right eye of fish 9.5 mm. long. B. Dorsal face of horizontal section of left eye of fish 25 mm. long. Optic nerve directed forward and inward. C. Cross-section of left eye of fish I 00 mm. long. D. Anterior face of transverse section of left eye of fish 123 mm. long. E. Transverse section of left eye of fish 1 30 mm. long. No definite structures are distinguishable aside from scleral cartilage. F. Transverse section of right eye of fish from which E was taken. G. Cross-section of right eye of fish 1 05 mm. long, showing large vesicle formed by pigment epithelium and remainder of retina as small nodule on its distal face. H. Eye of Troglichthys rosae showing large scleral cartilages and different layers of the eye. THE EYES OF TROGLICHTHYS. 133 The Retina: The elements of the retina proper, i.e. the ganglionic, nuclear, and reticular layers, form a vesicle arranged so that the cellular elements surround a central (the inner) reticular layer. These may be taken up seriatim. The cellular elements are of three sorts. (1) Behind the lens and behind the pigment layer, sometimes also over the side of the retina, lie a few cells with elongated nuclei (•»/,/, in figs. 45 a, b, 46 b, 49 a) and so arranged as to suggest an epithelial covering for the underlying structures. Some of these cells were supposed by Kohl to represent the choroid, with which they have absolutely no connection. It is possible that some of these lateral cells are modified pigment cells, but even this seems doubtful. I am unable to refer the cells of this nature situated laterally over the retina to any structure in the normal retina. Such cells are, however, found in the eyes of T. subterraneus between the pigment epithelium and the nuclear layers (fig. 41 a), and whatever their origin the two structures are unquestionably homologous in the two eyes. It is probable that the cells with elongated nuclei to be found behind the lens are of different origin and significance. They may be the remains of the elongated cells found in the inner surface of the iris of Clwlogaster, cells which are still present in both Amblyopsis and T. subterraneus. It is also possible that they are the remains of the hyaloid nuclei. (2) The ganglionic cells, which in Typhlichthys are arranged around the vestige of the vitreal cavity and in Amblyopsis form a central core and are distributed over the front of the retina, are in this species practically confined to the latter location. All there is left of the central core of ganglionic cells in Troglichlhvs roses is three cells in the most highly developed eye found (fig. 47 and plate 10, fig. n). In the other eyes no indication of these cells was detected. If these cells come to be formed at "all in the present eye, they migrate forward, where they form the anterior wall of cells surrounding the inner reticular layer. The fibers of the ganglionic cells extend directly from the ganglionic cells through the reticular layer to the exit of the optic nerve. The cells must, as Kohl has suggested, have undergone a rotation on their axes to send their fibers directly to the optic nerve, unless only the lineal descendants of those ganglionic cells immediately surrounding the entrance of the optic nerve in Chologastcr are here represented, a supposition not to be entertained. The ganglionic nuclei are occasionally notably larger than the nuclei of the rest of the retina, but they are by no means always so. (3) The cells of the nuclear layers join those of the ganglionic layer. The cells of the inner and outer nuclear layer and the horizontal cells are indistinguish- able from each other. They form, in the most highly developed condition, figure 47, 3-7, a layer three cells deep covering the sides and the proximal surface of the inner reticular layer. In some cases the layer is reduced to a single series of cells, and even these are occasionally absent. There is no sharp distinction between the nuclei of this layer and those of the ganglionic layer, so that the boundary between these cells and the ganglionic cells is not marked. In some instances these cells appear to be directly continuous with the cells surrounding the origin in the optic nerve. This condition led Kohl to imagine that the primary optic stalk had become filled with nerve fibrils. Of the reticular layers the outer (8, in fig. 47) is not developed. The inner reticular layer forms, with the optic fibers traversing it, the spherical or pear shaped 134 BLIND VERTEBRATES AND THEIR EYES. central mass of the retina. No cells are developed in the reticular layer. The optic fibers appear to pass directly through the reticular layer. This condition is probably apparent rather than real. First the vitreous cavity disappeared, bring- ing the ganglionic cells and the optic fiber layers together in the center of the eye. This condition has just been reached by T. subterraneus and Amblyopsis. In the present species the ganglionic cells have disappeared from the center, and only the optic fiber layer remains. This is represented by the individual fibers passing from the ganglionic cells to the exit of the optic nerve. They do not form a compact nerve, but the fibers pass individually to the exit in the most direct route from their respective cells. I have been unable to trace the optic nerve for any distance beyond the eye. In one case it leaves the eye as a loose bundle 12 ;u in diameter; in another case it is more compact, being but 4 ^ in diameter. It is surrounded by a sheath of varying thickness and complexity. In one case there are a few cells about the nerve, and these are covered by the tendon of the rectus muscle, which forms a complete covering. Measurements in /*: The scleral cartilages vary from 18 to 40 in thickness. The distance from the distal face of the retinal pigment to the ganglionic cells varies from 30 to 40. The pigment cells have a maximum depth of 14, dwindling from this to 2 or 3 on the sides. The nuclear layers reach a maximum thickness of but 10. The inner reticular layer, including the optic fiber layers, is about 40 in all directions, reaching a proximo-distal length of 70. The lens measures from 10 to 15. AMBLYOPSIS SPEL^EUS. The eyes of Amblyopsis have been described by Tellkampf, Wyman, and Put- nam. These authors gave general accounts of the eyes as far as this could be done without serial sections, and their accounts are far from satisfactory. It is therefore unfortunate that Kohl, who had less material of a supposed Typhlichthys from Missouri, should have based a criticism of the facts observed by Wyman in Amblyop- sis on what he saw, especially since scarcely a statement made by Kohl corresponds to a condition found in Amblyopsis, or even the Typhlichthys subterraneus from Mammoth Cave. An abstract of Kohl's result are given under Troglichthys. Tellkampf first pointed out the presence of rudimentary eyes and states that these can be seen in some specimens as black spots under the skin by means of a powerful lens. The statement that the eyes are externally visible in some speci- mens, which was afterwards thrown in doubt by Kohl, is perfectly correct. The ej (.• of .1 mblyopsis can be seen as a black spot with the unaided eye in specimens up to 50 mm. in length. Wyman, in Putnam, figured the optic nerve, a lens, and muscular bands attached to tlu- exterior of the globe, but did not recognize them as homologues of the muscles of the normal eyes of fishes. In a four-inch fish Wyman found the eye to be one- sixteenth of an inch in its long diameter. A nerve filament was traced to the cranial wall, but farther it could not be followed. The eye is made up of (i) a thin mem- brane,the sclera ; (2) a layer of pigment cells, the choroid, which were most abundant about the anlerior part of the eye; (3) a single layer of colorless cells larger than the pigment cells, the retina; (4) just in front of the globe, a lenticular-shaped transparent body, the lens; (5) the whole surrounded by loose areolar tissue. THE EYES OF AMBLYOPSIS. 135 Wyman was mistaken in his identification of Nos. 2 and 4, and part of 3. Of this species I have had an unlimited supply of fresh material from the Shawnee Caves in Lawrence County, Indiana. I shall first give the histology of the eyes of fishes from 25 mm. long to their maximum size, 135 mm. The details of the development of the eye will follow. In well-fed adult specimens of Ambiyopsis there is no external indication of an eye. In poor individuals the large amount of fat surrounding the eye and collected in a ball-shaped mass becomes apparent through the translucent skin. In young specimens, before they have reached a length of 50 mm., the eyes are perfectly evident from the surface. By this I do not mean that they are conspicuous, for the minute eyes would not be conspicuous were they situated just beneath the skin. The skin is not modified in the region over the eyes, but has the same structure it possesses in the neighboring regions. This condition is in strong contrast to the conditions described for Chologaster papillifcrus. The position of the eye can be determined from the surface in older individuals by certain tactile ridges, being I.e. It FIG. 4Q. (a) Section of Right Half of Head of Clwlogaster, through Eye. (b) Section of Right Half of Head of Ambiyopsis, through Eye. between a long longitudinal ridge (supraorbital) situated caudad of the posterior nares and two vertical (suborbital) ridges. They can also be approximately located by the mucous canals, being situated above the middle of the suborbital canal forward from the fork of the suborbital and rostral canals. The exact location in relation to these ridges differs, however, to some extent in different specimens. The skull is surprisingly little modified, there being deep orbital notches, large enough to accommodate a large eye. The maintenance of this skull structure long after the eye has dwindled is significant in the consideration of the causes of degeneration and will be referred to again. The change in the relation of the eye to surrounding tissues as well as the relative size can best be gathered from the accompanying figures or cross-sections of Cholo- gaster and of Ambiyopsis, drawn with the same magnification, but from different sized individuals (figs. 49 a, b). Beneath the dermis (black in the figures) a thick layer of connective tissue has developed in Ambiyopsis, The large fibrous capsule occupied by the eye, eye 136 BLIND VERTEBRATES AND THEIR EYES. muscles, and orbital fat in Chologaster has in Amblyopsis become largely filled with fat. There is no indication of fatty degeneration; it is simply the accumulation of fatty cells in the eye cavity. The eye is very small and lies on the floor of the optic capsule. The infraorbital and supraorbital fat-masses described for Cholo- gaster papillifcrits arc also large in Amblyopsis and form especially large masses in front and behind the optic capsule. In Chologaster the brain extends forward beyond the front of the eye, while in Amblyopsis the brain does not extend as far forward, the anterior portion of the brain cavity being filled with fat. Attention may also be called here to the presence and position of the suborbital bones which Kohl says are represented in Troglichthys by the cartilaginous masses forming a hood over the front of the eye. These cartilages (scl., fig. 49) are present in front of the . 1 ////>/ vo/>.s/.v eye, and it can readily be seen that they have nothing to do with the suborbital bones (sub. o). The adult eye of Amblyopsis with its appurtenances may now be taken up seriatim. The eye occupies the lower part of the eye cavity. It is surrounded by loose connective tissue, which is so associated with the eye that if contractions occur through reagents, as frequently happens, a space is left between the eye with its connective tissue and the septum forming the lower floor of the eye cavity. Above the eye with its connective tissue is the large accumulation of fat mentioned previously. From the eye to the inner wall of the orbit extends a continuation of the connective tissue surrounding the eye. In this continuation of the connective tissue the optic nerve and eye muscles extend. In the longest individual, 135 mm. long, the eyes were 5 mm. from the surface of the epidermis. The shape of the eye together with the pigment variously scattered in the con- nective tissue associated with it is very variable, differing from subspherical in the smaller individuals to long spindle-shaped in the old. Considerable difference is found in the shape of the eye itself. See table of measurements, page 144. Pigment is found in very variable quantity and variously scattered in the con- nective tissue surrounding the eye. The amount of this pigment seems to vary inversely with the amount of pigment in the eye itself and to increase with age. As Wyman has stated and figured, eye muscles are present in Amblyopsis, but, contrary to his statement, they are the homologues of the normal eye muscles. Not all preparations are equally good for tracing the muscles. They are best demonstrated in heads treated entire by Golgi's method and sectioned in celloidin. While the muscles have been noted in a variety of preparations the description will In' drawn from those treated by Golgi's silver method and stained at times with h;emalum or Biondi-Ehrlich's 3-color stain.1 Tn one individual the upper rectus and upper oblique muscles are inserted i her on the upper median surface of the eye, or more exactly on the upper posterior angle of the upper scleral cartilage. The lower oblique is inserted opposite this place From these places the oblique muscles extend inward and forward. Mi' origin ol the lower oblique is 0.72 mm. in front of its insertion, while the larger upper oblique- extends a little farther forward, being inserted 0.85 mm. behind its 1 1 takes its origin in the projecting angle of a cartilage above and in advance ol the origin of the lower oblique. In the inner part of the orbit a small muscle \-\n- the drsirrd re-suits for nervous sir in lures, but l>y staining with the above ethods the material was found excellent for general purposes. THE EYES OF AMBLYOPSIS. 137 extends from the inferior oblique horizontally backward, taking its origin with the rectus muscles. This muscle in its posterior extent has the characteristics of the inner rectus. But whether or not its fibers reach the eye, I was unable to determine. If they do, they reach it with the fibers of the lower oblique. The rectus muscles arise from the lateral margin of the bone forming the brain case, just behind the anterior end of the brain, the upper rectus taking its origin behind the others. They extend as four bundles forward in a connective tissue tube. Before leaving this tube they are reduced to three bundles by the union of a small bundle situated above the others in the tube with the largest bundle situated nearest the outer margin. One of these is the lower rectus. The largest one is the upper rectus and the one joining it, in all probability, the external rectus. The external rectus, if I am correct in the identification, is not distinguishable from the latter during the rest of its course nor in its insertion in the sclera. The entrance of the rectus muscles into the connective tissue sheath occurs 0.5 mm. behind their insertion in the eye. In this eye we have the two oblique muscles, the upper rectus, the lower rectus, a small bundle of fibers following for the most part the course of the upper rectus, the external rectus, and a small bundle of fibers extending from the origin of the rectus muscles forward to the lower oblique which may be the inner rectus. We have at least five, probably all six, of the muscles normal to fish eyes. But that this is not always the case is very strikingly emphasized by the fact that the eye of the opposite side of the same individual lacks the upper oblique. In another individual the superior rectus and superior oblique are the only muscles present on the left, while on the right the upper rectus is the only muscle present. The preparations of this individual are particularly favorable for tracing the muscles. They are stained with Mayer's haemalum and indigo carmine. The muscles are stained an intense blue, while the connective tissue through which they pass is light purple. In still another specimen both the oblique muscles are present on the left and three of the rectus muscles, one of which, the interior, extends forward in the inner part of the orbit and joins the lower oblique as in the first individual described. No fibers of this muscle reach the eye. On the right side of the same individual the upper rectus and but one oblique muscle are present. In still other individuals not suitable for tracing the muscles, their fluctuating number has been noted, and their varying method of attachment to the eye is also a matter frequently noticed. Inside of the loose connective tissue surrounding the eyes there is a more compact sheath. This is thickest in front of the eye where it contains spherical nuclei and holds one to three compact cartilages which usually are disposed to form a hood over the front of the eye. These cartilages, described by Wyman in this species and by Kohl in rosce, and taken by the latter as the remnants of suborbital bones, have nothing whatever to do with the latter structures. Their nature can be under- stood from their close association with the eye, by the fact that they are closely bound together by the scleral connective tissue, and by the fact that some, at least, of the eye muscles are attached to their outer surfaces. They are unquestionably scleral elements (scl.c. in figs. 49 to 52). There may be some hesitation in accepting this view of the nature of these cartilages since no cartilage whatever is found in the sclera of Chologaster. Their position, usually in front of the eye, is also anomalous if they are scleral cartilages. It may be stated, however, that the eye of Amblyopsis 138 BLIND VERTEBRATES AND THEIR EYES. is not simply a miniature normal eye. The whole eye has collapsed with the dis- appearance of the vitreous humor, and looked at in this light there is no difficulty in the position of the cartilages which have fallen together over the front of the eye. The presence of granular nuclei in front of the eye over the region of the iris has been noted by Kohl in Troglichthys and observed by me. These nuclei are probably the homologues of the nuclei found in the ligamentum pectinatum of Chologaster. In shape, number, and size the scleral cartilages differ very much. In one instance cartilages extend continuously from the exit of the optic nerve more than half-way over the side and around the front of the eye. In another a single cartilage lies directly in front of the eye, and on the opposite side of the same individual a single cartilage lies behind the eye. The sclera is much more developed than in Chologaster, consisting, aside from the cartilages, of an abundant fibrous tissue. .«•/. r. Fie. so. Section through the Eye of Amblyupsis spfl&us 75 mm. long, killed with Chromic Acirl and stained uilh liiondi-Ehrlich's three-color mixture. This is the most highly developed eye seen, 2 mm. and 4. The choroid is a thin membrane closely applied to the eye. It contains a few oval nuclei parallel with the surface of the eye. Pigment cells are few, irregularly scattered, and not at all uniform in different eyes. The pigment cells are rounded masses usually much thicker than the whole of the choroid in regions devoid of pigment. About the entrance of the optic nerve is frequently a large accumulation of pigment corresponding with the increase in the amount of choroidal pigment in Chologaster at the same place. Even this mass is not uniformly present. Some- times granular masses interspersed with pigment are found here, which give one the impression of a degenerating mass. An especially large accumulation of pig- ment is lound in the eye represented by figure 53. Blood-vessels are present in the choroid. They arc apparently as great in relative capacity as in Chologaster. In an individual with the vascular system injected, a vessel, o.oi mm. in diameter, approaches the eye with the optic nerve, but it does not enter the ball with the latter. THE EYES OF AMBLYOPSIS. 139 It breaks up into smaller vessels distributed in the choroid. A vessel is usually found in a groove of the pigment layer of the retina. This groove extends along the dorsal wall of the eye — otherwise it might be taken for the choroid fissure (fig. 50, cps.). A somewhat larger vessel than at other points is found near the iris, where this structure appears to be continuous with a deep choroidal groove (fig. 51 a, cps.). In the young a blood-vessel enters the hyaloid cavity at this point. chr. chr. ~ sd. c. nil. The eye itself, exclusive of choroid and sclera, differs greatly both in size and inner structure although the general ar- rangement of the retinal cells remains the same in all cases. In some cases the pig- mrnt layer of the retina forms a large mem- branous bag many times too large for the inclosed structures which lie as a small ball in this comparatively vast cavity. In such eyes found in old individuals the wall in many places is free from pigment. In general the pigmentation of the retina varies inversely as the pigmentation of the choroid. In other individuals the eye forms a compact mass of cells (fig. 53). To anticipate somewhat, the vitreous cavity with the hyaloid membrane and its blood-vessels have entirely disappeared, the ganglionic cells have in large part been brought together into a solid mass, and the irideal opening has usually become closed. Pigmentcd Layer and Cones. - - The pigment cells as they appear in the best preparations of the better-developed eyes may be described first (fig. 50). The cells are longest near the entrance of the optic nerve. They possess an outer seg- ment, not determinable in all cells, which is free from pigment. They have a homo- FlG. 51. From Amblyopsis 05 mm. long killed in Picric Acid and stained with a Mixture of Ha?malum and Indigo Carmine. Figures made with Bausch and Lomb ^a Immersion and 4 Eyepiece. (a) Section of Right Eye. Choroida! Groove with one of Scleral Cartilages in front of Eye. Nuclear Layers thinner than usual. Densely Pigmented Segments of Pigment Cells form a Conspicuous Layer just below Pigment Nuclei. (6) Next Section after 51 a, showing Group of Elongate Uveal Cells. 140 BLIND VERTEBRATES AND THEIR EYES. geneous, vesicular, ellipsoidal nucleus situated near the outer end of the cell. This nucleus is strikingly different in shape and constitution from the same structure in Chologaster. It stains but faintly and then homogeneously. Just within the nucleus there is a well-defined mass of dense pigment forming a cap over the inner side of the nucleus and at times encroaching on the rotundity of its inner outline. This pigment mass evidently has its counterpart in Chologaster where a solid band of pigment is found just within the nucleus. In depigmented cells this pigment cap is seen as a deeper-staining, more dense protoplasm than the rest of the cell. From this pigment segment a prolongation, much poorer in pigment and containing a central uniformly staining core, extends toward the interior of the eye. This core, which in reality extends also into the pigmented section, occupies the position of the cones in Chologaster. In no case have I been able to trace any connection between these bodies and the outer nuclear layer. They are sometimes in several esgments or in a number of spherical bodies, and occasionally two are seen side by side in the same cell in tangential section. In position they certainly suggest cones, i-hr FIG. 52. Section near Posterior Face of Left Eye of Small Individual, showing particularly Position of one of Scleral Cartilages behind Eye and Thick Choroid filled with more or less Angular Mass ot ( Ir.inular Pigment. This Eye shows one of the Largest Accumulations of Pigment noticed. and this suggestion is heightened by the presence in the inner end of some of the cells of a vesicular structure very similar to the nucleus, but frequently with an ;i uvular indentation on the surface. These occupy the relative position of the cone I K )i lies, they are by no means found in all eyes. The evidence seems to point most strongly in favor of the supposition that they are cones. One of the cells measures as follows: diameter of cell, 0.007 mm. ; nucleus, 0.007 by 0.007 mm. ; deeply pig- mented mass, 0.007 mm. ; total length of cell, 0.036 mm. No rods have been found. In many individuals, and strikingly so in two specimens 25 mm. (fig. 53, am.) and 35 mm. long (fig. 54 b) respectively, deeply staining spherical bodies, much smaller than the nucleus and staining much deeper, are present in the pigment cells. Those stained with rui-malum are quite dark and give the appearance of a large centro- some. These 1 take to be myeloid bodies noted in the pigment cells of the frog and other forms. In most individuals the high development of the pigmented region, above described, is not found. In some individuals the pigmented layer is composed of Hat pavement cells, forming a large vesicle (plate 10, figs. D and G). In others the THE EYES OF AMBLYOPSIS. 141 n. op. pigment is either entirely absent or very sparingly developed. As mentioned above, the pigmentation of the eye seems to vary inversely with the pigmentation of the surrounding structures. The pigment is in all cases granular and differs in this respect from the pris- matic pigment of the eye of Chologaster. Iris. - - The pigment cells decrease in height toward the irideal part of the eye, where they are replaced by a layer of pigmentless cells forming a thin membrane (fig. 50). The nuclei of these cells stain darker than the bodies of the cells, which is the reverse of the conditions seen in the pigmented cells. In individuals up to 35 mm. long similar cells ex- tend along the line of the vanishing choroid fissure (figs. 54, a and /). The pigmentless membrane is ap- parently the relic of the outer pig- mented layer of the iris. If so it has undergone greater changes than the rest of the pigmented layer, for it is well pigmented in all the species of Chologaster. The inner layer of the iris is fre- quently entirely separated from the outer layer and not infrequently is entirely obliterated. (A few rounded pigment masses are always found within the eye at this point.) In other individuals a minute opening is still present and the outer layer of the iris is continuous with the inner, which contains some of the elongate nucleated Cells found in the region Of Fie. 53. Horizontal Section through Right Eye of Specimen, 25 mm. long from above. A Large Branch of Optic Nerve is seen to pass the Ora SCrrata in CholOgaSter. These ^ f-ro?' °1 Cone.of GangBomic CeUa. This is not Constant, and are much more regularly present in Typhlichthys subterraneus. These nuclei are variously grouped in different eyes, as is represented by the figures 50, 51 b, 54 b, d, e. The exact significance of the various structures about this region in the eye can not always be determined owing to their presence or absence in different individuals and their great variability when they are present. In this region are sometimes a few cells with elongate nuclei that can not be identified with any of the structures considered. These may represent all that is left of the hya- loid. Blood-vessels are usually not found in the eye of the adult. Between this pigmentless membrane and the rest of the retinal structures, i.e. within the pigment epithelium, there is in the majority of the adult eyes an irregular mass of pigmented cells. I am entirely at a loss to account for this mass unless with the shrinking of the eye as the result of the loss of the vitreous body and lens and the consequent closing of the pupil, the margin of the iris is rolled inward and some of the pigmented cells of the outer layer of the iris come to lie within the eye after the closing of the pupil. The iris is seen to be rolled in the way imagined scl. in front of Cone of Ganglionic Cells. , in Left Eve of Same Individual the Largest Strand passes behind Number of Ganglionic Cells lying in front of Inner Reticular Layer and the Central Ganglionic Mass. 142 BLIND VERTEBRATES AND THEIR EYES. in many sections of Chologaster and the method of the closing of the pupil in Typh- lonwlge is as I have suggested. The Nuclear Layers. — Within the pigment and cone layer lies a nuclear layer made up of about four series of cells (3 to 7 in figs. 50, 54 e). The nuclei reach from 2.5 to 3.5 p. in diameter. Rarely I have succeeded in staining the smaller nuclei different from the larger. They are, in such cases, more refringent, the large nuclei being granular. The larger nuclei may be the spongioblasts. In a young indi- vidual this difference was well marked. Here the smaller cells were confined to the proximal part of the eye (fig. 53). A separation of the nuclear layer into an Fie. 54. (a) (») (c,d) CO From an Individual 35 mm. long killed in Perenyi's Fluid and stained with Mayer's Hsemalum. Outer Nuclear Layer in Center, Choroidal infolding on Left. Lower Part of figure passes through Choroidal Fissure Area Elongated Inner Nuclei i.s well shown. Passes near Center of Eye. Choroidal Fissure Epithelium seen below and Irregular Mass of Section through Elongated Irideal «/. /.). Passes throufih Optic Nerve and Pupil of Same Eye as fig. e. Figs, a to d are from I.rtt Kyr, c and / from Right Eye. All under Lenses s mm. and 4. inner and outer with an intervening outer reticular layer I have noticed but once. In this eye a slight separating space was found on one side, and here there were one or two cells that may be fulcrum cells. If so, it is the only indication of this layer in all the preparations made. The suppression, partial or total, of the sepa- ration into an outer and inner layer, has also been noted by Ritter in Typhlogobius. The Inner Reticular I.|, mi, UT of 1 ll. . II, I,T of Piemen t Layer Nuclear (Iranular Cian^lionic 1 1 \ \ i , 1 1 Eye, \iTiu.il. Posterior. Anterior. Layer. Layer. Layer. mm. 25 1 60 1 60 28 ? 16 24 12 60 6D ' 1 1 96 5" 108 1 08 142 j6 28 56 20 12 4 16 12 12 2 2 24 4 8 12 1 08 204 2OO 108 ' (-' 16 5<> 4 8 12 IO 2 5 2 4 1 08 ? 84 52 4 I.} X 120 \vrr.i 83 39 °I '3 SUMMARY OF AMBLYOPSID/E. 145 SUMMARY OF THE EYES OF THE AMBLYOPSID£. 1. There are at least 8 species of "blind fishes," Amblyopsida?, inhabiting North America; 3 with well-developed eyes and 5 with mere vestiges. 2. The 5 species with vestigial eyes are descended from 3 generically distinct ancestors with well-developed eyes. 3. The genera can be more readily distinguished by the structure of their eyes than by any other characteristic. 4. The most highly developed eye is much smaller and simpler than the eye of normal-eyed fishes. 5. The structure of their eyes may be represented by the following key to the genera and species of Chologaster: a. Vitreous body and lens normal, the eye functional. No scleral cartilages. Eye permanently connected with the brain by the optic nerve. Eye mus- cles normal. No optic-fiber layer. Minimum diameter of the eye 700 /J,. Chologaster b. Eye in adult more than i mm. in longitudinal diameter. Lens over o.:; mm. in diameter. Retina very simple, its maximum thickness 83.5 H in the old ; the outer and inner nuclear layers consisting of a single series of cells each ; the ganglionic layer of isolated cells. Maximum thickness of the outer nuclear layer 5 n ; of the inner layer 8 p cornutiis bb. Eye in adult less than i mm. in longitudinal diameter. Lens less than 0.4 mm. Outer nuclear layer composed of at least 3 layers of cells; the inner nuclear layer of at least 3 layers of cells, the former at least 10 fJ. thick, the latter at least 18 /*. c. Pigment epithelium 65 fj. thick in the middle-aged, 102 in the old . pa pill i ferns cc. Pigment 49 /j, thick in the middle-aged, 74 in the old ; 24-30 per cent thinner than in papillifcnis. Eye smaller . . . agassizii aa. The eye a vestige, not functional; vitreous body and lens mere vestiges ; the eye collapsed, the inner faces of the retina in contact; maximum diame- ter of eye about 200 p. d. No scleral cartilages; no pigment in the pigment epithelium ; a minute vitreal cavity; hyaloid membrane with blood- vessels. Pupil not closed. Outer nuclear, outer reticu- lar, inner nuclear, inner reticular, ganglionic, and pig- ment epithelial layers differentiated. Cones probably none. No eye muscles. Maximum diameter of eye 180 /u. Eye probably connected with brain throughout life . . Tvplilii'hthvs dil. Scleral cartilages; pigment in the pigment epithelium; vitreal cavity obliterated ; no hyaloid membrane. Pupil closed. Some of the eye muscles developed. No outer reticular layer. Outer and inner nuclear layers merged into one. Eye in adult not connected with the brain. e. Pigment epithelium well developed; cones well developed; ganglionic cells forming a funnel-shaped mass through the center of the eye. Pigment epithelium over the front of the eye without pigment. Maximum diameter of eye about 200 fj. . ... .-1 nil>lyopsi<; ee. Pigment epithelium developed on distal face of the eye, rarely over the sides and back. No cones. Nuclear layers mere vestiges; the ganglionic layer restricted to the anterior face of the eye just within the pigmented epithelium. Maximum diameter of eye about 85 //. . Trngliclitlivs 146 BLIND VERTEBRATES AND THEIR EYES. 6. The steps in degeneration are seen in figure 66, page 176. 7. The structure of the vestigial eyes differs much in different individuals. 8. The eye of Chologaster is an eye symmetrically reduced from a larger, normal fish eye. 9. The retina in Chologaster is the first structure that was simplified. 10. Later the lens, and especially the vitreous body, degenerated more rapidly than the retina. 11. The eye of Typhlichthys has degenerated along a different line from that of Awblvopsis, its pigmented epithelium having been most profoundly affected. 12. The eye muscles have disappeared in Typhlichthys. 13. Troglichthys shows that the steps in the degeneration of the muscles were in the direction of lengthening their attaching tendons, finally replacing the muscles with strands of connective fibers. 14. The scleral cartilages have not kept pace in their degeneration with the active structures of the eye. 15. The lens in the blind species, if present, is, for the most part, a small group of cells without fibers; in Amblyopsis it disappears early. 16. The proportional degeneration of the layers of the retina is shown in figure 67, page 179. 17. With advancing age the eye of Amblyopsis undergoes a distinct ontogenetic degeneration from the mature structure. 18. The phyletic degeneration does not follow the reverse order of development. None of the adult degenerate eyes resemble stages of past (phyletic) adult condi- tions. 19. The degenerate eyes do not owe their structure to a cessation of develop- ment at any past ontogenetic stage, i.e., at any stage passed through in developing a normal eye. 20. Cessation in development occurs in the reduction of the number of cell generations produced to form the eye and in histogenesis, not in cessation of mor- phogenic processes. 21. In some cases (Typhlichthys) there is a retardation in the rate of develop- ment, the permanent condition being reached later in life than is usual in fishes. (It is possible that the pigment of the pigment epithelium never comes to develop at all. It is, however, impossible to assert this until the embryos of this species are examined. It is possible that the pigment degenerates before the stages that I have rx;i mined are reached.) DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 147 DEVELOPMENT AND LATER HISTORY OF THE EYE OF AMBLYOPSIS. The present chapter describes the developmental stages of the eye of the blind fish Amblyopsis speheus and gives the history of the eye during growth, maturity, and old age. Questions of special interest in the history of this very degenerate organ are: 1. Do the rudiments of the eye appear as early as usual or later? 2. How much does the eye grow from the time of its appearance? 3. When does each part of the eye reach its maximum (a) in size, (5) in mor- phogenic development, (c) in histogenic development ? 4. When does the eye as a whole reach its maximum development? 5. Are there evidences of a slowing down of the rate of the developmental processes: (a) cell division, (b) cell arrangement, (c) cell differentiation? 6. Are there evidences of a cutting off of late developmental stages, that is, are there any parts of the normal eye that are not developed? 7. Does the eye develop directly toward the condition of the adult or does it follow palingenetic paths and then retrograde to the condition found in the adult ? 8. What parts of the eye degen- erate first ? y. What is the comparative rate of the ontogenetic degenera- tive modifications of the vari- ous parts of the eye, and how does their rate compare with the rate of phylogenetic de- generation implied by the structure of the adult eye of Amblyopsis and the dif- ferent stages of degeneration reached by other members of the family? 10. Is there any evidence for or against the dictum of Sedg- wick that structures which have disappeared from the adult organization are re- tained in the embryo only if the organ was of use to the larva after it the adult ? w Flo. 56 (a}' Outline of Head of Embrvo between 1.3 and 1.5 mm. long. (6) Outline of Brain and Optic Thickening in Mounted Embryo 1.6 mm. long, with 4 Protovertebrae (2.30 p.m.. M.i> 0. Outline of Brain and Optic Thickening in Living Embryo 1.92 mm. long with 10 Protovertebra (12 p.m.. May 5). Outline of Brain and Optic Vesicle of Living Embryo 2.4 mm. long with 10 Protovertebra? (12 p.m., May 5). had ceased to be of use to EARLIEST STAGES TO A LENGTH OF THREE MILLIMETERS. The development of the eye has been followed in several series of living embryos and in sections of these embryos. The earlier stages of the eye as they were observed in the series obtained on May 4, 1901, will be described.1 Where advisable other series will be described also. The first indications of the eye are seen in living specimens when the embryo is about 1.5 mm. long, at about the time of the forma- tion of the first protovertebra. This size was reached in the present series in 2.5 to 1 For an account of the general development of this series see p. 95. I IN BLIND VERTEBRATES AND THEIR EYES. 3 days from fertilization. The degree of development when the eye begins to form is exactly as in fishes with normal eyes. At ii a. m., May 5, 1901, the head was slightly raised so that its outlines appeared definite and clear, while the remaining outlines of the embryo were hazy. It was not possible at that time to distinguish eyes (fig. 560). At 2'' 30'" p. m., when the embryo has reached a length of 1.6 mm., the eyes form prominent lobes on either side of the brain. The lobes are distinguishable in living embryos, but stand out much more prominently in embryos mounted entire. In an embryo prepared in this way, a camera outline of which is reproduced (fig. 56 6), the eye protuberance (IK .) has a length of So /A and projects 36 /u beyond the lateral margin of the brain. Sections of embryos at this stage of development show the brain to be still joined with the ectoderm. There is no indication of any cavity in the central nervous system at this time and the eye lobes arc solid, symmetrical, lateral protuberances with their anterior margins but 48 ^ from the tip of the brain. At 6 p. m. the embryo had reached a length of 1.76 mm. and 6 protovertebrae had been formed. The eye was no longer a symmetrical swelling on the side of the brain, but its outer, posterior angle was now distinctly farther back than the pos- terior inner angle. In other words, the lobes had grown laterad and were bent backward. The lateral projection of the eye beyond the contour of the brain amounts to 48 p. and has a longitudinal extent of 100 p. (fig. 56 c]. The greatest diameter — measured from the anterior inner angle of the eye to the posterior outer — was 116 p.. Sections show the nervous system, including the eye, to be still a solid mass of cells, which anteriorly is still continuous with the ectoderm. Histologically there is no differ- ence between the cells com- posing the optic lobes and those composing the brain. There is a slight indication in the arrangement between the two optic lobes suggesting a lateral traction of the cells. At 9 p. m. the characters of the eye shown at 6 p. m. had be- come intensified without other material change. The embryo had reached a length of 1.92 mm. and 10 protovertebrae had been formed. The optic lobe was still broadly united with the brain, but its lateral growth was largely represented in the lobe extending back. There was no cavity as yet in the nervous system. A little later the canal of the central nervous system made its appearance, for at 12 p. m. it was well formed. There was probably some Iliicluation as to lln- rate of growth in length and the degree of differentiation Fir.. 57- 'I""1 "I Hi-iin .,,„) i i^ij, \Vsj,l,. ,,f l.ivini; T-'.mbryo l>i-l\vi-i-n si/cs - •/ a nil =;7 li (s.so ;t.ni., May 6). i and Optii Vcsich "I Living Embryo 2.4mm. long, with i. ProtovertCDra (x a.m., May M. i"M through l. and Optic Vesicle of Embryo 1.68 mm. long, with 5 Protovei tcbra imm I .ivm,.- S|.T, iiitcn. DEVELOPMENT OF THE EYE OF AMBLYOPSIS. I I'.l the tissues reach, for, in embryos of another series, some individuals had a well- developed canal, while others of the same size did not. At 12 p. m. the embryos had reached a length of 2.4 mm. (fig. 56 (/). At 5'' 30™ a. m., May 6, the eyes had become a pair of flaps lying along the sides of the brain or diverging from near its anterior end and connected only in front by the contracted optic stalk (fig. 57 a). The split in the optic lobe which separates it into an outer and an inner layer had developed to such an extent that it could readily be made out in living embryos. At 8 a. m. some of the embryos were still only 2.4 mm. long and 12 to 13 protovertebrae had been formed (fig. 57 &). The changes in the eye from 12 p. m., May 5, to 12 noon, May 6, were not very great, and consisted chiefly in the constriction of the optic stalk and the consequent gradual separation of the optic lobe from the brain. The skin had not yet begun to thicken to form the lens (figs. S7c,d). The changes from noon till 6 p. m., May 6, when the last embryo of this series was preserved, consisted largely in the shifting of the optic vesicles as the result of the development of the olfactory pits. Seventeen protovertebra? had de- veloped and the embryo was about 3 mm. long. \ FIG. 58. Horizontal Sections through Optic St;i!k (fig. a) and Optic Vesicle (fiy. '») of Embryo of Second Series. For later stages I am compelled to draw on another series of embryos which I also observed through the earlier stages described above. They were taken from a female that was captured March n, 1898, and that contained eggs in the early stages of gastrulation. The eyes had reached a stage seen at about 2.5 to 3 days from the beginning of development. An outline of the development may be given to connect this series with that just described. The rate of development was considerably slower than in the preceding series. Figure 57 e (March 13, 10 a. m.) was taken from a living specimen, showing 5 protovertebrae. Sections demonstrated that at the stage repre- sented by figure 57 e the neural tube was still a solid structure. The distance from edge of eye to edge of eye measured 164 /*. About a day later the larva? were 2 mm. long. The neural canal had been formed and extended out into the now well-formed vesicle through a distinct optic stalk. Sections showed that the epidermis was still unmodified over the eye, with no indication of a thickening to form the lens. 150 BLIND VERTEBRATES AND THEIR EYES. Figures 58 a and b show horizontal sections through the base of the optic stalk and through the middle of the optic vesicle respectively. The embryo is 2 mm. long and in about the same stage of development as those 2.8 mm. long of first series. During the next 24 hours the embryo grew to a length of 2.4 mm. At this stage the tail was free for 0.4 mm. of its length. Embryos 24 hours older than the last were found to be 2.5 to 2.8 mm. in length. The latter, while not longer than the oldest embryos of the first series described, are evidently farther along in the development of the eyes. In all of these specimens (figs. 59 a, r) the eyes have become greatly modified. The secondary optic vesicle has been formed by the thick- ening of the skin to form the lens. The retinal wall of the vesicle is three series of cells deep, while the wall destined to form the pigment epithelium has become He. 50. (a) Horizontal Section of Head of Embryo 2.5 mm. long, two Sides at Different Levels. (6) Left Eye of Same Embryo as that from which lig. 5g a was taken, showing First Indication of Lens. (c) Transverse Section through Dorsal Part of Optic Stalk of Embryo 2.7 mm. long. (d) Optic Vesicle and beginning of Lens in another Specimen 2.7 mm. long. (c) Transverse Section of Optic Vesicle and beginning of Eye of a Cymatogaslcr larva, 1.5 mm. long. thin and is composed of a single series of cells. The eye, at this stage, does not differ materially from that of a Cym/it/i^istcr1 larva about half as long. (Com- pare figs. 59 r, <7.) There is no indication of a differentiation of an iris. The secondary cup is a -shaped structure, the depression being entirely filled by the thicken- ing of the skin which is giving rise to the lens (figs. 59 b and' J). FOUR-MILLIMETER STAGES. In specimens 4.4 mm. long the eye had become a deeper cup than it was during The lens, which no longer fills the entire cavity, has become with l.irnc an. I «<:-]|-ili-vdopcrl (.-yes. Figures 60 a, b (Cvmatogasler) should lie DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 151 a spherical mass of cells, solid in some cases (fig. 60 mm. long the cells giving rise to the oblique muscles and those lor at least _> of the recti can be distinguished. Scleral cartilages are not yet formed . "I I he specimens sectioned there was no indication of a lens. In others it »vas well developed. Cell division was still going on in the retina. The optic vesicle was very shallow. The rim of the vesicle was wide and still continuous ,vith the choroid fissure, which showed as a shallow groove along the ventral surface. The choroid fissure, instead of leading into a central secondary DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 153 optic cavity, led to the mass of ganglionic cells (fig. 62 c). This condition of the choroid fissure and its relation to the interior of the eye leads me at this point to say a few words concerning the general structure of the eye. In the description of the eye of the adult I considered that the central ganglionic mass was the result of the collapsing of the eye with the disappearance of the vitreous body and cavity. I was justified in this conclusion by the process of degeneration going on in the eye of Typhlomolge, Typhlichthys, and Typhlogobius. Whatever may have been the -e'Uufig. - n.oft. FIG. 62. (a) Anterior Face of Transverse Section of Left Eye of L.irva 5 nun. long. Sections run obliquely in such a way that Right Eye is cut first, series beginning in front. Divergence from Spherical Outline is due to Pressure of Brain on Proximal Face and Epidermis on Distal Face. (/i) Anterior Face of Transverse Section of Left Eye of Larva 6 mm. long. No Lens in connection with this Retina. (c) Parasagittal Section of Eye of Larva 6 mm. long, showing Ventrally Choroid Fissure represented by space between Pigmented Layers and Vitreous Cavity represented by Shallow Depression on Ventral Face. Retina differentiated into (Janglionic. Inner Reticular, and Nuclear Layers. ((/) Anterior Face of Transverse Section through Right Eye of Larva, 7.^ mm. long. (e) Horizontal Section through Middle of Eye of Larva, 7 mm. long, showing Choroid Groove. phylogenetic process in Amblyopsis, it is evident that ontogenetically the mass of cells does not arise as imagined. It appears from the embryos that the condition of the adults arises more as the result of a contracting of the retinal area without a cor- responding decrease in the size of the eye as a whole than as the result of the col- lapsing of a vesicle followed by the coalescence of the walls brought together by the collapse. Sagittal sections of the eye (fig. 62 c) show the lips of the choroid fissure drawn apart with the contraction of the retina, only the dorsal two thirds of the eye reaching full development. From a study of the embryos of this size the point 154 BLIND VERTEBRATES AND THEIR EYES. of exit of the optic nerve which marks the proximal end of the choroid slit alone gives evidence that potentially, at least, we have to do with an eye from which a central cavity has disappeared, i. e., in which it does not develop. The optic nerve is well developed, arising apparently from the ventral cells of the ganglionic mass, that is, those immediately lining the potential optic cavity. The pigment cells are well developed and have a varying depth in different parts of the eye. They are low and without pigment over the front of the eye and the ventral surface near the choroid slit. The retinal layers proper are differentiated into the ganglionic layer or mass which occupies the central and lower part of the interior of the eye. Apparently only the more centrally placed cells of this mass give rise to fibers. The inner reticular layer surrounds the ganglionic mass above and partly on the side, not at all below. The nuclear layers are well developed, without a differentiation into outer and inner layers or any indication of an outer reticular layer. The latter structure is apparently never formed at all. SEVEN-MILLIMETER STAGES. The variability in the rate of development of the eye is well seen in a series of specimens about 7 mm. long and whose eyes are little if any beyond the stage of development reached in other specimens only 5 mm. long taken from another female. In the former the eye is in contact with the dura proximally, but is withdrawn from the epidermis by 36 /u or more. A strand of cells extends from the eye upward and outward to the thinnest part of the epidermis. The epidermis is distinctly thinner over the eye than in neighboring regions. The eyeball is subspherical, with a shallow groove along its ventral surface representing the choroid slit (fig. 62 e). In half of the specimens of this size examined no lens could be detected. In one the lens was a comparatively large pear-shaped structure whose cells were undergoing degeneration, if the numerous dark granules in them were indicative of degeneration. In one individual in which no lens could be found on one side, that of the other side was probably represented by a small group of cells lying between the eye and the skin (Ins. 63 c). The cells were breaking apart and the outline of the structure as a whole was irregular. In all cases the lens lies out- side the iris, and in fact the entire vitreous space is not large enough to hold the in such eyes as still show this structure. The pigment layer is pigmented over the dorsal part of the eye. In vertical no pigment appears below the entrance of the optic nerve. The iridian part of the layer is, as usual, without pigment. The ganglionic cells, as in the last stages described, are exposed to the exterior through the choroid fissure, or vhere ihis is not evident there is no differentiation into different layers along the line of the choroid fissure. The ganglionic cells placed at the distal face of the eye give off fillers to the optic nerve. Fibers have not been definitely traced to the cells ol (lie sime series occupying the proximal or middle position. The optic nerve reaehes a thickness of 20 /A and breaks up into bundles a short distance within the These bundles radiate, forming an incomplete funnel-shaped structure. The incomplete inner relicular layer only partially separates the ganglionic and the DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 155 nuclear layers. The relative development of the pigment layer and the inner reticular layer both show a less degree of differentiation than the same layers in the eyes of another series of larvae only 6 mm. long. This is due to the individual variation in the rate of development, not to degeneration since the last stage. Dividing cells are found in the nucleated layer. In the nuclear layers some nuclei elongated in a vertical direction are probably the nuclei of the Miillerian fibers. FIG. 63. (a) Horizontal Section 10 /x Dorsal to that given in fig. 60 f, and showing Iris and Vitreous Cavity. (6) Outline of Lens of Same Eye as that shown in Jigs. 60 e and 61 a but at a Level Dorsal of tig. 61 a, (c) Region between Eye and Epidermis of Larva 7.5 mm. long, showing Degenerating Lens. (rf) Lens of Larva about 7 mm. long. (el Vertical Section near Center of Right Eye of Fish 9.5 mm. long. (/) Anterior Face of Transverse Section through Eye of Fish 9.5 mm. long. (g) Horizontal Section through Left Eye of Fish 0.5 mm. long. NINE TO TEN-MILLIMETER STAGES. In larvae 9 to 10 mm. long the eyes lie from 60 to 100 ^ removed from the epi- dermis and in contact with the brain capsule or but little separated from it. Their average measurements are: longitudinal diameter, 114 /LI; antero-posterior, 98 /^ ; vertical, 106 /x (figs. 63 c, 64 b). The epidermis over the eye has assumed the thickness found over neighboring regions, and from now on till death by old age there are no external modifications to indicate the former position of the cornea. The pupil is still open, and also the choroid fissure in the region of the pupil (figs. 63 e, g). In the proximal portions the choroid fissure is indicated by the absence of pigment along the ventral line (fig. 63 /). The vitreous cavity is a 156 BLIND VERTEBRATES AND THEIR EYES. shallow depression in the distal face of the eye with a very narrow slit, sometimes a line, separating the iris from the solid mass of cells representing the retina. The vitreous cavity formed by the ventral invagination, that is, proximal of the iris, is obliterated in some individuals except in so far as the absence of pigment along a median line and in the union of the ganglionic layer with the pigmented layer along this line indicates its presence. The choroid fissure has been noted in an individual over TOO mm. long, so that evidently in some cases it may not close. Blood-vessels are still present in the vitreous cavity as far as it is developed. The distance from the exit of the optic nerve to the ventral margin of the pupil is considerably less than the distance between the exit of the optic nerve and the dorsal margin of the pupil. A few nuclei, probably the remnants of the hyaloid membrane, lie over the distal face of the retina. In 10 specimens sectioned, all of them from 9.5 to 10 mm. long, the lens has disappeared without leaving any trace. The pigmented layer increases in thickness from the iris to the exit of the optic nerve. Its pigmentation also increases from the iris to the optic nerve. Within any one cell the pigment is uniformly distributed. In the dorsal part of the eye the pigment reaches to the iris, while in the ventral it does not reach so far, and in fact in a line from the optic nerve to the iris very few (only about 3) cells are pigmented. The maximum thickness of this layer is 12 p. The inner cells of the iris have taken on their elongate shape which distin- guishes them in the eye of the adult, where the region of the iris and pupil can not otherwise be distinguished. The layers of the retina are now well developed except that the ganglionic mass of cells occupying the center of the eye is continuous with the outer nuclear and the pigmented layers along the ventral line. The outer and inner nuclear layers are represented by about 4 rows of nuclei immediately within the pigmented layer. The cells represented by these nuclei are not separable into an outer and an inner layer histologically, nor is there any break indicating the presence of any outer reticular layer. The cells form a compact layer of approximately uniform thickness. There are no indications of cones in any of the eyes 'examined. The inner reticular layer is well developed except along the region of the cho- roid fissure, where, as has been said above, the nucleated layers of the retina meet. There is possibly one exception to this in one of the eyes, in which the reticular layer surrounded the optic nerve at its entrance to the eye (fig. 63 f). The space ventral to the central axis of the eye is occupied by the mass of ganglionic cells. This mass is irregularly trumpet-shaped, with the narrow end of the trumpet at the entrance of the optic nerve and the wide end at the distal • of the retina, where its cells are continuous with those of the nuclear layers. In the distal face of the trumpet, in what would be its hollow end, there is a dis- linit conical area free from cells and abundantly supplied with fibers (fig. 63 #)• It is possible that this represents the optic-fiber layer. The optic nerve is well developed, but its libers seem to go to their respective cells directly without first going to this apparent optic fiber layer. The outer nuclear layers measure about o fj., the inner reticular about 8 p., and the ganglionic layer about 32 /* in thickness. The changes taking place between 10 and 25 mm. are insignificant. DEVELOPMENT OF THE EYE OF AMBLYOPSIS 157 THE EYE OF THE ADULT. The eyes of adult individuals from 25 to 75 mm. long were fully described in a previous chapter, and the eyes of very old individuals were mentioned briefly. The most highly developed eye found was that of an individual 75 mm. long. This eye is much above the average in the development of its pigmented layer, etc. Perhaps 25 mm. represents the stage at which the eye as a whole reaches its maximum development. GROWTH OF THE EYE FROM TIME OF ITS APPEARANCE. The question of the rate and amount of growth of the eye from the time it appears can best be answered by the following table of measurements of the eyes of successive sizes of embryos. Attention should be called to the great varia- bility of the size of the eye in any one stage or in successive stages of development. It is seen from this table that the eye reaches the full vertical and longitudinal diameter of the adult when the embryo is only 2 mm. in length. Since the eye does not make its appearance till the embryo has reached a length of 1.5 mm. and the lens does not begin to develop until i mm. has been added to the length attained by the embryo after the eye has reached its full size, that is, not until it has reached a length of 2.5 mm., it is apparent that from the beginning the eye is in longitudinal and vertical diameter equal to the full adult eye. Table of Measurements of the Eye from the Time of First Appearance to Maturity. [All measurements are given in micra, except lengths of embryos, which are in millimeters.] Condition nf embryo; living, or if preserved, direction of the sections. Length of embryos. Number of embryos measured. Longitu- dinal diameter. Vertical diameter. Axial diame- ter from cor- nea to optic nerve. Diameter. Diameter of optic nerve. Alive I 6 80 -6 \live I 76 48 Alive Alive 2 C IOO -> 8 Alive Alive t 7* 16 tn jS Sagittal Transverse . . . f) 6 136 88 IOO Horizontal 6 136 So and icS Mounted entire 6 z, to 7 1 60 ifto Transverse 126 QO i<> In 36 or none i<; Horizontal 18 to 50 or in inc.- 17 Sagittal n to O ^ 108 88 icS (JO n Horizontal . . . uS 12 Sagittal I ""O 112 Transverse 2 1 08 IOQ 12 Mounted entire I 3O Horizontal 128 1 6O 1 60 Horizontal . 144 60 torcS "5 '39 -• * The following gives the individual measurements of the eyes of the seven specimens whose average is here noted : No. i. No. 2. No. 3. No. 4. No. 5. No. 6. No. 7. LongiluJinal diameter . . . Vertical diameter 176 144 1 60 128 136 112 172 1 60 1 60 144 i no 128 128 128 48 16 1.-.S BLIND VERTEBRATES AND THEIR EYES. THE HISTORY OF THE LENS. The lens begins to develop when the embryo is about 2.5 mm. long (fig. 59 b). It forms as a thickening of the skin where the optic vesicle is in contact with it. It is still connected with the skin when the embryo has reached a length of 4.5 mm. (Compare figs. 59 b, 59 c, 60 d, 60 e, 63 c with figs. 60 a, 60 b, the latter repre- senting the development of a normal lens.) The history of the lens after this stage is somewhat uncertain. It is well established that the cells composing it never lose their embryonic condition, that they are never differentiated into fibers. In n.anv eyes, certainly in all in which a lens could be detected in later stages, the k-ns becomes separated from the skin (fig. 60 c). The separation is completed when the larva has reached a length of 5 mm. (fig. 62 a). From this stage on, the lens begins to be resorbed ; in some 6-mm. larvae it could no longer be found (fig. 62 b). In 7-mm. larvae exactly half the eyes were without a lens (figs. 63 b. c, d), and in 9 to lo-mm. larva? no trace of a lens could be detected. The his- tory of the lens is completed. Judging from this rapid and universal disappear- ance of the lens in the young I am inclined to the opinion that the structure described in the adult eye as a lens is not a lens. The lens is the first organ to stop developing, the first to begin to degenerate, and the first to disappear. THE HISTORY OF THE SCLERAL CARTILAGES. Attention was called to the variation of the scleral cartilages. A study of the development of the cartilages has enabled me to detect perhaps a greater degree of uniformity of plan, if not of structure, in these carti- lages than I was able to make out from a study of the adult alone. It would seem that there are normally two cartilaginous bars of variable shape developed. One or both of them may be replaced by two or more smaller cartilages. One of the cartilages is found over the distal i oi saSe¥fsh°as that face of the eye and the other on the posterior face caudad from whith lig. 63 g w;is , , . _. .. ... . taktn. o| the optic nerve. I he earliest stages at which carti- (b) Sili-ral Cartilage of Left ° • ish of lages were noticed were 9.5 to 10 mm. (figs. 63 g, 64 a, b) long. In one fish 10 mm. long there were in the right eye about jo cartilage cells, all directly over the pupil and iris. In the left eye there were about 22 cells, all over the dorsal part of the iris, none of them in Iront of the pupil. There were no traces in these eyes of scleral cartilages elsewhere. The cartilage cells were still for the most part isolated, not bound together into a definite cartilage. In another fish 10 mm. long the cells were definitely bound together into a small cartilage in each eye, that of one side encroaching on the pupil, that of the other side not. In a fish 25 mm. long there were two cartilaginous masses in each eye. One of these was over the distal face of the eye, the other over the caudal face of the rye caiidad of the exit of the optic nerve (plate 10, fig. B). The one over the distal face curved ventro caudad. In a fish 30 mm. long the cartilages were confined to the caudal half of the eye and were developed in such proportions that they encroached on the eye. DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 159 The development of these cartilages to such unexpected size indicates that these cartilages are self-determining and not conditioned by the stimulus to growth by the eye with which they are in contact. In the right eye of this fish there were two cartilages in close contact with each other over the distal face. A third car- tilage lay on the dorsal surface of the proximal part of the eye. The larger one of the two distal cartilages measures 63 by 32 by 65 p., with a maximum diameter of the eye of 12 /x. In a fish 33 mm. long there were no cartilages on the proximal faces of the eye. In the right eye there was a cartilage 128 p, long by 40 p. thick, curved along the ventral part of the distal face. In the left eye there were two much smaller cartilages on the distal face of the eye. In a fish 35 mm. long there were two cartilages in the left eye placed as in the fish 25 mm. long, but they were larger. In the right eye the distal cartilage was represented by two cartilages in contact with each other. From the above it is seen that the distal cartilage arises first (10 mm. stage), the proximal ones not till much later (25 to 30 mm. stage). The cartilages do not reach their maximum size till later.1 The distal cartilage in older fishes is frequently nodular and lies in front of the eye, where it was taken to be the lens by one of the earliest observers. In a specimen go mm. in length a globular cartilage 62 p. in diameter lay just over the pupil of the eye, which had a total diameter of 84 p.. One or the other car- tilage not infrequently encroached on the general outline of the eye. In the left eye of an individual 105 mm. long there were no traces of a scleral cartilage; the right eye was not examined. In the right eye of an individual 108 mm. long there was a single large cartilage, 134 p, by 208 p., lying at one side of the center of the distal face of the eye. In the right eye of an individual 123 mm. long a minute cartilage was found on the proximal face of the eye. It was not determined whether one occurred over the distal face. In the left eye of the same fish a large cartilage lay over the distal face (plate 10, fig. D). In the left eye of the largest fish a single large cartilage 64 p, by 96 p. in sec- tion occupies the region to one side of the distal face (plate 10, fig. D). In the right eye (plate 10, fig. F) the distal cartilage measured 48 p. by 160 p- in section, and two smaller proximal ones were also present, one of them 24 p. by 32 p- in section. The scleral cartilages are the last structure to appear in the development of the eye ; they grow during the greater part of life and retain their structure to the end. THE HISTORY OF THE OPTIC NERVE. The details of the formation of the optic nerve have not been followed. No indications of it were seen in the eyes of the embryos 4.4 mm. long. In the eyes of embryos 5 mm. long it is well developed, forming a solid strand of fibers 12 p. in diameter which is readily traceable to the brain. The optic nerve increases but little, if any, after its formation. Its development is rapid. In subsequent stages it is not always traceable from all the cells forming the ganglionic mass. In the 6-mm. larvae its fibers wrere distinctly traceable from the cells nearest the choroid fissure, while in later stages they were more distinctly traceable from the 1 In the original the words " and there is no evidence of degeneration in them even in the oldest fish " completed the sentence. This is not strictly true and is omitted. 160 BLIND VERTEBRATES AND THEIR EYES. distal cells of the ganglionic group. The optic nerve can be followed to the brain in all the larval stages and in the young fish up to 25 mm. in length (plate 10, fig. B). The optic nerve is evident within the eye in older stages up to about 100 mm.; in the very oldest ones it could not be found. In individuals much more than 25 mm. long it was not possible to follow the nerve to the brain, though it could usually be followed for some distance from the eye. The fibers are never meclullated, and so far I have not been able to give them a differential stain. HISTORY OF THE DEVELOPMENT, MATURITY, AND DEGENERATION OF THE EYE. The history of the eye may be divided into four periods: The first period extends from the appearance of the eye till the embryo reaches 4.5 mm. in length. This period is characterized by a normal palingenetic devel- opment except that cell division is retarded and there is very little growth. The second period extends from the first till the fish is 10 mm. long. It is characterized by the direct development of the eye from the normal embryonic stage reached in the first period to the highest stage reached by the Amblyopsis eye; its latter half is further characterized by the entire obliteration of the lens. The third period extends from the second period to the beginning of senescent degeneration, from a length of 10 mm. to about 80 or TOO mm. It is character- ized by a number of changes, which, while not improving the eye as an organ of vision, are positive as contrasted with degenerative. There are also distinct degen- erative processes taking place during this period. The fourth period begins with the beginning of senescent degeneration and ends with death. It is characterized by degenerative processes only, which tend to gradually disintegrate and eliminate the eye entirely. It is questionable whether these changes should be called senescent. It may be urged that they are the result of disuse in the individual, or that the end product of these degenerative changes is the typical structure of the eye of Amblyopsis. First I'criod. — During the first period the eye arises as a solid outgrowth from the solid central nervous system when the embryo is about 1.5 mm. long. The outgrowth increases rapidly in size during the next 0.5 mm. of growth in length. The solid lateral outgrowth is bent back along the side of the brain, and its connection with the brain becomes constricted into the optic stalk. A cavity approximately central arises in the optic lobe at the same time that a cavity ap- pears in the central nervous system, which occurs when the embryo is about 2 mm. in length. The two layers of the optic vesicle formed by the appearance of the cavily are of about equal thickness. A little later the secondary optic vesicle is lormed by the thickening of the skin over the eye and the consequent cupping of the distal hue <>f the eye. The process reaches its culmination when the embryo has a length of 4.4 mm. The lens is still connected with the skin, and the two layers of the secondary vesicle have become very different, the proximal one being one layered, the distal one several-layered. The details of the changes of this period have been given in the preceding pages. At any lime up to this length the eye might, as far as its structure is concerned, give rise to a perfect eye in the adult. The eye so far follows phylogenetic paths \\illi the reservation that no adult ancestor is supposed to have had eyes like these embryonic stag< DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 161 The Second Period. - - The development during the second period is direct and leads to the condition obtaining at the end of that period. Some of the processes are palingenetic, some are of purely ontogenetic significance, while still others (if I may make the distinction) are degenerative. The optic nerve develops at the beginning of the period in an undoubted phylo- genetic way. As in the case of the eye as a whole, the nerve develops directly into its full size. The details of its history are given under the head of the optic nerve. The latter half of the history of the lens belongs entirely to this period. Its his- tory is also given under another head. The changes the lens undergoes during this period are all katagenic, and some time before this period closes the lens has disappeared. The direct development of the optic vesicle of the beginning of this period into the eye as found at the end of this period is very difficult to interpret satisfactorily. A comparatively very narrow marginal part of the secondary optic vesicle is converted into the epithelial part of the iris. The lens is almost always entirely excluded from the optic cup when the iris develops. The extreme shallowness of the optic cup and the comparative thickness of the retina would lead one to expect the choroid fissure proper to be a very short structure. The shallow cup develops into the adult eye by processes like those that take place in normal eyes. These purely palingenetic processes operating on so deficient material give rise to condi- tions that are not palingenetic. In the closing of the choroid fissure of the normal eye the thing of chief concern is the union of the infolded margins of the optic cup from the margin of the pupil to the point of exit of the optic nerve and the closing in of the retina around the optic nerve at its exit from the eye. In Amblyopsis the former process has become insignificant, and the latter the prominent process. This is further complicated by the fact that the vitreous cavity has ontogenetically disappeared nearly as much as phylogenetically, so that, while the processes of changing the optic cup into the eye are palingenetic, the material operated upon being quite different from that normally obtaining in fish embryos, the resulting stages of the eye are not palingenetic. The choroid fissure, which is distally a distinct slit leading into what remains of the optic cavity, becomes proximally a groove in a solid mass of cells. The closing of this groove takes place at various times, or it may remain permanently open. This condition has undoubtedly been brought about by a contraction of the area of the retina and the consequent heaping up of cells, either concomitantly with, or as the result of, the obliteration of the optic cavity. The funnel-shaped mass of cells in the center of the Amblyopsis eye is thus the result of the phylogenetic rather than the ontogenetic disappearance of the optic cavity. I must confess that an easier way of explaining the developmental stages would be reached by assuming that the central mass of cells, through which the optic nerve passes, is not really ganglionic -- that only the distal cells of the mass are ganglionic — and that the proximal ones are the homologues of the cells found at the point of entrance into the eye of Chologaster (fig. 65, z). This would imply that a cavity has not disappeared from the center of these cells (because there never was one), and that the entire vitreous cavity has been reduced to that now found in the embryo, and that no part of the cavity has disappeared in toto. This interpretation is especially suggested by figure 62, c. This would account for the 162 BLIND VERTEBRATES AND THEIR EYES. '' in •"'•"'• FIG. 6S. Exit ot optic Ner« from Eye of showing Ganglionic Cells and (s) cells at Entrance of Optic Nerve. fact that the optic nerve does not form a central strand through the funnel of ganglionic cells, but passes through it in several strands as it does through the mass of cells at the entrance of the optic nerve (fig. 65). The objection is that it would not account for the position of the ex't °f the optic nerve, which should, according to this view, be at the proxi- rnal end of the choroid fissure. The second objection is found in the phylo- genetic stages of degeneration indicated in different eyes, notably that of Typh- lomolge. Furthermore, it would not account for the groove that is un- doubtedlv found alons; the ventral side - of the larval eye, nor would it account for the presence of the inner reticular layer around the optic nerve. It would, moreover, make it necessary to assume that the cells found about the entrance of the optic nerve in Chologaster have been retained in Amblyopsis out of all pro- portion to the other structures of the eye. These objections seem to me fatal to this second supposition. During this period the differentiation of the several layers of the retina also takes place. At the beginning of the period the pigmented layer is represented by a layer of thin cells without pigment. At the end of the period it is composed of cylindrical cells 12 ju, high which are markedly pigmented. Pigment granules first make their appearance when the larva is about 5 mm. long. The remainder of the retina is at the beginning of the period several cells deep without any dif- ferentiation into layers. The inner reticular layer first appears as a number of irregular spaces separating the ganglionic from the nuclear layer when the em- bryos are 5 mm. long. These spaces soon unite into a single layer, but this does not occur till the very latest stages of the period when the choroid fissure has been closed for some time, and in fact they may never form a layer entirely around the central ganglionic cells. In earlier stages the layer extends between the dorsal and lateral parts of the ganglionic and nuclear layers. The nuclear layers never become separated into outer and inner ones, nor is an outer reticular layer ever formed. There is no indication of cones such as are seen in some adult eyes. Miillcrian fibers are well formed in older individuals at this period. The development of the scleral cartilages described under another head also takes place toward the close of this period. No dividing cells have been found in tin- eves of specimens more than 7 mm. long. The nuclei of the retina in the lo-mm. stage are all granular and measure 4 to 5 p. in diameter. Tin- Third Period. -- This extends from the time the fish has reached a length of 10 mm. till marked senescent changes begin, which take place when the fish approaches 100 mm. in length. The nuclei of the retina, when the fish has reached a length of 25 mm., are no longer alike. There are two types of cells in all layers : cells with larger granu- lar nuclei, and cells with smaller compact or dense nuclei. The difference is per- haps due less to histogencsis than to the process of degeneration which has already DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 103 set in. The cells with smaller nuclei are probably degenerate. In the oldest fish only cells of the second type are found. A number of changes take place during the third period, some of which can be classed neither as progressive nor as retrogressive. As the fish grows, the eyes are farther and farther removed from the surface. In the fish 25 mm. long they are nearly i mm. below the skin, and in the largest specimen examined they are as much as 5 mm. beneath the surface of the skin. The scleral cartilages develop progressively probably during the entire period, in some cases encroaching on the regular outline of the eye. Other processes which are progressive nevertheless do not tend to make the eye a more perfect organ of vision. The pupil, for in- stance, becomes closed in many cases, or reduced to a very minute opening. The vitreous cavity, which was still evident, becomes, concomitantly with the closing of the pupil, entirely obliterated. The pigmented layer becomes a variable struc- ture, the pigment granules being in many cases entirely absent. Rarely the pig- ment layer changes to a high columnar epithelium. The stages of this period have not been successively observed as in the younger period, and the genetic relation- ship of different stages is not always apparent. The Fourth Period. - - This extends from the time the fish has reached a length of about 100 mm. to the end of its life. There are distinct features that charac- terize the eye of this stage (plate 10, figs. C-G). The fibrous capsule enveloping the eye is distinctly thicker than in younger stages. The scleral cartilages are as well developed as at any time.1 The eye- muscles, as far as present, show no indication of degeneration and their striation can readily be made out in all individuals. The most marked changes take place in the size of the eye itself. The pig- mented layer becomes distended to form a thin-walled vesicle of two or three times the diameter of the eye in previous stages (plate 10, figs. F and G). This develop- ment of the pigmented layer beyond the requirements of the retina has also been seen in the eyes of Rhineura and other blind vertebrates. The cells of this layer become spherical or attenuated and the columnar epithelium converted into a thin epithelium thickened in places. Within this vesicle, whose sides may be compressed, as in figure F, the rest of the retina forms an insignificant little ball of tissue. In an eye of an individual 105 mm. long whose pigmented epithelium forms a vesicle 320 JJL in diameter, the rest of the eye forms a small sphere 60 p in diameter in contact with the iridian part of the pigment (plate 10, fig. G). The elements composing this little ball and representing the retina have also under- gone a marked senescent modification. The optic nerve is no longer evident.2 The ganglionic cells no longer form a compact mass, but are either unidentifiable or irregularly scattered. The cells of the outer nuclear layer are also less regular. While in the second period and up to 95 mm. in length two sorts of nuclei are distinguishable, some of them small and dense, others larger and granular. In these later stages they are all small and dense, no granular ones being present, and their outlines are less well defined than in the young. 1 In the left eye of a specimen 105 mm. long no cartilages were found. It is not possible to say whether they had disappeared or were never developed. Because of the irregularity in the development of these cartilages and their large size in other individuals of this period, I am inclined to think cartilages never appeared in this specimen. 2 The optic nerve can be traced as a very delicate filament through the pigment layer in an individual 123 mm. long. In this eye the choroid fissure was still open. 164 BLIND VERTEBRATES AND THEIR EYES. In a fish 25 mm. long the smaller nuclei measure 2.5 /A, the larger ones measure 3.5 to 5 p. In the specimen 123 mm. in length the nuclei measure but 2 to 3 p.. Evidence that the smaller nuclei in the younger specimen are degenerate is fur- nished by the fact that optic fibers can not be traced to the smaller ganglionic nuclei in a 25-mm. specimen. The most disorganized eye found is the left one of the largest fish examined, 130 mm. long (plate 10, fig. E). The fibrous sheath (sclera) is thick; the cartilage is large, 64 by 96 ^ in section. The eye itself is a disintegrated mass abundantly provided with granular pigment and without well-defined outline or structure. The right eye of the same specimen is less degenerate (plate 10, fig. F). It is an elongated vesicle 60 by 256 p. in section, with a large cartilage to one side of its distal half, 48 by 160 p. in section, and two smaller proximal ones, one of which measures 24 by 32 \L in section. Associated with the retina of this eye is a struc- ture that I described as a possible lens in my first paper. It consists of a few nuclei about which there are concentric layers of a homogeneous tissue. Consider- ing the fate of the lens in all the young fishes examined, it seems very doubtful, if not impossible, that this structure should be a lens. That the eyes of these largest individuals belong to the fourth period is seen in the fact that they become distended vesicles whose parts are finally resorbed after undergoing degenerative changes. The scleral cartilages offer an exception to the general fate. Summary of the Origin, Development, and Degeneration of the Eye and its Parts. Earliest appear- ance or differ- entiation. End of cell division. End of morpho- genesis. End of histogenesis. Beginning of degeneration. Disappearance. Eve mm. mm. mm. mm. mm. mm. Choroid fissure Pigmented layer. .. 2-5 2 ? 2-5 10 ? 100 or before ? 10-130 Beyond 130 Outer nuclear Outer reticular.. . . Horizontal cells. . . Inner nuclear ( ianglionic . . . after 10 4-4-5 Never Never 4-4-5 5-7 — IO IO IO Before 25 Before 25 Beyond 130 130 mm. and beyond Optic fiber layer or nerve . . Scleral cartilages.. Lens Q-IO ? ? 75 ( f division is very much retarded. In the retina it stops altogether at the time the lisli has readied a length of 5 to 7 mm., and very rarely more than two dividing cells are found in any eye. In its first stages the eye is thus about equal in size to the adult eye. Cell division stops earlier in the lens, where no new cells are formed afler it is cut off from the skin. The lens is at this time relatively as well developed as the retina. In both the retina and the lens cell division ceases in late stages, and the total number of cell generations is very much limited. The lens is looked upon as phylogenetically a new structure, and we have, by the stopping of its later stages of cell division, a step in the elimination of a phylogenetically new structure. This is, however, of no consequence because it is not differential, for the retina, a phylo- •etically older structure, suffers a similar stoppage. There is no evidence, then, DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 1G7 that phylogenetically younger structures lose their power of cell division earlier than phylogenetically older ones. The retardation of the morphogenic processes, cell arrangement, movement, union and separation, etc., is conspicuous in the delay of the closing of the choroid fissure and all that this implies. There is no conspicuous stopping of this process except in the occasional failure of the choroid fissure to close at all. Histogenic processes are also distinctly retarded, and in conspicuous instances suffer an entire stoppage. While the eyes of 3-mm. specimens of Cymatogaster or Cam ssi us and Amblwpsis are nearly alike, in the former two the tissue differ- entiation has progressed vastly farther by the time the fishes have reached a length of 10 mm. Histogenesis is carried surprisingly far in many degenerate eyes. In Rhi-iiL'itra, for instance, the layers of the retina are differentiated far beyond the requirements of the case. In Amblyopsis the process, as far as it can be made out with the methods available, falls short of the normal development.1 The cells of the lens never lose their embryonic characters ; they are never transformed into lens fibers. Cones are rarely if ever developed in the retina, and an outer reticular layer never. In normal development the cones and the outer reticular layers are the last to differentiate, so that we have certainly a cutting off of late ontogenetic stages. The question whether these are also phylogenetically young may be passed over. The total evidence from the three processes is that none of them proceed with the push and rapidity found in normal structures, and though they are normal, they grow weaker with development and frequently give out altogether. But with all this lack of vigor, while there is more variation in each structure developed than has been noted in normal eyes, the point to which cell division, cell arrangement, and histogenesis are carried, in different individuals, is about the same. The causes leading to the changed development are of approximately equal value in different specimens from the same locality. CAUSES OF RETARDATION AND CESSATION IN THE DEVELOPMENT OF THE EYE. The retardation and arrest in the ontogenetic development of the eye of Amblyop- sis may be due to one of several possible causes. They are either conditioned by something outside the cells composing the eye, or they are inherent or predeter- mined in the egg cell from which the eye is ultimately derived. The conditioning factor, if it lie outside the eye, may be a peculiarity in the physical and chemical environment in which the fish lives, or a lack of stimulation or an inhibition exer- cised by some other part of the body. Unless we assume that the eye of Ambly- opsis has reacted and does now react differently to the physical and chemical environment from that of some of the relatives of Amblyopsis, physical and chemical factors may readily be eliminated as contributing directly to the retarda- tion and cessation. Although, in discussing the phylogenetic degeneration of the eyes of cold- blooded vertebrates in general, I have insisted that cross-country conclusions must be guarded against, I then saw no objection, and now see none, to considering the different members of the Amblyopsidae as homogeneous material within the bounds of which we may expect similar causes to effect similar results. The different stages 1 The difficulties, for instance, of differentiating with Golgi methods the bipolar cells of an eye whose total diameter falls short of 0.2 mm. can readily be imagined. 168 BLIND VERTEBRATES AND THEIR EYES. (phyletic) of development found in the eyes of the different members of the Ambly- opsidae are all referable to the difference in time in which they have been subjected to their present environment. The only environmental condition surrounding the developing eggs of Amblyopsis to which the peculiarities of development might be attributed is the total absence of light. Temperature, oxygen pressure, chemical composition, etc., of the surrounding medium may be entirely excluded from the possible agents affecting the eye, inas- much as normal eyes are developed by other fishes in the same water and under all possible fluctuations of the above conditions within the limits of the possibility of fish life. But the same objection holds in attributing the lack of development to the absence of light. Clwlogaster agassizii, a member of the Amblyopsidas, which always lives in caves in exactly the same conditions under which Amblyopsis lives, has nevertheless normally developed, though small, eyes. While guarding against the possibility of attributing too much weight to the results obtained in other families of animals, it still may be mentioned that many fishes living perpetually in total darkness develop normal eyes. This is also true of the young of all viviparous animals which develop in more or less complete darkness. If, then, so closely related fishes as Chologaster and Amblyopsis are subjected to the same environment which is minus a certain element and both develop their normal parental structure, one developing a normal eye, the other a very abnormal degenerate one, it is scarcely warrantable to say that the abnormal structure in one of them is due to the absence of the one element (light) from the environment. Moreover, if the development is controlled by the absence of light, there is no reason why development should be normal, even to the extent of forming a normal start and should then be arrested or retarded. The fact that the presence or absence of light is not the controlling factor in the retarded development of the eye of Amblyopsis does not vitiate the supposition that a certain amount of change may not be pro- duced on the eyes of an individual by rearing it in the light. Such change would, however, stand on a par with the ontogenetic degeneration of the eye with age in the absence of light ; that is, it would be a functional adaptation due to use. Experiments have been in progress to test the effect of light. So far only nega- tive results have been obtained. One young has been reared till it was 6 months old. It was obtained from the caves at a time when it was ready to swim about freely ; that is, when the eye was already fully formed. There was no difference in the gross anatomy of the eye of this individual as compared with that of others. The minute anatomy, as the result of an accident, was not available for study. The others examined in earlier stages have not been reared beyond a length of a few millimeters, and the effect of the light, if any, was not appreciable. From the observations on the development of the eyes — which show that some processes are arrested very early - it would seem that the only rational way to determine the elleet of light on the total development is to colonize the adults in an outdoor pool A!HH tlie young can be reared, from the fertilization on, in normally lighted waters. The lark of development of the eye not being chargeable to any factor in the environment, is there any factor within the fish that inhibits its development, or whose absence fails to furnish the stimulus necessary to the development? If so, this factor must be present or absent at the time the retardation begins or some time In I'ore. DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 169 The inhibition, if any, might operate through a mechanical crowding on the part of a neighboring organ or the greater selective power in eliminating the food requisite for the development of the eye. The first cause may be eliminated, for there is no evidence whatever of crowding other than that found in normal eyes; in fact, in ah1 stages beyond the earliest, the eye is much smaller than the optic sockets can easily accommodate. The question of selective food elimination is not so readily disposed of. The ophthalmic artery provides the eyes abundantly with blood, so it is not an absence of this that causes the supposed starving. Indeed if the retardation were due to a lack of blood supply we would be removing the problem one step from the eye with- out solving it. Besides, Loeb's experiments have shown that the action of the heart may be greatly diminished without affecting the rate of growth of the larval fish. The blood supply being abundant, is there any other organ that may drain it of the nutriment necessary for the proper growth of the eye? Leaving aside the question whether an organ can be starved by having the nutriment requisite for development withdrawn from the blood by another organ, I can think of no organ or set of organs that attain an unusual growth aside from the tactile organs of the skin. This system of organs is undoubtedly very highly developed in the adult and has also attained a remarkable degree of development at the time the fish is 10 mm. long. It is, however, not unusually developed in the earlier stages before hatching and shortly thereafter when the cessation of cell division, the most important element of the stunted optic development, takes place. Besides this, the tactile organs of Chologaster, which possesses normal eyes, are very highly, if not so elab- orately, developed as in Amblyopsis. I have experimentally determined by eliminat- ing the eyes altogether that the tactile organs in Chologaster papilliferus are amply developed to enable the fish to live indefinitely without the use of its eyes. The same must also be true of Chologaster agassizii, which lives permanently in caves. While not impossible, it seems, therefore, very improbable that the tactile organs affect the development of the eyes in Amblyopsis and not in Chologaster.1 I know of no other organs in Amblyopsis whose development differs from that of Chologaster in a degree sufficient to make it a successful contestant for a food supply in Amblyopsis and not in Chologaster. What has been said concerning organs whose presence might affect the develop- ment of the eyes is equally true concerning organs whose absence might deprive the eye of the necessary stimulus to reach normal development. I know of no organ, either in Amblyopsis or Chologaster, whose absence in the one and presence in the other might account for the difference in the degree of development reached by the eyes in the two fishes. The conclusion is forced upon us by the above considerations that neither in the environment nor in the fish itself is there a factor sufficient to account for the early arrest in cell division, the retardation of the morphogenic processes, and the stopping of the histogenic processes. We are therefore entirely justified in assuming that the determining cause of the method of development lies in the cells themselves and is inherited. The great development of the scleral cartilages beyond the needs of 1 As an example bearing on this subject attention may be called to the tactile apparatus of the Siluridu', which is certainly in many instances more elaborate than that of Amblyopsis, and yet the eyes are normal, though small. 170 BLIND VERTEBRATES AND THEIR EYES. the eye also tend to locate the formative or hereditary power in the cartilages them- selves rather than in the stimuli to their development that they receive from their contact with the developing eyes, for they develop entirely beyond the needs of these eyes.1 The causes operating in ontogeny and phylogeny that have led to the limited power of development and differentiation I have fully considered in the concluding chapter, which was also published in the Popular Science Monthly.2 The conclu- sion is reached that the phylogenetic degeneration, which is equivalent to saying the limited power of development found in the cells entering into the eye of the indi- vidual, is the result of functional adaptation during the lifetime of past individuals to the total disuse of the eye. This adaptation, it was concluded, was transmitted to a certain extent to the succeeding generation through the usual vehicles of trans- mission. There has always been and is yet a serious objection to this conclusion, because the method of the transmission of functional adaptations to the organiza- tion of the egg so as to limit or extend its powers is not known. Recently, while admitting that functionally adaptive structures arise develop- mentally without reference to function, Driesch has maintained that: "Wer hier von 'Vererbung' friiher einmal functionell 'erworbener' Eigenschaften reden will verlasst den wissenschaftlichen Boden, denn wir wissen von solcher Art der Verer- bung gar nichts." Possibly we might find a warrant for the assumption of the transmission of func- tional adaptation to the germ cells in the writings of Driesch himself, though he might not thank us for it. He maintains that certain developmental results whose proximal cause he is not able to determine may be produced by factors working in a distant part of the embryo. Without entering into a discussion of the validity of these factors working at a distance, if they are really factors and capable of acting, as Driesch imagines, why may not functional modifications effect changes in the hereditary cells in a similar manner? I conclude that retardation and cessation in development are not due to onto- genetically operating causes, but they are inherent in the fertilized ovum — they are inherited. THE EYES OF AMBLYOPSIS AND THE LAW OF BIOGENESIS. During recent years the law variously termed von Baer's law, Agassiz's law, I laeckel's law, or the law of biogenesis, has been frequently called into question. Its genrral it -nets arc : (i) every individual in its development repeats in brief the devel- opment of the race ; (2) closely related forms have a similar ontogeny, and the nearer two animals are related the longer their embryos are alike; (3) the embryos of high animals pass through stages resembling the adult stages of lower animals; and (4) in every ontogeny there are, among the truly ancestral stages, stages which are adaptive and have been acquired during ontogenetic development. \d objection has been raised to the fourth tenet in so far as its acceptance does not commit to the acceptance of the first. In objection to the first of these proposi- tions Hurst writes : I do not deny that a rough parallelism exists in some cases between ontogeny and phylogeny. 1 do deny that tin- |,1i\]occifs of AmblyopMd.c and Typhlinimlgr, d c, g. h. and t drawn under same i ('it t 'liit!i^;,rl i ccrttitlu^ (b) L Itiilogasler papiUtJerus, (c) Cluilogasttr agassizii, drawn to scale; (rf) Retina of Cltologasler cornutus: (r) Kttina of Chuiagasltr fapillijerus; (/) Eye of Typhlo- molgf under lower magnification than d~f\ (g) Eye of Typhlichthys subierrattcus ; (ft) Eye of Amblyopsis spdaus; (i) Eye ot Troglichlhys rasa. simplification of the retina. There was at first chiefly a reduction in the number ol many times duplicated parts. Even after the condition in Chologaster pa pill if - cms was ivachnl the degeneration in the histological condition of the elements did not keep pace with the reduction in number (vide the eye of cornutus'). The SUMMARIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID^. 177 dioptric apparatus disappeared rather suddenly, and the eye, as a consequence, collapsed with equal suddenness in those members which, long ago, took up their abode in total darkness. The eye not only collapsed, but the number of elements decreased very much. The reduction was in the horizontally repeated elements. The vertical complexity, on which the function of the retina really depends, was not greatly modified at first. In those species which took up their abode in total darkness the degeneration in the dioptric apparatus was out of proportion to the degeneration of the retina, while in those remaining above ground the retinal structures degenerated out of proportion to the changes in the dioptric apparatus, which, according to this view, degenerates only under conditions of total disuse or total darkness which would necessitate total disuse. This view is upheld by the conditions found in Typhlo- goblus, as Ritter's drawings and my own preparations show. In Typhlogobius the eye is functional in the young and remains a light-perceiving organ throughout life. The fish live under rocks between high and low tide. We have here an eye in a condition of partial use and the lens is not affected. The retina has, on the other hand, been horizontally reduced much more than in the Amblyopsidae, so that, should the lens disappear, and Ritter found one specimen in which it was gone, the type of eye found in Troglichtkys would be reached without passing through a stage found m Amblyopsis; it would be simply a horizontal contracting of the retina, not a collapsing of the entire eye. The question may with propriety be asked here : Do the most degenerate eyes approach the conditions of the pineal eye? It must be answered negatively. RESULTS OF THE PHYLETIC DEGENERATION ON THE DIFFERENT PARTS OF THE EYES OF THE AMBLYOPSIDAE. The different structures of the eye may now be taken up in detail. (a) The eye muscles are normally developed in Chologasler. They are present to a greater or less extent in Amblyopsis. They have been reduced in number in Troglichthys, where the half nearest the eye has been replaced by bundles of fibrous tissue. In Typhlichthys they have vanished. (b) The scleras of the different members are not comparable on account of the presence of cartilage in some species and not in others. Both this layer and the choroid are insignificant in Chologasler and Typhlichthys. In Amblyopsis cartilages different in size and number are found anywhere about the eye, being frequently present in shape and position to suggest a displaced lens. In thickness the cartilages are disproportionate to the size of the eye. In Troglichthys we have a still more evident misfit, for the scleral cartilages are both too long and too thick. Evidently the scleral cartilages have not decreased in size in the same ratio as the eye, or, what amounts to the same thing, they develop beyond the present needs of the eye. (See also Lucifuga.) (c) The choroid is thin in all cases except where pigment cells are situated These are frequently several times as thick as the rest of the choroid. In Ambly- opsis the pigmentation of the choroid is inversely proportional to the pigmentation of the retina. (d) The lens has already received sufficient attention. It is merely necessary to insist again that, as long as an eye is functional to any extent, the lens — in fact the dioptric apparatus in general — does not degenerate and that when absolute disuse 178 BLIND VERTEBRATES AND THEIR EYES. comes, the lens, both phylogenetically and ontogenetically, disappears rapidly. In Typhlogobius Ritter found the lens absent in one very old individual, and Cope found that in Gronias the lens is sometimes present on one side, while not on the other. In Amblyopsis and Typhlichthys it has degenerated to a mere vestige, or is gone altogether. Ritter, after considering the structure of degenerate eyes as far as known at the time, came to the conclusion "that the lens disappears before the retina; and that, where degeneration takes place at all in ontogeny, the lens is affected first and most profoundly." With the first part of this statement the more recent observations are in full accord. It is, however, doubtful whether the lens is ever the first part affected ; in fact the retina always leads, but certainly the lens, if affected at all, is affected profoundly. (f) There is more variety in the degree of development of the pigment epithelium than in any other structure of the eye. Ritter has found that in Typhlogobius this "layer has actually increased in thickness concomitantly with the retardation in the development of the eye, or it is quite possible with the degeneration of this particular part of it. An increase of pigment is an incident to the gradual diminu- tion in functional importance and structural completeness." There is so much variation in the thickness of this layer in various fishes that not much stress can be laid on the absolute or relative thickness of the pigment in any one species as an index of degeneration. While the pigment layer is, relative to the rest of the retina, very thick in the species of Chologaster, it is found that the pigment layer of Chologaster is not much if any thicker than that of Zygonectes, but exception must be made for specimens of the extreme size in papilliferus and agassizii. In other words, primarily the pigment layer has retained its normal condition, while the rest of the retina has been simplified, and there may even be an increase in the thickness of the layer as one of its ontogenetic modifications. Whether the greater thickness of the pigment in the old Chologaster is due to degeneration or the greater length of the cones in a twilight species I am unable to say. In Typhlichthys, which is undoubtedly derived from a Chologaster-like an- cestor, no pigment is developed, the layer retains its epithelial nature and remains apparently in its embryonic condition. It may be well to call attention here to the fad that the cones are very sparingly developed, if at all, in this species. In Amblyopsis, in which the degeneration of the retina has gone farther, but in which the tones are still well developed, the pigment layer is very highly developed, but not by any means uniformly so in different individuals. The pigment layer reaches its greatest point of reduction in rosce where pigment is still developed, but the layer is fragmentary except over the distal part of the eye. We thus find a development of pigment with an imperfect layer in one case, Troglichthys, and a full-developed layer without pigment in another, Typhlichthys. In the chologasters the pigment is prismatic; in the other species granular. (/) In the outer nuclear layer a complete series of steps is observable from the two layered condition in papilliferus to the one-layered in cornutus,lo the undefined layer in Typhlichthys and the merging of the nuclear layers in Amblyopsis, and their occasional total absence in rosce. The single cones disappear first, the cones long before their nuclei. (.(,') The outer reticular layer naturally meets with the same fate as the outer nuclear layer. It is well developed in papilliferus and agassizii, evident in Cholo- SUMMARIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID^E. 179 9 10 gaster cornutus, developed in spots in Typhlichtliys, and no longer distinguishable in the other species. (h) The layers of horizontal cells are represented in papillifcrus by occasional cells ; they are rarer in cornutus and beyond these have not been determined with certainty. (i) The inner nuclear layer of bipolar and spongioblastic cells is well developed in C. papillifcrus and C. agassizii. In cornutus it is better developed in the young than in the older stages, where it forms but a single layer of cells. There is evi- dently in this species an ontogenetic simplification. In the remaining species it is, as mentioned above, merged with the other nuclear layer into one layer which is occasionally absent in Troglichthys. (j) The inner reticular layer is relatively better developed than any of the other layers, and the conclusion naturally forces itself upon one that it must contain other elements besides fibers of the bipolar and ganglionic cells, for, in Amblyopsis and Troglichthys, where the latter are very limited or absent, this layer is still well developed. Hori- zontal cells have only been found in the species of Chologaster. (k) In the ganglionic layer we find again a com- plete series of steps from the most perfect eye to the condition found in Troglichthys. In papiUiferus and agassizii the cells form a complete layer one cell deep except where they have given way to the optic fiber tracts which pass in among the cells instead of over them. In cornutus the cells have been so reduced in number that they are widely separated from each other. With the loss of the vitreous cavity the cells have been brought together again into a continuous layer in Typhlichtliys, although there are much fewer cells than in cornutus even. The next step is the formation of a solid core of ganglionic cells, and the final step the elimination of this central core in Troglich- thys, leaving but a few cells over the anterior face of the retina. (/) Mtillerian nuclei are found in all but Amblyopsis and Troglichthys. In C. cornutus they lie in part in the inner reticular and the ganglionic layer. Cells of this sort are probably also found among the ganglionic cells of Typhlichtliys. We thus see that the simplification or reduction in the eye is not a horizontal process. The purely supporting structures like the scleral cartilages have been retained out of all proportion to the rest of the eye. The pigment layer has been both quantitatively and qualitatively differently affected in different species. There was primarily an increase in the thickness of this layer, and later a tendency to total loss of pigment. The degeneration has been more uniformly progressive in all the layers within the pigment layer. The only possible exception being the inner reticular layer, which probably owes its retention more to its supporting than to its nervous elements. Another exception is found in the cones, but their degree of development is evidently associated with the degree of development of the pigmented layer. As long as the cones are developed, the pigmented layer is well developed, or vice versa. FIG. 67. Diagram showing per cent'of Total Thick- ness of each Layer of Retina in I, Zygoncctes iwlatits; 2 and 3, Chologaster cornutus, 27 mm. long and 43 mm. long; 4, Oio/ogiii/ir ptipilli- feriis, 29 to 39 mm. long, and 5, 55 mm. long; 6, Chologaster agassizii, 38 mm. long and 7.62 mm. long; 8, Amblyopsis; 9, 10, Troglichthys. 180 BLIND VERTEBRATES AND THEIR EYES. ONTOGENETIC DEGENERATION. The simplification of the eye in comutus has been mentioned in the foregoing paragraphs. It may be recalled that the nuclear layers are thinner in the old than in the young. There is here not so much an elimination or destruction of element as a simplification of the arrangements of parts, comparatively few being present to start with. The steps in ontogenetic degeneration can not be given with any degree of finality for Amblyopsis on account of the great variability of the eye in the adult. While the eyes of the very old have unquestionably degenerated, there is no means of determining what the exact condition of a given eye was at its prime. In the largest individual examined the eye was on one side a mere jumble of scarcely distinguish- able cells, the pigment cells and scleral cartilages being the only things that would permit its recognition as an eye. On the other side the degree of development was better. The fact that the eyes are undergoing ontogenetic degeneration may be taken, as suggested by Kohl, that these eyes have not yet reached a condition of equilibrium with their environment or the demands made upon them by use. Furthermore, the end result of the ontogenetic degeneration is a type of structure below anything found in the phlyogeny of the vertebrate eye. It is not so much a reduction of the individual parts as it is a wiping out of all parts. PLAN AND PROCESS OF PHYLETIC DEGENERATION IN THE A M BLYOPSI D^E. Does degeneration follow the reverse order of development or does it follow new lines, and if so, what determines these lines ? Since the ontogenetic development of the eye is supposed to follow in general lines its phyletic development, the above question resolves itself into whether or not the eye is arrested at a certain stage of its morphogenic development, and whether this causes certain organs to be cut off from the development altogether. In this sense the question has been answered in the affirmative by Kohl. Ritter, while unable to come to a definite conclusion, notes the fact that in one individual of Typhlogobius the lens which is phyletically a new structure had disappeared. This lens had probably been removed as the result of degeneration rather than through the lack of development. Kohl supposes that in animals placed in a condition where light was shut off more or less, every succeeding generation developed its eye less. Total absence of light must finally ] irevent the entire anlage of the eye. Time has not been long enough to accomplish this in any vertebrate. Phyletic degeneration is looked upon as the result of a long series of " H> mmungen " which in successive generations appeared in ever earlier time of ontogenetic development in always lower stages of the development of the individual eyes. The eye develops after the vertebrate type. At certain stages the rate of progress is diminished and in most cases finally completely ceases. A retardation lias developed which after a shorter or longer period ends in the cessa- tion of all development. The first appearance of the retardation falls in a time of embryonic or post-embryonic development that in the phylogeny corresponds to the moment when the lack of light became operative. The period in ontogeny .vhich lies between the first disturbance in development and its cessation corre- sponds to the phyletic time during which the development of the eye is checked at a continually lower stage of development. The point of cessation in ontogeny cor- responds to the time when the eye reached its equilibrium. If in ontogeny there SUMMARIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID/E. 181 is undoubted degeneration, it is always an indication that the eye has not yet reached the point where it is in equilibrium with its functional requirements. Cessation of development does not take place at the same time in all parts of the eye. Those not essential to the perception of light are disturbed first. The retina and the optic nerve are the last affected, the iris coming next in the series. Because the cornea, aqueous and vitreous bodies, and the lens are not essential for the performance of the function of the eye, these structures cease to develop early. The processes of degeneration follow the same rate. Degeneration is brought about by the falling apart of the elements as the result of the introduction of connective tissue cells that act as wedges. Abnormal degeneration sometimes becomes manifest through the cessation of the reduction of parts that normally decrease in size so that these parts in the degenerate organ are unusually large. Kohl's theoretical explanation here given somewhat at length is based on the study of an extensive series of degenerate eyes. He has not been able to test the theory in a series of animals living actually in the condition he supposes for them, and has permitted his erroneous interpretation of the highly degenerate eye of Troglichthys to lead him to this theory of the arresting of the eye in ever earlier stages of ontogeny. It has been shown in previous pages that this most degenerate eye is in an entirely different condition from that supposed by him. The mere checking of the normal morphogenic development has done absolutely nothing to bring about this condition, and it could not have been produced by the checking of development in ever earlier and earlier stages of ontogeny, for there is no stage in normal ontogeny resembling in the remotest degree the eye of Troglichthys. The process of degeneration as seen in the Amblyopsidae is in the first instance one of growing smaller and simpler — not a cutting off of late stages in the develop- ment. The simplified condition, it is true, appears earlier and earlier in ontogeny till it appears almost along the entire line of development, even in the earliest stages. But the tendency for characters added at the end of ontogeny to appear earlier and earlier in the ontogeny is well known, and there is no inherent reason why an organ disappearing in the adult should not eventually disappear entirely from ontogeny. The fact that organs which have disappeared in the adult have in many instances not also disappeared in the ontogeny and remain as so-called rudimentary organs has received an explanation from Sedgwick. For a discussion of this see the chap- ter on the Law of Biogenesis. In Amblyopsis, where the eye has not been functional at any period of ontogeny for many generations, where degeneration begins at an early period and continues till death, the degenerate condition has reached the early stages of the embryo. It is only during the first hour or so that the eye gives promise of becoming any- thing more than it eventually does become. The degree of degeneration of an organ can be measured as readily by the stage of ontogeny when the degeneration becomes noticeable as by the structure in the adult. The greater the degeneration, the farther back in the ontogeny the degenerate condition becomes apparent, unless, as stated above, the organ is of use at some time in ontogeny. It is evident that an organ in the process of being perfected by selection may be crowded into the early stages of ontogeny by post-selection. Evidently the degenerate condition is not crowded back for the same reason. How it is crowded back, I am unable to say. A satis- factory explanation of this will also be a satisfactory explanation of the process 182 BLIND VERTEBRATES AND THEIR EYES. by which individually acquired characteristics are enabled to appear in the next generation. The facts, which are patent, have been formulated by Hyatt in his law of tachygenesis. Histogenic development is a prolonged process, and onto- gcnetic degeneration is still operative, at least, in Amblyopsis. Degeneration is not the result of the ingrowth of connective tissue cells as far as I can determine. It is rather a process of starving, of shriveling, or resorption of parts. From the foregoing it is evident that degeneration has not proceeded in the reverse order of development, rather the older normal stages of ontogenetic develop- ment have been modified into the more recent phyletic stages through which the eye has passed. The adult degenerate eye is not an arrested ontogenetic stage of development, but a new adaptation, and there is an attempt, in later ontogeny at least, to reach the degenerate adult condition in the most direct way possible. THE CUBAN BLIND FISHES THE CUBAN BLIND FISHES.1 HISTORY OF THE WORK. The Cuban blind fishes were discovered by the surveyor D. Tranquilino San- dalio de Noda. They were described as Lucifuga subterraneus and Lucifuga dentatus by Poey, in his "Memorias sobre la Historia Natural de la Isle de Cuba," tomo 2, pp. 95-1 14, 1856. Poey recorded them from the cave Cajio, near La Guira de Melena, La Industria, half-way between Alquizar and Guanimar, the Cave of Ashton, the Cave of the Dragon, on the cattle farm San Isidro, near Las Mangas, La Concordia, a cave near the bee house of the coffee plantation La Paz, and a well near the tavern Frias. Poey stated that Lucifuga dentatus from some of the caves had vestiges of eyes, while those from others were without the least vestige of eyes. Poey later added some notes on their distribution in his "Enumeratio Piscium Cubensium." In 1863 Gill (Proc. Acad. Nat. Sci. Phila., 1863, p. 252) recognized Lucifuga dentatus as the type of a distinct genus, which he called Stygicola. No additions were made to the knowledge of these fishes until March, 1902, when I visited Cuba with Mr. Oscar Riddle expressly to secure material for the study of their eyes. We visited several of the caves mentioned by Poey and many others, securing 119 specimens of both species. One of the specimens contained four young, making in all 123 specimens. The discovery that the blind fishes are viviparous, and that the young have fairly well developed eyes, made it seem very desirable to secure a full series of embryos and also if possible to rear some of them in the light. The expenses of this trip were defrayed in part by a grant from the American Association for the Advancement of Science and in part from subsidiary work on the fresh-water fishes of the western end of the island. The results as far as pub- lished are included in an article on the "Fresh-water Fishes of Western Cuba" (Bull. U. S. Fish Com., 1902, pp. 211-236, plates 19-21, 1903). Grant No. 64 of the Carnegie Institution made additional work in the field possible. It was planned to spend the entire breeding season near the caves and rear young in the light, but for reasons that will appear the grant was exhausted in apparently determining that these fishes do not breed in the places visited. My trip to Cuba in March, 1902, made it seem probable that the blind fishes give birth to their young in February. Many recently born young of Lucifuga were obtained at that time, and one of the females caught contained young nearly ready to be born. The California viviparous fishes, with which I had extensive experience2 and which give birth to young in a similar degree of maturity, carry their young about 5 months. On these premises I concluded that early stages of the young of the blind fishes should be found during the middle of September. Allow- ing a month for the probably more rapid development in the tropics, I visited the caves the latter part of October and first part of November. 1 The specimens were numbered as they were collected, i, 2, etc., and when referred to are given by their serial number. 2 On the viviparous fishes of the Pacific coast of North America, Bull. U. S. Fish Com., 1892, pp. 381-478, 27 plates, 1894. 185 INI, BLIND VERTEBRATES AND THEIR EYES. Aside from obtaining young it was planned to build cages in a well-lighted cave in which the adult would be compelled to carry and give birth to their young in the light. The body walls in the majority of individuals would offer little or no obstacle to the penetration of light to the embryos. Dr. J. W. Beede, of the Geological Department of Indiana University, acted as volunteer assistant and rendered very valuable aid in collecting fishes, making the cages, and taking the traverse to the various caves in the chief cave region about ("anas. Only a single individual with young was obtained and one other with nearly mature eggs. Two cages were built and fishes were confined in them and the cages sunk in the Modesta, a well-lighted cave in which fishes were naturally abundant. On December i a few fishes were collected and sent me by Mr. F. Martinez, of Canas. Although these promised little better success than the ones collected in October and early November, I started for Cuba again on December 18, 1903, accompanied on this trip by Mr. John Haseman, as volunteer assistant. It was again found that this was not the breeding season, as no fishes with young were found at all. The cages were found intact and received a new supply of fishes. On May i a number of fishes were sent me by Mr. F. Martinez, and as these promised no young the trip planned for May was abandoned. On June i, when Mr. Martinez was again to send me samples, he was unable to obtain any fishes on account of high water. Between June and August I could not get away from my routine work, but this period was later covered by Mr. Haseman. On August 15, 1904, I started again for Cuba, accompanied by Mr. Hankinson as volunteer assistant. I returned Sep- tember 7. On this trip, which was more extensive than the former, I obtained two females with young, one a Lucifuga containing 10 young, and one a Stygicola con- taining i young. On this occasion I visited two new localities. At one of these, Jovellanos, from which Poey reported Stygicola, I obtained nothing. At the other, the Carboneria farm, on the north coast near Matanzas, I obtained my first speci- mens from the northern slope of Cuba. I am under many obligations to Dr. Felix Garcia, the harbor health officer of Matanzas for the opportunity to visit the Carboneria. At this time the cages in the Modesta were found to be entirely spoiled, the wire screening having corroded in large pieces. I succeeded in bringing living fishes to Indiana, but it was not possible to bring large numbers. There was great mor- tality en route on account of the extreme sensitiveness to cool water, which rules entirely out of court the idea of colonizing them in some of our northern caves. In June, 1905, two of my students, Mr. J. Haseman, who had accompanied me on one of the trips, and Mr. Norman Mclndoo,^made another tour of the caves, but with no better success as far as embryos were concerned. They secured but one female with young. The following papers have appeared on material gathered during the various Cuban trips : i. Tin.' lilim! Fish of Cuba. Srienre, N. S., xvi, p. 347. Eigenmann, C. H. The fresh water fishes of western Cuba. Bull. U. S. Comm. Fish and Fisheries, , |i|i. 211-236, plates 19-21. 3. Tin- water supply nf Havana. Sc ience, N. S., xvm, pp. 281-282. Aug. 28, 1903. Fish in Cuba. Woild To-day, V, pp. 1129-1136. 1 Sui li nai li blimlen l''ischen in Cuba. Die Umschau, VII, pp. 365-367. 1 '" •' mall ' ollei ti.m <,f ( rusiac cans from the island of Cuba. Proc. U. S. Nat. Mus., xxvi, pp. 429-435, Feb. 2, IQ03. lures of the viviparous blind fishes, Lucifuga and Stygicola. Biological Bulletin, vi, pp. 38-54, 1903. EIGENMANN Carboneria beach near Matanzas. Dividing line between naked beach (on right) and sand-filled area (on left). Rift separates the two zones. Bushes on extreme left mark line of older beach. Cave of the Insurrectos, near the Carboneria, from entrance. Pool of water showing at bottom of cave. LUCIFUGA AND STYGICOLA. 187 8. Muhse, E. F. The eyes of Typhlops lumbricalis (L.), a blind snake from Cuba. Biol. Bull., v, pp. 261- 270, Oct. 1903. 9. Pike, F. H. The degenerate eyes in the Cuban cave shrimp, Paltzmonetes eigenmanni Hay. IJiol";'ii .il Bulletin, xi, pp. 267-276, 1906. 10. Payne, F. The eyes of .-{mphisbtnia functata (Bell), a blind lizard from Cuba. Biol. Bull., xi, pp. 60- 70, plates I and II, July 1906. 11. Weckel, A. L. The fresh-water Amphipoda of North America. Proc. U. S. Nat. Mus., xxxn. De- scribing a new Amphipod, Gammarus ccecus, from the Modesta Cave, Cuba. pp. 47-49, 1907. 12. Haseman, J. D., and Mclndoo, Norman N. On some fishes of Western Cuba. Proc. A< ad. Xat. Sci. Phil., 1906. ZOOLOGICAL POSITION OF LUCIFUGA AND STYGICOLA. Lucifnga and Stygicola are members of the Brotuliclae, of which Jordan and Evermann say: "These fishes are closely related to the Zoarcida?. In spite of various external resemblances to the Gadichv, their affinities are rather with the blennioid forms than with the latter." They are most closely related to the genera Brosmophycis and Ogilbia, with which they have a distinct caudal peduncle in contradistinction to the numerous other American genera of the family. Brosmophycis niarginatiis (Ayres) occurs on the coast of California in moderate depth. Ogilbia ventralis (Gill) occurs in rocky pools about the Gulf of California and at La Paz. The other member of the genus, Brosmophycis cayorum, was taken on a shoal covered with algas at Key West. Other members of the family are found at great depths in various parts of the world; one, Brotula barbata, occurs about Cuba in water of moderate depth. The genera Lucifnga and Stygicola differ from each other in their dentition. Stygicola has teeth on the palatines; Lucifnga has none. In Stvgicola the nape is more strongly arched than in Lucifuga. The maximum recorded size of Stygi- cola is 152 mm. ; of Lucifuga, 104 mm. PRIMARY AND SECONDARY SEXUAL CHARACTERS. The male organ of Stygicola consists of a conical papilla, two-lobed at the tip and surrounded by a dermal pouch. It reaches to the second or third anal ray, being turned either to one side or the other of the anal. It is pigmentless, but is covered from in front by a pigmented dermal flap. In color, Lucifuga varies from a faint pink to lilac-pink and lilac. There is, in general, an increase of pigment with age. Stygicola varies from pinkish lilac to steel-blue, with transparent edges to the fins. There is no regular increase of color with age in this species nor is there any distinction in the sexes. Both black and light-colored individuals are found side by side in caves. It is possible that light - colored individuals have lived in the remote recesses of the cave and that the black ones have remained in the lighted chambers, but there is no direct evidence on this point. The males of Stygicola are distinctly larger than the females. The average length of 137 females caught is 98.2 mm., the largest one being 140 mm. The average size of the 82 males is 107 mm., the largest one being 152 mm. long. In the first lot secured the males were in excess of the females in the ratio of 100 females to 115 males. In all I have 137 females to 82 males. Counting the first 43 specimens secured, there is but an appreciable difference in the average of the fins as far as these could be counted, the average formula for the female being, D. 91.4; A. 74; and for the males D. 91.1; A. 73.3; or the average for the two, D. 91.2; A. 73.6. 188 BLIND VERTEBRATES AND THEIR EYES. Of Lucifnga ' 74 males have an average length of 63.5 with a maximum of 104, and 82 females have an average length of 58 mm., with a maximum of 95. Only specimens over 50 mm. in length were considered. While the average number of rays differs considerably in the two species, the number in each varies so much that the numbers in individual cases overlap, the individuals of Lucifnga reaching as high as 88 dorsal rays, and the individuals of Stygicola as low as 87. The same is true with the anal. DISTRIBUTION OF STYGICOLA AND LUCIFUGA. Slygicola is known to be distributed from Jovellanos and Alacranes on the east to Canas. Lucifuga is confined to the region from Guira de Helena westward to Canas. The entire region between Alacranes and Canas on the southern slope is drained by underground rivers. In the Canas region, the two species live side by side with apparently no choice, except that while the young of Lucifnga are abun- dant in shallow water among the roots of trees I have not been able to see or secure Stygicola shorter than 60 mm. except as larvae from the mother. Stygicolas are perhaps more abundant in the deeper, darker caves, though they are also found in the shallowest, while lucifugas are more abundant in shallower, more open caves, they in turn being found in the deeper caverns. Blind fishes resembling Stygicola or Lucifnga have been reported to me from well-like caves at Merida, Mexico. None have been captured. Other blind fishes which may be related to them are said to occur in Jamaica. NATURE OF THE HABITAT OF STYGICOLA AND LUCIFUGA. Within the area over which they are distributed the blind fishes of Cuba live, as far as known, in well-like caves in coralline limestone. The character of the region in which they live can best be understood from an examination of the Finca Carboneria, just outside of the Bay of Matanzas. There is here a coral strand about on a level with high water. At the point of contact between ocean and land there is an abrupt wall, 5 to 10 feet high, profusely covered with seaweed, the nearly tideless water coming to the .top of the wall where there are shallow, panlike pools replenished by waves and spray. Immediately on top of the wall follows a low, naked, jagged mass of rock resembling a huge sponge with its numerous pits and points. This area is in- habited by innumerable mollusks. This low area is separated by a cleft (plate u, fig. A) forming a sharp line of demarcation from a second zone similar to the first, but in which the pits and depressions in the rock have become filled with sand which gives foothold to tufts of plants. Over this lizards scamper from rock to rock. Following this there is an abruptly sloping beach, the outer half of which is rocky and sandy, partly covered with cactus and other low-growing plants, the inner or land halt" being covered with shrubs and trees. All of these zones occupy per- haps 100 yards. They are followed by the level, practically treeless, meadow, I In- following :i> i mint was published of the first 53 specimens of Lucifuga secured: The females are dis- tinctly larger than the males. In making the average for the size of the sexes, individuals less than a year old ;e ililfi-u-ni os in the sexes, if present, could be but very slight, and because in such young the sex could not always be- determined with certainty. An examination of all specimens makes it probable that .it the i-nd of a year after birth the young are about 50 mm. long. In obtaining the average size of the sexes only ovei qo mm. were considered. The males above this size measure 59.7 mm. on an average, with a maximum of 94 mm. ; the females measure 71.1 mm. on an average, with a maximum of 93 mm. Of the speci- mens over 50 in ; wore males and 22 females, or 100 females for every 104.5 males. Counting the fin rays of I In- firs! i ; spa iim-us over 50 mm. long, we get males, D. 82.1, A. 67.4; females, D. 81.9, A. 68. The average formula for those less than 50 mm. long is D. 83; A. 67.2, or for all together, D. 82.6; A. 07.5. HABITAT OF STYGICOLA. 189 perhaps 0.75 mile wide and less than 10 feet above sea-level. It is such a beach as is shown in figure A, plate n, raised to a little higher elevation. There is here but little sandy soil, the underlying rock coming near the surface. The slope of the hill behind this level stretch is composed of bare rocks very similar to those of plate n, figure A, except that the gnarled roots of the densely growing stunted shrubs and trees twist about the rocks and into the crevices. The character of this area was very well described by my host, who dryly remarked, when I asked him whether I should go on horse to the caves on top of the hills, "No, you will go on your hands and knees." The disagreeable impression that these hills make on one traversing them on foot in the heat of the day is heightened by the innumerable hermit-crabs that lurk in every cranny and scamper over the rocks. At an eleva- tion of about 100 feet is another level stretch of rocks with a thin layer of sandy soil. Within less than a quarter of a mile from the ocean is a natural well, improved somewhat with the chisel. It is circular, with a diameter of about 6 feet and is less than 10 feet deep. It is evidently situated along the line of an original fissure in the coralline rock such as is shown in plate n, figure A, for there are openings in opposite sides of the deeper part of the well that have an indeterminable extent. The surface of the water in this well is near sea-level, about 4 feet below the level of the land. The water, over 5 feet deep, is perfectly fresh and blind fishes were more abundant in this well than in any other area of the same extent. FIG. OS. Diagrams of Cave uf the Insurrcctos and Uie CarburiLTia \Vrll (lig. B, plate u) taken from X. 3. Depression about Mouth of Cave; 2, Dry Cave; i, The Pool of Water near Sea-level, S.L, and with Submerged Stalactites and Stalagmites; r, Side Rifts in Carboneria Well. There are a number of caves on the plateau over the hills and I visited two of these. They are within 4 or 5 miles of the seashore. Their mouths lie at an ele- vation of about 100 feet (87 and 93 by barometer). In general character these caves are like others visited in Matanzas province, i. e., at Matanzas and at Alacranes, or Alfonso xn. They occur in a level area and from a distance there is nothing to indicate their presence. There is first a slight depression in the level country (fig. 68 (3) ). From one side of this depression a fissure, whose upper and lower surfaces are approximately parallel, extends down at an angle of about 45° or more (plate n, fig. B). The slope is in all cases very steep, though not always regular. In horizontal section the walls appear to form sections of a circle so that these caves all suggest fragments of hollow cones. At a depth of about 80 feet 100 BLIND VERTEBRATES AND THEIR EYES. water is encountered in a crescent-shaped pool. The caves extend down for ai indeterminable distance below the water-level. The surface of the water in th< caves is near sea-level. Light penetrates to all the recesses of these caves, one o which is called Cueva clos Insurrectos from the fact that a company of Cuban: was quartered in it during the Revolution. Figure B, plate n, gives a glimpst down the Cave of the Insurrectos from the entrance X in figure 68 to the pool o: water at the bottom, at a vertical distance of 83 feet. These caves are inhabitec by Slygtcola, but in very much fewer numbers than the well near the seashore One specimen was secured. A cave in the side of the hill at the edge of Matanzas shows essentially the same character. The slope is very much steeper and the cave is much smaller. There is the same sort of pool at the bottom as in the Cave of the Insurrectos. I secured no fishes in the Matanzas Cave, though it probably contains them. We were told that into this cave the Cubans, shot during the Revolution, were thrown by the guardians of Matanzas. On the southern slope of the island, both at Alacranes and westward aboul Cafias, are formations very much like each other and very much like the condition represented in figure 68, with these exceptions : the territory is farther from the sea ; the pockets corroded in the surface rocks are much deeper and larger, and are filled with a stiff red clay. Bananas are grown in the pockets of soil about the caves at Alacranes. Aboul Cafias most of the territory is still in its primitive condition, covered with manigua, a straight-stemmed, smooth-barked, but irregular-surfaced, sapling that grows in such abundance mingled with other bushes and vines that it obscures the nature oi the' ground and makes progress through it impossible without the machete. Frequent clearings made to convert the manigua into charcoal and prepare the soil for seed tobacco reveals the nature of ground to be a series of jagged rocks with pits and depressions filled with the aforesaid red clay. The roads through this region are simply trails along which the manigua has been removed. The rocks are in the natural condition or worn a little by the two-wheeled vehicles which alone are usable here. The wheels of these are so large that they bridge most of the pits between rocks. Traveling over the roads in the manigua in one of the two- wheelers is quite a serious performance. Where the soil is a little thicker, tobacco, casava, and other things are grown. I do not know whether the formation is con- tinuous from Cafias to Alacranes, but it seems quite certain that we have to deal with the same sort of structure in both places. It is a raised coral beach somewhat shattered and with a thin, in many cases interrupted, layer of soil. The entire southern slope of the area from Alacranes and Union to Canas is drained by underground streams which, for the most part, are inaccessible. The underground drainage begins further north than the northern edge of the manigua. At San Antonia de los Banos ' a stream is seen to enter the ground, and a few yards Irom tliis place, where the thin limestone roof of the underground channel has given way, the stream can be seen. (See frontispiece.) I ''or reasons to be mentioned at once the streams are inaccessible. In August ol i no i a ury heavy rain caused a small torrent to run in the road leading south from Cafias for a distance of about a mile to the Finca Rosa, where the water d out over a depression of several acres, so shallow that the depression was I'hr rlrvation of llie railroad track is 62.92 m. EIGENMANN Root breaking up into rootlets in Ashton Cave. Young of Lucifuga are found among these rootlets. Cave Isabella, showing group of roots coming through crack in roof. Taken with artificial light. CUBAN CAVE REGION. 191 not perceptible to a casual observer. Mr. Francesco Martinez, who lives within a mile of the place and has been my guide about the caves of Cafias, informed me that the water would all disappear in a day, but that there was no distinct opening to any stream below the surface. Though I have been able to get to the ground- water in many caves about the neighborhood, none of the caves had any intimate connection with an underground stream, for, while the surface water was extremely muddy and abundant and all of it was carried off as rapidly as it would have been in a surface-drained area, the water in the caves to the south, in which direction the drainage flows, remained limpid and showed no appreciable rise. I was told, however, that during an unusual freshet in 1886 the entire region about Modesta Cave became flooded and, naturally, the cave was overflowing.1 The underground streams come to the surface in a series of "ojos de agua." I visited two of these. One of them is in the Cienaga near the Playa of Guanimar. The water simply rises here in a pool 20 feet across in a swamp and is conducted in an artificial canal by the side of a road to the sea. I did not make extensive observations in this neigh- borhood, for the Cienaga has a great number of soft places with unknown depth, from which even the highway with a ditch on either side was not altogether free. One of the ditches showed the ground to be permeated with canals up to a foot in diameter. In this Cienaga many of the southward-flowing subterranean streams find their exit, doubtless others have a subaqueous exit in the ocean; two others are found at Batabano on the coast just south of Havana. On the northern slope the most famous of the exits of the underground rivers is the Vento Springs, which supply the city of Havana with water. I have described these in Science (N. S. xviii, pp. 281-282, 1903). I should say that this spring does not yield half as much water as that at Guanimar. Underground streams and tunneled mountains are not rare in other parts of Cuba, though I have not connected them directly with the blind fishes. I was told a cave passes through a hill west of Matanzas, over which the United Havana Railroad runs. I was also told that at Cardenas, only 10 or 15 miles from the Cave of the Insurrectos, there are underground streams with blind fishes, but this information reached me too late to make a personal inspection. The most famous of the underground streams and tunneled mountains in all of Cuba is the Sumidero which I visited. This region is half a day's travel by horse from Pinar del Rio. I found no blind fishes here, and it is extremely doubtful whether any occur in the main stream which twice pierces mountains in the course of a mile amid the most impressive cave scenery I have seen. In the blind-fish area drained by underground streams the surface water reaches the underground streams through sink-holes, fissures, and "caves." The sink holes are shallow and imperceptible. One at Finca Rosa, I have described above; another is at Aguada on the United Havana Railroad, where, in extreme cases the water rises to stand several feet over the railroad track and then gradually disappears entirely.2 The difference in the nature of the sink holes of 1 Mr. Martinez gave me the following facts: Rain unless protracted makes no impression on the water in the caves — as measured by visual standards. After a rain of 3 days and nights it rises 6 or 8 inches. In 1886, after a long rain of 5 days and nights the water in the well at Isabella rose to within 5 feet of the surface. Ordi- narily it is about 50 feet from the surface. In the Modesta Cave in which the water is normally 15 feet from the surface the water rose to the top and over, till it stood i foot in the house of Modesta, and between the houses at Isabella and Modesta the water was in places 5 to 6 feet deep. The rain water does not run off in surface streams, but all of it sinks into the ground. At the time of the high water the water disappeared from the surface at Modesta in 2 days, while in the deeper places it did not disappear for 5 or 6 days. 2 The lowest part of the land at Aguada del Cura is 45.77 m. above the Nueva R. R. station in Havana. The railroad track is 2.82 m. higher. 192 BLIND VERTEBRATES AND THEIR EYES. Cuba and of Indiana seems due to the difference in the thickness of the soil, which, as stated, is extremely thin in this part of Cuba. In the manigua frequent fissures or narrow wells lead down to the groundwater. There are, finally, the so-called "caves" which also lead down to groundwater. As stated above, the caves at Alacranes are of essentially the same character as those of the Carboneria. There are several of these. I have visited three, but obtained fishes from only two, the "M" and Donkey. Into the deeper parts of one of the caves visited, the Pedregales, light does not penetrate ; stalactites and stalagmites are clear, tinted rosy, and pure in tone when struck. The usual pool of water did not contain any fish at the time of our visit. An amusing incident occurred at this place. Our guide evidently thought our chief object was to view the marvels of cave formations. When we asked whether there were any caves in the neighborhood with fishes in them, he remarked, "Yes, but the fish don't amount to anything, they haven't any eyes." The "M" cave consists, first, of the slight depression in the general surface, and second, of the opening at one side of the depression leading down to the water. The slope is here gentle enough for a zigzag path in the shape of the letter " M " and enables cattle to get to the water at a vertical depth of 83 feet- Light penetrates this cave, and indeed the part directly down from the opening is well lighted. The pool FIG. 68a._ Diagram of the Kentucky Cave Region, after Shaler. ^.Sandstone and limestone showing ordinary topography. B. Sink holes. C. Domes below large sink holes. D. Upper line of caverns first formed. E. Lower line of caverns. F. Cavern filled with stalactite. G. Lowest line of caverns filled with water. H. Masses of pebbles. of water leads off to the left, so that the remote part of the pool is in perpetual dark- ness. This condition makes this cave an ideal place to observe the reaction of the blind fishes to light. As in the Cave of the Insurrectos the caves extend down for an undetermined distance below the surface of the water and blind fish could fre- quently be observed here far below the reach of our id-foot dip net. The Donkey Cave is similar to the "M" Cave, but the descent is steeper and there is a large shallow expanse of water on the left of the shaft of light from the opening. The depression at the mouth of the cave is here g feet below the general surface and the water is reached at 64 feet below the surface. Water was formerly pumped from this cave for purposes of irrigation. The caves about Cafias differ from those of the Carboneria and Alacranes. They are < islern-shaped sink- holes rather than caves in the ordinary sense of the word, but on account of the absence of soil there are no funnel-like depressions on the surlnre to indicate their presence. There is absolutely no general surface indi- -> rib 209 247 248 175 ^64 227 1 60 215 266 262 I87 1 60 224.5 158 211 256 25S 181 9 162.16 210.83 142.58 202.18 = 55.18 252.13 181.2 Gabriel 77 47 47 86 100 61 89 63 63 1 08 69 §3 55 55 104 65 76.65 52-77 57-33 99.22 I04-53 Arroyo Narranjo. . Alquuar Rancho Boyeros.. Santiago de las Cahas House of Finca Isabella . .. Artemisa 124.31 Salud 1 Accepted engineer's determination. The engineer of the United Havana railroads furnished the following eleva- tions of stations in the cave region. The elevations given are above the Villa Nueva station at Havana, not above sea-level. As the line crosses the Western Railroad at Rincon and the elevation of its rails above sea-level at Rincon is 252.13 feet, I estimate Villa Nueva to be 23 feet above sea-level. Locality. Elevation above Villa Nueva. Locality. Elevation above Villa Nueva. 16 Si 12 68 Almendares station 27 06 Melena station Rio Almendares water-level 22.O7 Palenque station Rio Almendares water-level to the face Guines station of the superior rail . . 4 84 Rio Seco station 48 44 Depression of land at Aguada del Cura, Bermeja station 4.C 77 Union station Height above this point to the face of 2 S-> Rincon station 6081 Goven station So 17 So s<: Seborucal station 44 I s Saladriyas station Guanajav station At Canas there is a well in the yard of a store about 100 yards from the railroad station. On August 26, 1903, the surface of the water in this well stood very near sea-level,?', e., exactly 100 feet below the surface of the ground. Mr. A. P. Livesey, general manager of the Western Railroad, kindly furnished me with the depths of 3 wells.1 Tabulating these and the depths obtained in the "He wrote: " Regarding the depths of wells along our line, I may say that these vary very considerably, not only in the different localities, but also during the two seasons, viz., wet and dry, but for your information and guidance I give below the average depths of 3 of our company's wells, which are used to obtain water for our locomotives. They are as follows: Salud, 100 feet; Guira, 50 feet; Artemisa, 80 feet." 196 BLIND VERTEBRATES AND THEIR EYES. various caves, together with the elevations of the mouths of the caves, we get the following results: Stations. Elevation of station. Elevation of surface of groundwater above sea- level. Stations. Elevation of station. Elevation of surface of groundwater above sea-level. Salud 181 Si Isabella Finca caves : re c 5 Open pool at Canas 104 4 Hawey 18 6 Isabella 6e 16 18 2Q 14 8. Frias ^8 8 9. Ashton No i . . . 7O C2 3. Hawey (new) . . 26 2C ii 18 10. Ashton (new). . . 5° 22 12. San Pedro 46 The elevations of the water of the caves together with the well at the house Isabella fall into two groups : first, those from numbers i to 8 in which the elevation of the water does not vary more than 1 1 feet. This amount may easily be due to change in barometric pressure during the various readings and to the personal equa- tion. It seems probable that the water in these caves, most of which are south of Finca Isabella, is at a level, and that this level is between 8 and 19 feet above sea- level. The Finca Isabella is about 15 miles north from the coast, or 10 miles from the Cienaga, in which some of the underground rivers rise to the surface. The second group, from 9 to 12, are east of Isabella; 9 and n are near each other; 12 is 2 miles or more east of 9 and n, and I am not certain about the loca- tion of 10. These readings were taken August 25, 1903, in the order: 10, 12, n, 9. The first reading at the house was at 6h 30™ a. m., when the barometer stood at 1,114 feet- The trip consumed all of the morning. About 3 p. m. the barometer stood at 1,179 at the house, so it is very probable that the high elevations may in part be due to the change in barometric pressure. For comparison we have the data for the caves, Adolfino and Insurrectos, at the Carboneria, near the north coast. The Cave of the Insurrectos is about 93 feet above sea-level according to barom- eter. The water is 83 feet below the surface and according to that 10 feet above sea- level.1 The top of Adolfino is 87 feet above sea-level, the water is 80 feet below, or 7 feet above sea-level. The surface of the Carboneria well is about 4 feet above sea-level, the surface of the water is at sea-level and more than 5 feet deep. There is every indication that the water has risen about 10 feet in the caves in very recent geologic times. In all the caves stalagmites are seen to rise out of the \v;iler, in some cases from a depth of at least 10 feet. . As these could only have been formed on ground free from water it is evident that the water must have risen in the caves. As the water is now near sea-level, this rise is probably due to the subsidence of the western end of the Island of Cuba. This subsidence is general, as M;ilagmites are found submerged on the northern and southern sides of the island. 1 The water in the Donkey and " M " caves, according to barometric readings from the railroad station at Union, •ra li-vH. It is not at all probable that this reading is correct, but it indicates that the groundwater is hen- again very nr t ea-Ii rel. At " M," according to barometric reading, it is 83 feet below the gen- nal Ic-vi-1 ni' i In- surface and at the Donkey it is 73. EIGENMANN PLATE 13 B D A. Drawing of black individual of Stygicola. B, C, D. Stygicola. Photographs of preserved specimens. ORIGIN OF CUBAN BLIND FISHES. 197 ABUNDANCE OF STYGICOLA AND LUCIFUGA. The number of fishes in any cave differs very greatly. They are rare in caves entirely inclosed ; in those entirely open and not connected with hidden recesses they are also very rare or absent. They are most abundant in caves with both well- lighted and dark portions and those that are continued subterraneously. The den- sity of the distribution of the fishes evidently varies greatly, directly with the food supply. The food supply itself varies with the openness of the cave to the external world. The question arises whether the caves visited are independent pockets or form part of a continuous underground system of channels, and whether the fauna of the caves visited may be easily exhausted or continuously replenished from the extensive subterranean channels and reservoirs. Collections made in the same caves indicate that there is an undoubted decrease in the numbers and that the decrease is not usually compensated by immigration from the underground reser- voirs. It has rarely proved worth while to visit the same cave twice on any of the stays in the cave region. The results of three visits to the "M" and Donkey Caves on October 25, November 2, and December 23 illustrate the point. In 1904, I secured 15 fishes in the Donkey Cave on October 25 ; 5 on November 2 ; and 3 on December 23. In the "M" Cave I secured 20 in March, 1902 ; 19 on October 25 ; 14 on November 2 ; and 9 on December 23, 1904. Equal efforts were made on each occasion and an equal amount of time was given to the caves. On June 24, 1905, Mr. Haseman secured 4 fishes in the Donkey Cave and 7 in the "M." The Donkey thus yielded 15, 5, 3, and 4 fishes respectively, on succes- sive visits; the "M," 20, 17, 14, 7, 7. Both of these caves are with deep recesses in which fishes could be seen but not secured. THE ORIGIN OF THE CUBAN BLIND FISHES. Without doubt the remote ancestors of the Cuban blind fishes lived in the ocean and were adjusted to live in the light and to make use of it in detecting their food, their enemies, and their mates. Equally without doubt, their less remote ancestors became adjusted to do without light and lived in total darkness, either at a depth in the ocean or more probably in the crevices in Cuban coral reefs. If in the former, they entered the subaqueous exits of Cuban rivers ; if in the latter, they are older than the rivers themselves, having remained in their original habitat in the crev- ices of the coral reefs as these were elevated to their present and even greater heights. The latter seems to me the more plausible theory. The fresh-water blind fishes of Cuba are as old as the parts of Cuba they inhabit. They are part of the result of the formation of the island. The deeper recesses of the crevices and rents in the naked reef at the Carboneria already described are probably now inhabited by fishes of some sort, possibly by Ogih'ia among others. Attention has been called to the fact that within less than a quarter of a mile from them, in a coral reef raised only 4 feet above the ocean-level, there is a rift essentially like those found in the naked reef skirting the ocean. This rift contains fresh water, and blind fishes are abundant at a place where a circular opening has been cut to form a well. It is entirely within the range of probability that the ancestors of these fishes lived in this rift when it was 5 feet lower and contained salt water and that they 198 BLIND VERTEBRATES AND THEIR EYES. gradually, as the reef was raised, became adapted to fresh-water conditions. But if this rift with its well contain descendants of its original marine inhabitants, there is no reason why the same should not be true of the wells and caves and rifts of the more elevated coral reefs of Cuba. In other words, there is no reason why the blind fishes should not have developed over the entire area and risen with the entire area over which they are now known to be distributed. Stygicola is found from Canas at least as far east as Jovellanos ; Lucifuga only west of Guira and at least as far as Canas. There is, furthermore, no special reason why the blind fishes which have been reported from the natural wells at Merida in Mexico and from Jamaica should not be identical or related to the Cuban species, why they should not have been independently derived in different places from one or more species widely distributed in cracks and crevices of coral reefs. PHYSICAL ENVIRONMENT OF STYGICOLA AND LUCIFUGA AND THEIR REACTIONS TO IT. LIGHT. Whatever conditions may have been in the past, at the present light is entirely absent from some of the places inhabited by the blind fishes while others are as well lighted as any stream. In the only cave I entered which light does not penetrate, the pools of water, in every respect similar to those in other caves, contained no blind fishes. On the other hand, in Ashton Cave, parts of which are as well lighted as any stream, blind fishes live side by side with eyed fishes. In a few of the best- lighted caves no blind fishes were found, but in Modesta, where an n-foot opening in the ceiling lights a space 35 by 45 feet so that pebbles and fishes can be seen with perfect distinctness at a depth of water of 10 feet and more, blind fishes are abundant. The same is true of similar caves, well or partially lighted. Blind fishes were abundant in Tranquilidad, a dark cave into which light pene- trates through a narrow shaft over 20 feet deep and then only illuminates the margin. They were also abundant in the open well at the Carboneria, about 6 feet in diameter and with a total depth of about 10 feet. It is to be emphasized that blind fishes are abundant in well-lighted caves only when these are connected with underground channels that extend into the dark. Such caves contain many more fishes than caves that are totally dark. The reason for this lies entirely in the much greater abundance of the food supply in caves open to the surface; the lighting of the cave is incidental. The reaction of the blind fishes to light can be as well studied in the " M " Cave as in any aquarium ideally constructed for the experiment. The pool of water in this cave varies from 5 to about 20 feet across, and from a few inches in depth to many fed - certainly over 10 feet and possibly 50. The pool is probably between 150 and 300 feet long. A direct shaft of light reaches the pool near one end so that the water is well illuminated within this shaft. The right end, near which the shaft ot light reaches, is shaded by rocks and is so dark that a lamp is of distinct assistance in exploring its 2 to 3 feet of depth. The other end of the cave is in total and per- petual darkness. Fishes are abundant in this cave. I have seen very few within the shall ot light and most of those were driven there by my movements. In tin- shade of the rocks to the right, on the contrary, they are abundant, and in the larger dark parts of flu- cave to the left they arc also abundant though relatively less so than on the right. I lere we have a very distinct reaction to the light — all the fishes CAVE ENVIRONMENT. 199 avoiding it. Cattle come down to drink in this cave within the shaft of light. The indirect result of this is a great abundance of blind-fish food. In the movements and distribution of the fishes in this cave we have a clear balance struck between the positive attraction to the food and the negative response to the light. The same reactions demonstrating perception and tropic relations to light are seen in the Donkey Cave near by. In this cave I have never seen a blind fish within the shaft of light, but have seen and caught them in numbers in the expanse of shallow water in the shadow and total darkness to the left of the shaft of light. While fishing in Ashton in December, 1903, I caught 3 specimens in the lighted part of the cave and about a dozen in the dark recesses to the right of the entrance. Unfortunately, on account of the difficulty of getting about over the jagged country, I have been able to visit but few caves at night, but the observations in the Carboneria well were exceedingly instructive. A few bushes growing over the well shade it to a certain degree. As stated elsewhere, poles and fence rails were placed slanting into the water crossing each other and in sufficient number to form a teetering foothold that enabled me to stand waist deep in water. From this position every part of the well was within reach of my net, except pockets in the sides too small for the net and the indefinitely extend- ing side rifts I have mentioned. On visiting the well about 9 a. m. perhaps as many as 10 stygicolas were seen swimming about or resting on the wood or sides of the well. I entered the well but succeeded in catching only one fish ; the others readily escaped either by making for the dark side rifts or by hugging the walls of the well and entering the small pockets where I could not get them. There seemed to be no hesitation in their actions. I again entered this well the same night. Liberally discounting the result for the experience already gained in entering the well and knowledge of the location, the result alone is evidence of a distinct difference in the actions of the fishes at night and in the day — I caught twelve. Their actions were quite different. While in the daytime they seemed able to locate the dark recesses and make for them with precision, their action at night gave distinct evidence of confusion and lack of ability to readily escape. They could be easily followed with the pencil of light from the lamp and picked up with the net. TEMPERATURE. The fluctuations in the air temperature of caves with small openings are, in a climate like that of Indiana, reduced to a few degrees Fahrenheit, and must be re- duced to a minimum in a climate like that of Cuba. The temperature of the water will also fluctuate but little. The air of caves that are open like that of Ashton will, on the contrary, fluctuate to nearly the same extent as that of the epigean neighborhood. The nights of the Cuban winter are cool and the temperature of the water in the open pools of these caves may be reduced a few degrees. No direct observations are at hand on this point. The temperature of the water in 18 caves containing fishes, taken in June, 1905, showed a total range from 74° to 76.5° Fahrenheit. Only 2 caves had a temperature as low as 74°; 3 of 75°; 5 over 75.5°; 6 of 76°; 2 of 76.5°. 200 BLIND VERTEBRATES AND THEIR EYES. Observations between August 22 and 25 showed slightly higher temperature for open caves, thus: in June, 1905, the temperature at Bafios was 75.8°, at Ashton, 75.6°, and on August 25, 1904, it was 77° at Bafios and the same at Ashton. In the "M" Cave, a closed one, the temperature was the same, 75° Fahrenheit. The blind fishes are adjusted to withstand slight fluctuations in temperature. Some were kept in aquaria and the water became distinctly chilled over night and warmed during the day. While they lived for several days in these aquaria, they were always sluggish or numb in the morning. A more distinct reaction of the same sort was noticed in the only fish I succeeded in bringing home alive. It could scarcely move after an early September night in Indiana. A still greater reaction was noticed in several I succeeded in bringing alive to Louisville and which suc- cumbed to the frosty weather on the way from the depot to the hotel. TRANSPARENCY OF WATER. In all caves in which collections were made the water is clear as crystal. It will easily rank with the water of Lake Tahoe and of the limestone springs of Florida, as among the most transparent natural water in the world. Fishes can readily be seen at depths of 15 and 20 feet or more, with the aid of a bicycle acetylene lamp. The water at the Vento Spring is of the same nature, but I was informed that it becomes slightly roiled after heavy rains. CHEMICAL COMPOSITION OF WATER. The water is everywhere highly charged with salts of lime and magnesium In all cases where the surface of the water is not disturbed by breezes, a crust of these salts forms like a thin ice over the surface of the water. When one disturbs the water, the crust breaks up into small fragments which fall through the water like snow through the air. Occasionally a larger flake, a foot square, may fall to the bottom ; sooner or later they are dissolved again. With falling of the level of the water some of the crust is left on shore and gives an index of the amount of rise and fall in the water during a year. FLUCTUATION IN AMOUNT OF WATER. The ordinary fluctuation in the amount of water in the caves is very small - about one foot during a year — judging by the flakes of lime left on the banks. I have mentioned elsewhere that, after long-continued rains, water flooded the entire region about Modesta, the cave was full to the top, and the water stood several feet over the ground. All of this retreated in a few days. Such fluctua- tions arc very rare. SIZE OF ENVIRONMENT. Concerning the size of the environment little can be said. The pools accessible arc easily measured, none of them exceeding a few square meters in surface, but the si/c of the underground connections is naturally unknown. The rapid disappearance of the water after heavy rains indicates extensive underground channels. EIGENMANN PLATE 14 Living Stygicolas. Position of body and fins m swimming and differences in color of different individuals. EIGENMANN PLATE 15 sri !•? D_ O 03 £j — — 3 £1) 3 "" OQ O §- rc, o •< o ^* <» • o •' ^i p prj O^ Z O £» rt d ,"Tr" 0) 3 *< Z. era -—, C n £: era x "2.3 TO S. S o S ? O" vj 3" n . n r o CAVE ENVIRONMENT. 201 BIOLOGICAL ENVIRONMENT OF STYGICOLA AND LUCIFUGA. ASSOCIATES. During March small frogs are found abundantly at the margins of the pools in some of the caves. I do not know that these affect the lives of the blind fishes in any way. Tadpoles were found in the Carboneria well. It is possible that these may form some part of the food of the fishes during some seasons of the year. They are but casual associates of the blind fishes in some of the caves. Fishes other than the blind ones were found in Ashton and some of the small open caves about Modesta. They were all Girardinits uictallicus Poey, a species very abundant all the way to Pinar del Rio. The female reaches a maximum length of 79 mm., but is usually much smaller; the maximum length of the male is 45 mm. The largest specimens taken in Ashton are 41 mm. and 38 mm. These fishes are active swimmers, living near the banks, and while a few may be cap- tured by the blind fishes, they are themselves too small to attack even the young of the blind fish. FOOD OF STYGICOLA AND LUCIFUGA. The blind fishes are carnivorous, securing living prey. Their food consists largely of 4 species of crustaceans, 3 of which are blind cave forms. Probably every living animal of the proper size is used by the blind fishes for food. Cirolana cubensis Hay. This species was described by Hay in Proceedings of the National Museum, vi, page 430, as follows: Body oval, a little more than twice as long as broad, widest a little behind the middle, rather strongly convex, and perfectly smooth. Head a little broader than long, slightly produced in front. Mesosome broader, with its greatest width at the fifth segment; coxal plates of the second, third, fourth, fifth, and sixth segments successively more enlarged and more strongly produced backward at an acute angle. The plate of the seventh segment is about the same size as the one preceding it. Metasome narrower than mesosome, of five segments, each of which, except the last, has the lateral angles strongly produced posteriorly ; telson as long as the metasome, its margins gently curved and convergent for about two-thirds of its length, and then rather abruptly strongly convergent to form a short, obtuse tip. The eyes are altogether wanting. First antenna with three basal seg- ments and a short flagellum which, when extended backward, reaches slightly beyond the posterior margin of the first thoracic segment. Second antenna with five basal segments, and a long, slender flagellum which may extend slightly beyond the middle of the body and is composed of about twenty-nine segments. The mandible, maxilla?, and maxillipeds do not present specific characters of importance, being of the type usual in the genus. The appendages of the mesosome are of mod- erate strength, and are armed with a few rather stout spines and stiff seta?. The branchial append- ages of the metasome are membranaceous and small; the uropoda are well developed, the outer branch lanceolate in outline, the inner much broader and very slightly longer, and with the tip somewhat accuminate; both branches and the margins of the telson as well bear a rather dense fringe of hairs. Color in alcohol, white, with no markings of any kind. Length, 5 mm. Of the species of Cirolana known to inhabit American waters, C. mayana, which occurs on the coast of Yucatan and Colombia, is the nearest relative of the present species. Between the two, however, there are several important structural differences. The physiological differences between this species and all the others of the genus must be very great to admit of its living in the subterranean streams of fresh water. It may be added that Cirolana ciibcnsis is very distinct from Cirolanides texensis Benedict, which occurs in the waters which flow from the large artesian well at San Marcos, Texas. 202 BLIND VERTEBRATES AND THELR EYES. This species is everywhere abundant and may attack the fishes if it succeed in attaching itself to them. I have not caught any fish with them attached, but in small aquaria in which many of them were placed as food for the fishes they soon turned the tables and fastened themselves upon the fishes. In some of the caves cirolanas exist in vast numbers. At the base of the shaft of Tranquilidad they were so numerous and voracious that it was impossible to stand in water long enough to light our lamp. They fastened themselves in numbers on the feet and went to work with such a will that it was impossible to stand still. Palcemonetes eigcnmanni Hay. This extremely slender and graceful shrimp is abundant in all the caves. It is essentially pelagic in habit, though it is frequently seen resting on various objects on the bottom. Its eyes have been described by Pike. The species was described as follows in the Proceedings of the U. S. National Museum: Carapace thin, very delicate and transparent, in form slightly compressed near the middle of the body but rather broad anteriorly ; the anterior border, below the eye, is produced as a broad, obtuse angle, which bears, near its lower margin, an acute, forwardly directed spine; this spine is the anterior end of an obscurely marked ridge, which extends obliquely downward and backward along the sides of the carapace. The rostrum is long, slender, compressed, and rather markedly upcurved; on its superior margin it bears a row of 6 or 8 slender, acute teeth, which begins well back on the carapace and extends forward to the rostrum; these teeth are directed obliquely for- ward ; the inferior margin is unarmed ; the tip of the rostrum is acute and reaches forward to a point opposite the distal extremities of the antennal scales. The eyes are much reduced in size, are with- out pigment, and the corneal surface comes to an obtuse point in front. The first antenna has the basal segment well excavated above and provided with a small, acute spine at the outer distal angle; there are two long and one short flagella, the short one slightly exceeding the rostrum, the long ones somewhat longer than the body. The second antenna has the basal segment provided with a small spine near the distal end ; the antennal scale is broad and with subparallel margins ; the tip is slightly rounded, and there is a small, obtuse spine at the outer distal angle; the flagellum is slender and about twice as long as the body. The mandible has an incisor portion with three or four sharp teeth, a small molar surface with several obtuse teeth, but is without a palpus. The third maxilliped is not strongly developed and presents no characters of importance. The first pair of pereiopods is chelate, and except for its much smaller size is exactly like the second ; the chela is slender and weak ; the carpal segment is long and slender; the meros is of about the same length, but stouter; the remaining segments short and rather thick. The remaining pereiopods are very long and slender. The abdomen is of the form usual in this genus, but the sixth segment is neither elongate nor com- pressed; the telson narrows gradually from the base to the obtusely angulate tip; on the upper surface there is on each side at about the middle and again about one-fourth the distance from the tip ;i small, appressed spine; at the tip there is on each side one minute and one long, slender spine, and in the middle' a fringe of setce. Color in alcohol, white. Length, 23 mm. They differ very markedly from Palamonetes anirorum Benedict, hitherto our only known blind l'(ilirmi»i<-lcs, in the shape of the rostrum and the character of the chelae. The shape of the eye is rather remarkable, even in a group, where through atrophy the eye tends toward the conical form. I know of no other in which it is produced into a blunt point. So far as I have been able to ascer- tain, this is the first record for this genus in Cuba. In the material from San Isidro there is one speeimen which agrees in every way with the types, but the other two differ in such a manner as to lead me to believe that a second species may be found to inhabit the subterranean waters of Cuba. The two specimens just mentioned have the sixth segment of the abdomen 2.5 times as long as deep, and the antennal scale is more slender and acute. Unfortunately, the rostrum of one is entirely gone, while nf the other only the abdomen remains. BIOLOGICAL ENVIRONMENT. 203 Epilobocera cubensis Stimpson. This crab, which reaches a width of several inches, was observed in many of the caves. It is probably found in all of them though not in great abundance. If the adult affects the blind fishes at all, it is to feed on them. I have found the young of this species in the stomach of Stygicola. Gammarus caecus Weckel. The following technical description will be found in Proc. U. S. Nat. Mus., xxxn, page 47. Eyes absent. First antenna? more than half as long as the body; second segment of the pe- duncle slightly longer than the first and about three times as long as the third; flagellum composed of twenty to thirty elongated segments, each bearing a few short hairs at the distal end; secondary flagellum reaching slightly beyond the third segment of the primary flagellum, composed of four segments, the distal one short and furnished with long hairs. Second antenna? are about two-thirds as long as the first pair with the peduncle extending far beyond that of the first pair; ultimate seg- ment of the peduncle only slightly longer than penultimate which is greatly elongated and about equal in length to the antepenultimate; flagellum composed of about twelve segments, which are shorter than those of the first antenna? and furnished with more hairs. The carpus of the first gnathopods of the male is triangular and elongated, with the anterior margin furnished with a few long hairs and numerous short ones; propodus narrower than the car- pus, twice as long as broad, with the anterior margin concave, armed sometimes with a fascicle of hairs, the posterior margin convex, and the palm almost straight, slightly convex, and armed with four or five spines and a few short hairs; dactyl as long as the palm and fitting it closely. Second gnathopods with a carpus broader than in the first pair but similarly armed; propodus not so broad as the carpus, about twice as long as broad and larger than in the first gnathopods; posterior mar- gin almost straight ; anterior margin slightly convex and usually furnished with one or two fascicles of hairs; palm very oblique, slightly concave at the center, armed with five or six spines at the tip of the closed dactyl, and one or two spines and a few short hairs on the margin; dactyl strongly curved, as long as the palm. Both margins of the coxal plates of the third, fourth, and fifth pergeopods are serrate and fur- nished with spines, those on the anterior margin being smaller than those on the posterior. Postero- lateral angles of the third and fourth abdominal segments are produced backward and end in a blunt tooth. The last two or three abdominal segments are furnished dorsally with a few short spines. The first uropods project slightly beyond the second pair. In both pairs the rami are about equal in length and slightly longer than the peduncle. The third uropods were broken off in the few specimens which I had for examination. Telson cleft to the base, armed distally with a few short spines. I found this blind amphipod in Modesta in the roots of trees. It was not abundant and was not observed in any of the other caves in which no special search was made for it. It was hidden among the rootlets of Ficus in a way in which it would not be noticed unless special care was taken to look for it. It is quite probable that it may be found in many of the caves. In addition to the above mentioned species dragon-fly larva" were found in the stomachs of some of the fishes. PLANTS. In parts of Ashton a green alga forms a dense mass over many square feet of bottom. Young lucifugas are abundant in the alga, but this is the only instance of its occurrence in association with blind fishes and it scarcely deserves considera- tion as part of their normal environment. The only plant worth considering as forming part of the biological environ- ment of the blind fishes is the tree sending roots to the water. The roots break 204 BLIND VERTEBRATES AND THEIR EYES. up into innumerable rootlets harboring numberless cirolanas and many young and small lucifugas. These trees are found in all the caves of the Canas region. The roots sometimes extend vertically as much as 40 feet before striking water. At other times roots run along the ground down the slope of the cave as in Ashton, finally breaking up into rootlets (fig. 70 and plate 12). GENERAL HABITS OF LUC1FUCA AND STYCICOLA. The position in the water and action of body and fins in swimming of Stygicola are amply indicated in plate 14, which is from instantaneous exposures on fishes confined in a 5-gallon aquarium. It is seen that the posterior part of the body moves from side to side, eel-fashion. The long dorsal and anal fins move in the same way, waves of motion passing from in front back. These fins, on account of this motion, are not well shown in the photographs. The pectorals move in- dependently of each other. One may be forward, the other back. They are used in guiding largely. When the fish is swimming very slowly, the wave-move- ments passing along the dorsal and anal fins are the chief means of locomotion. In swimming rapidly the motion of the body comes chiefly into play. The fishes swim indifferently up or down, with the back up or lying on their sides. The ac- tions of Lucifuga are essentially like those of Stygicola. These fishes are much more readily disturbed than Amblyopsis of the Indiana caves, and when disturbed they swim swiftly in a less distracted way. On the whole they are much harder to catch than the Amblyopsis. The action of the stygicolas in the Carboneria well in daytime and at night has been detailed. Two instances that seem to indicate that fishes "remember" localities must be put on record for what they are worth. One of these is of a fish at the right end of the "M" Cave, and the other in the left, dark part of the Donkey. In the "M" Cave the same fish, three times within an hour and a half, apparently made straight for an opening under the wall of the cave and escaped. In the Donkey Cave the same thing happened about a big stalagmite that rises out of the water. Several times within half an hour the fish came out, but each time it darted back among the nooks in the stalagmites with apparently as much decision as a mouse in seeking its hole. Perhaps in both cases the action was a n-action merely to the vibrations set up by my net. Perhaps the location of the solid stalagmite and the wall were perceived by the approaching fish and the escapes into nooks below the wall were simply necessary sequences in following along the solid wall until an opening was reached. Whatever it was, the repeated escape of the two fishes was as interesting as it was aggravating. Very frequently when dis- turbed they descend in the water and escape into depths beyond the reach of the net. The character of food has been detailed under the head of Biological Environ- ment. I am unable to give any direct observations on the securing of this food. BREEDING HABITS OF STYGICOLA AND LUCIFUGA. In M;irch of 1902, on my first trip, Mr. Riddle secured a female lucifuga con- taining 4 young, lacking but 3 or 4 mm. of being as long as the smallest lucifugas caught in the caves (plate 15, fig. c). This was the first intimation we had that these l'ishe< are viviparous. No other embryos were obtained at that time. An examination of the ovaries of all the females caught and the size of the young led me to suppose that March was the close of the breeding season. With the grant BREEDING HABITS. 205 from the Carnegie Institution I expected to remain in Cuba during the entire breeding season to secure a full series of embryos and to rear young in the light. Unfortunately for this plan the fish seem to have no general breeding season, and the appropriation was exhausted in determining that fact. I visited Cuba late in October, which was supposed to be the beginning of the breeding season if March was the end, but there was no indication that this time was near the breeding season. I had collections made early in December and again visited Cuba late in that month. But while, as before, there were indications that some individuals were ready to breed, there was no indication of the approach of a general breed- ing season. I next had collections made the first week in May without results. I revisited the caves late in August and early in September and finally, near the end of June, sent two of my students, Mr. John Haseman and Norman Mclndoo, to the caves. The former had accompanied me on one of the trips, and both were in every way thoroughly competent to get everything possible. To summarize : The caves were examined by myself and Mr. Riddle early and late in March, 1902; by Mr. Martinez early in May, 1903; by Mr. Haseman and Mr. Mclndoo late in June, 1905 ; by myself and Mr. Hankinson late in August, 1904 ; by myself and Dr. Beede late in October, 1904 ; by Mr. Martinez in Decem- ber i, 1903; and by myself and Mr. Haseman late in December, 1903. The net results of these numerous trips for Lncifiiga are : Late in March I secured one female with young about 20 mm. long, or nearly ready to be born; the ovaries in most of the other females were minute, the largest eggs measuring 356 ju, ; in two ovaries there were eggs 560 fi and 850 /u. in diameter, both of these containing spermatozoa. Late in June a female with 15 young, 12 mm. long, was obtained ; the ovaries of the remaining fishes were small. On August 23 a female with 10 nearly grown young was obtained. The ovaries of all the others were min- ute. Late in October and December the ovaries of all females secured were minute. The young from the female in March were at least 3 months old. This would give a breeding period whose outside limits would extend from December to the end of August. The examination of numerous ovaries does not indicate a general breeding season, though a larger per cent contained large eggs in March than in other seasons. The best season to get material is probably March to May. The net results for Stygicola are : In March the ovaries of Stygicola are mostly small, with eggs not exceeding 200 /A. One female taken at this time contained eggs 600 to 700 p- in diameter and her ovary was abundantly supplied with spermatozoa. In May no mating females were secured. In June the ovaries were mostly minute. Two of those secured contained turgid ovaries in which the structures were distinctly lobulated. On September i, I obtained a female with one young from the Carboneria. Other females had large ovaries, probably recently freed from young. Most females had small ovaries. One contained large eggs. The rest contained small eggs. On October 30, I obtained a Stygicola from Alacranes containing two young. The mother was 92 mm. long and her ovary contained eggs 880 ^ long, which were evidently mature.1 At the same time I obtained 47 other females from 77 to 1 In an ovary containing spermatozoa in abundance, days if not months before the ripening of the eggs, an occasional early ripening should naturally result in the development of the embryo. The present case is probably one of this sort. Two eggs evidently started to develop long before the others were mature. The ripening of the eggs at different times may lead to different sized larvae in the same ovary unless the earlier larv:e digest the sper- matozoa present before the other eggs become ripe. 206 BLIND VERTEBRATES AND THEIR EYES. 115 mm. from Alacranes and Canas, in all of which the ovary was empty and in most cases at its minimum. In December all the ovaries but two were minute. In one ovary a single large egg 720 p was found, in the other the ovary was large and the eggs reached a maximum of 640 p. Thus, nearly mature eggs were found in December and March, and young in September and October. If the species breed annually and irregularly throughout the year and the young are carried but 3 months, at least one-fourth of all the females caught at any season of the year should be with young. If the young are carried but 2 months, one-sixth of all the females should be with young. If the species breed at some definite season of the year and this period is not more than 3 months long, all of the females should be with young near the middle of the breeding season. The results are wide of any of these marks ; and the only conclusion possible is that either there is no definite breeding season, but individuals breed at any time during the year, or the fishes breed only at longer intervals than a year, and in either case while breeding they migrate to undetermined regions. That these regions are not far away is shown by the fact that occasionally breeding females reach the upper accessible parts of the cave. Between breeding times they are found in the upper, readily accessible parts of the cave. I found that while Amblyopsis probably breeds throughout the year a larger per cent breed in March than in other seasons. A similar condition may exist in the Cuban blind fishes. THE OVARIES OF STYGICOLA AND LUCIFUGA. The minute structure of the ovary of Lucifuga is elsewhere described. The ovary consists of a pair of delicate walled sacks united behind and with the ovif- erous tissues attached along the middle of its dorsal and ventral wall except for a short distance behind. It is placed in the mesentery between the dorsal wall of the body cavity and the rectum and stomach. In enlarged ovaries the oviferous tissue is seen to be lobulated, the lobules being attached anteriorly and free pos- teriorly. These lobules are arranged like shingles, the anterior ones overlapping the posterior ones. When the ovaries contain no larvae or ripe eggs, they extend far forward, the posterior oviferous tissues reaching but little behind the stomach. \Yhrn eggs mature, the ovary becomes turgid and the oviduct apparently shortens, so that the posterior part of the stomach comes to lie in the fork near the anterior end of tlie ovary. 'I 'he spermatozoa are evidently, as in Cymatogaster, which is another vivi- parous fish, transferred to the female long before the eggs are mature. When mature the eggs are probably 850 /A in diameter, or even larger. Spermatozoa were found in an ovary containing eggs but 560 p. in diameter. The number of young found in Liir/fn^n were 4, 15, and TO respectively. The young were nearly all turned with their heads toward the front of the ovary, a condition duplicated in the ovary of Cymatogaster with nearly mature young. The condition of the young in the ovary with 4 young is well shown by the photograph (plate 15, fig. c). There were 2 young on each side. The largest eggs in this ovary were 200 /* in diameter. BREEDING HABITS. 207 The condition in a female 90 mm. long containing 15 young, about 12 mm. long, was as follows: there were u on the left side, one of which had an ovarian lobe in its mouth, and several had the gill covers hooked over ovarial lobes, the rest being free in the cavity (plate 15, fig. D). There were 4 on the right side, one of which had the head turned to the rear, and one was so firmly attached to the ovarian lobe by the gills that it was practically impossible to get it loose without damage. One ovary of Slygicola containing 1 1 large eggs, at least one of which is free in the ovary, is distended much more than the few eggs would warrant, being 16 mm. long and 12 mm. wide. The outer tunic is quite thin. The eggs are nearly of the same size and measure 848 p, in diameter. The general features of the ovary of this species are given in plate 27, fig. A. The details of the structure are given in another chapter by Lane. 208 BLIND VERTEBRATES AND THEIR EYES. THE EYES OF LUCIFUGA. The snout of Litcifnga is broad and depressed to the posterior edge of the max- illaries — duck-bill shaped. The eye is distinguished without difficulty in the trans- lucent living individuals, and even in specimens preserved in formalin or alcohol it is readily distinguished up to very old individuals. In the older specimens the skin over the eye readily discloses the location of the organ. There is over the eye in these specimens a hemiovate elevation sepa- rated from the rest of the skin of the head by a distinct groove. The skin in this ovate arch is not any less abundantly supplied with pigment than any other part of the head, and there are no other distinguishing features to indicate that it is better adapted to admit light than any other part of the skin of the head. In some cases it is even more densely pigmented than neighboring regions. The region is proportionately larger in young individuals than in old, but is more conspicuously de- marked in the older than in the young. Removing the skin shows that beneath the ovate arch lies a mass of orbital fat, approx- imately in the center of which the eye lies embedded. The orbital fat-mass seen from above has an oval shape, considerably longer in the axis of the head than transversely. Behind, the mass touches the orbital process FIG. 72. (A) Outline Camera Drawings of Eye of 4 Young of Female shown in plate P , ,. . . ~, from Sides, Left Eye on Left, Right Eye on Right, so that Middle of OI tllC frontal DOIie. 1 llC 6y6 IS placed approximately over the middle of the maxillary. Pairs is Anterior, a. Fish 18 mm. long; b, 18.5 mm.; c, IQ mm.; d, 20 mm. For details of these Eyes see figs, plates 16 to 18. 16 mm. and 4. (B) Eyes of Mother of 4 Young, shown in A, drawn to Same Scale: a, from above; b, from sides. For sections, see plate 21. (C) Outlines of Eyes of No. 95, a Fish 53 mm. For sections see plate 20 < :i!nl plate 24 A, 16 mm. and 6. The proportion of the or- bital space or socket occupied by the eye differs greatly in individuals of different sizes. In younger individuals, just about to be born, the eye fills a large part of the socket (plate 16, fig. B), while in the old it forms an insignificant dot in a mass of fat and connective tissue, hundreds of times larger than the eye (plate 21). The relation of the eye to the surface is similarly conditioned with age. In the young it lies near the surface, while with age it becomes farther and farther removed, retaining however its relative position in the orbital fat-mass until old when possibly it may move nearer to the skull. Seen from the surface, that is without sectioning, the eye presents great fluctu- ations in size. These are in part conditioned by the size of the individual, but in part are independent of size. Other things being equal, the eye decreases in size progressively from birth to its disappearance in extreme old age. This process is accompanied by, if it is not responsible for, the appearance of pigment masses. These are either intimately associated with the eye, as in the development of great EIGENMANN pa > " ^ S S. 3-1 I! IT CfQ 3™ 3 *-" re ^ TO gr 3 3" g_^ N' Q- r& n> o " a, -T o -o •< n re ^ ° - a, w x§' ° „ s g3 ^ ^ I' F 3 S- S^ TO a o: ^5' § -a. d- X ^ < tf ^ 1 §"- 2_ e ui y* *"* ^i •2. ^% o' re o 2 §' ^«W9hr /! A ^ » 4y •*? \ ^-^"^ V X • • > *®ntet ? v EIGENMANN PLATE 1 7 03 > O 01 (I o *" J §' 3 g n S. X-3^ W s r ^ o » " rt 1 EIGENMANN pa > ^_X TO3 £ § >l e! rD o_ 03 XS£ "~i f» 2 t ts ^j 2. ™- o 02 B> o' 03 " 3 O W <-!• p§r B»- ^ .. ™ O • u-~* % §-3 m p n> |- ^XZ ^ 3 ^P 2 31- ^J^i o S ^S ^ 8 ^ - S2 3" S-; 5 o c =r g w 3 S" n = (t ft •D O- > V * %•. • *?W5S3 '•'*^i i V, , j t, ^C t ^ -& E1GENMANN F'LATE 19 * Eyes of Lucifuga, 25 mm. long. A. Right eye, showing vesicular arrangement of pigment layer and retina and folding of sclera. From above. X I 00. B. Left eye, shriveled and sclera similarly folded. EYE OF LUCIFUGA. 209 pigment cushions on the eye, or in extra ocular regions at times in contact with the sclera, at other times in the orbital fat some distance removed from the eye. While, other things being equal, we find a progressive decrease in the size of the eye with age, we do not find that individuals of the same size have eyes of the same size. On the contrary, the eyes of individuals of approximately the same length may be very different in size and, as we shall see later, in structure also. For instance, of 4 young taken from the ovary of one mother and differing from each other by not more than 2 mm. in total length, we have the eyes of two individ- uals without a lens and the eyes of the other two with large lenses. The eyes measure 272, 320, 384, and 416 p respectively, or, after clearing in xylol, which permitted a more minute measurement, 260, 280, 375, and 425 (fig. 72, A). De- tailed measurements of these eyes will be found in the following table: Measurements in //. of Eye of Female Litcifnga and of Four Young contained in her Ovary. f.v, as they were taken from the ovary ; y, cleared in xylol ; z, sectioned.] ] .EFT KYI No. 1 i n 'j.l h In mm. Condi- tion. Longitu- dinal. Verlkal. Medic- distal. Between cartilages. Pupil longitu- dinal. Pupil vertical. Lens. Optic nerve in eye. Gptic nerve outside. 6c: •76 l 18 ai6 ^68 26* s 41 •> 5 I2O 80 76 - $1A 2*6 cc 260 CQ 76 3 z 240 2t-6 88 320s 80 36 80 OO 12 280 7^ z 76 ' 384 148 68 No. Length in mm. Condi- tion. RIGHT EYE Longitu- dinal. Vertical. Medio- distal. Between cartilages. Pupil longitu- dinal. Pupil vertical. Lens. Optic nerve in eye. Optic nerve outride. 76 76' 75 2 76 ^ 76 ' 65 18 18.5 '9 20 X X y X y X y z X y 225 362 360 ? 250 320 275 400 375 340 208 356 250 212 ? 22^ 248 288 2IO 368 315 200 12 152 '35 128 ? 25 36 56 40 1 60 140 OO 20 208 2OO ? 5° ? 64 60 224 185 176 1 68 376s 80 88 300 5 00 8 136 420 212 20 20 1 Lower embryo of right ovary. a Upper embryo of left ovary. 3 Lower embryo of left ovary. 4 Upper embryo of right ovary. s Vertical distance between inner margins of scleral cartilage. 210 BLIND VERTEBRATES AND THEIR EYES. Still more striking is the variability in the size of opposite eyes in the same individual whatever its length. There are minute differences in the size of the eyes of the two sides at all times, the individual with two eyes exactly alike is probably not to be found, but the differences in mind are of a much larger order. For in- stance, in the mother of the 4 young mentioned above, the left eye had a longitudinal diameter of 170, the right eye 225 ; that is, the right eye was a third longer than the left. Instances of this sort are by no means rare, there being a marked difference in a number of the individuals secured. In one of the oldest secured, the eye of one side is all but gone, that of the other still well defined (plate 23 and plate 24, D). In a much younger one, 43 mm. long, I have found no eye in one side. In another the left eye bears the ratio of i to 3 to the right eye, which is therefore almost nine times as large as the left (plate 24, figs. A and B). Such big differences between the eyes of the two sides, fluctuating in amount in different individuals, but readily seen in living specimens, are found in about 10 per cent of individuals. Sections usually showed that such differences whenever they existed were largely to be found in the pigment layer which in the large eyes was vesicular and the retina shriveled and retracted to the pupil, leaving a large space between the pigment epithelium and the rest of the retina (plate 21, fig. B; plate 22, fig. A ; plate 24, figs. A and c). NOTE. — One element of error is present in the exposition of the eye of Lucifuga. Lucifuga and Stygicola live together in the same caves. There is no difficulty in distinguishing these after they reach acertain size. What that size is I can not say, but at 60 mm. they are conspicuously different. The smallest specimen of Stygicola unquestionably determined is 60 mm. in length. Possibly the two species are superficially indistinguishable when young, and some of the young specimens mentioned below 60 mm. and used in preparing the following account may in reality be stygicolas. All specimens below 60 mm. secured had the characters of Lucifuga. The probability of this pos- sible error is not as great as it may appear at first sight, as an analysis of the origin of the specimens less than 60 mm. will show. Seven of the specimens less than 60 mm. sectioned are from the cave of Jaiguan. From this cave 23 fishes were taken, 5 of which were stygicolas. The smallest of the stygicolas was 81 mm. and considerably larger than the smallest undoubtedly distinguishable spec- imens. If there were no specimens of Stygicola between 60 mm. and 81 mm. long when they could have been readily distinguished, it is probable that there were none smaller. The 3 smallest specimens of Stygicola measured 81, 90, and 97 mm. respectively. From Hawey I secured only Lucifuga, at least 3 of them being larger than the smallest specimens, permitting an unquestioned determination. From La Fria the only 2 over 60 mm. long were Stygkola, while those below 57 mm. were apparently all lucifugas. Two of those sectioned, 54 and 57 mm. long, may be considered lueil'ugas without a doubt. This leaves one 27 mm. and one 28 mm. in doubt. In Los Banos we secured no large specimens ; all the small ones were referred to Lucifuga. In Ashton large and small were all referred to Lucifuga, the smallest one sectioned from this place being 53 mm.; it is undoubt- edly a Lucifuga. The proportion of stygicolas to lucifugas among individuals over 60 mm. is: stygicolas, 43; lucifugas, 36. Liicifiigu does not reach a size over 104 mm., and comparing the ratios of lucifugas to i olas, between ihr smallest determined Stygicola 60 mm. and the largest Lucifuga 104 mm., we ^<-t ,S'/ v^'/i ulii T,I mm., I. in ifugii 36 mm., or a ratio of i to 1.25. But of the 32 stygicolas between 60 mm. and at mm., 10 came from the " M " Cave which is remote from the region where lucifugas were found. Eliminating these, we would get a ratio of 22 to 36, or i to i 7Ti f°r tne region where both are found. This, other things being equal, would give us the probability that any of the younger specimens found in the region where both species were found was a Stygitola or a Luci- fuga. More than this, in the "M" Cave, about 60 miles removed from any cave in which Luci- fi• ' . ' • •-:•:• a - y' V £3 <* -^'r -:--^ . -V . . B Eye of an old Lucifuga, 94 mm. long. A. Left eye-socket with contained eye and pigment-mass at its left. X 60. B. Part of same section, X 375, showing fibrillar network about eye. Eye a nodule of cells in which distinction can only be made between pigment and retina. Part of pigment mass at extreme left. (For right eye see plate 24 D). THE RETINA OF LUCIFUGA. 217 unlike white corpuscles that have been abundantly fed with pigment granules. Whether they carried these in with them or whether the remnant of the lens had undergone a pigment degeneration, I am not able to say. The eye of the other side is much damaged in sectioning, but is essentially the same. In a young 24 mm. long, evidently just born, the lens capsule of both eyes is a large balloon, little wrinkled, and containing but little stainable material, all of it of the same nature as that described above. In a young 27 mm. there is no remnant of a lens in the left eye, while in the right there is the filmiest suggestion of the lens capsule, but nothing more. In an individual 28 mm. long the lens of the right eye is represented by a nearly empty capsule, that of the left is shriveled, contains pigment, and is entirely in the vitreous space, the pupil having closed. In an individual 38 mm. long the left lens is represented by a large empty col- lapsed capsule, that of the right being small and collapsed. The lens capsule is the last part of the lens to disappear. In specimens over 40 mm. long, it was observed in only two doubtful cases ; in all others there was no trace of it left. It is quite evident from the structure of the lens displayed in the unborn young 18 mm. long that it had passed its point of highest organization and was obviously far along on the route of degeneration. Indeed the lenses of the young (12 mm.) show no signs of fiber formation and also show indications that they have begun to degenerate. Conspicuous and remarkable are the fibrous lens capsule which persists after its contents have disappeared, the irregularity of the contained cells in their highest development and their irregular distribution, and finally the pigment-fed phagocytes in the capsule. THE RETINA. On account of the fluctuation in the size of the eye it is difficult to determine whether the end of its development is reached with a length of 12 mm. or not until a length of 20 mm. In the 4 embryos, 76 a, b, c, and d, about 20 mm. long, the eyes fluctuate from a maximum 425 p. in longitudinal diameter in the longest, to 260 p. in the shortest. If the embryo with the smaller eye had been of smaller size, it would have been but natural to come to the erroneous conclusion that the eye increases with age till the fish reaches a length of 20 mm. The same is true in respect to the differentiation of the retina. One can not say in general that the retina progresses in any respect between the length of 12 mm. and 20 mm. I can only say that the most highly developed retina was found in an unborn individual 20 mm. long (plate 18, fig. A, and plate 24, fig. E). In the retina of the youngest individuals (12 mm.) there is a distinct differentia- tion into a ganglionic layer occupying 0.24 of the total thickness, an inner fibrous layer of the same thickness, a nucleolar layer 0.32 of the total, and a pigment layer occupying 0.20 of the entire retina. The boundaries of the different layers are not equally regular at all places, and the nuclear or ganglionic layer sends a connect- ing series of nuclei in an irregular manner through the reticular layer in different places. The pigment layer is well pigmented. The inner cell layer of the uvea is 218 BLIND VERTEBRATES AND THEIR EYES. not pigmented and forms a distinct ciliary process. Between the latter and the rest of the retina there is an accumulation of elongate nuclei. This retina has reached a stage in an irregular process of histogenesis, or it has earlier stopped at such a stage of differentiation, or finally, it has reached its present condition as a degeneration from an earlier, more highly differentiated stage. From the material at hand it is impossible to determine when the ret- ina reaches its highest stage of development and when it begins to degenerate. A slightly higher stage of differentiation is found in one of the eyes of one of the unborn young of 76. In this eye, the retina has about the same total thickness. There is found in places a very distinct separation of the outer layer of nuclei into an inner layer, a reticular layer, and an epithelial layer. To one of the epithelial nuclei a cone is found attached (plate 24, fig. E). A ciliary process is not seen in this eye nor in the group of elongate nuclei so conspicuous in the younger stage. The inner layer of the uvea, as well as the outer, is pigmented. Beyond birth only general processes can be described without entering into a minute description of each eye. The retina degenerates progressively and it seems to do this accompanied by one of two modifications in the general structure of the eye. The eye may shrivel (plate 20, figs. B, c; plate 21, fig. A), the pig- ment layer lying close against the rest of the retina ; or the pigment layer may sepa- rate itself from the rest of the retina and become very greatly distended, the retina itself forming but a small segment of the eye vesicle (plate 22, fig. A; plate 24, figs. A, c). Plate 22, figure A, represents such an eye, in which the retina is well con- tracted and the pigment layer shriveled. The optic nerve passes through the vesicle. The beginning of such a modification is probably to be seen in plate 21, figure B. In other cases the retina is drawn out laterally (plate 24, figs. A, c). Such vesicular eyes were also found in old individuals of Amblyopsis. There does not seem to be any increase in the amount of pigment, and, since it is scattered over a larger area, the pigmented layer of these vesicular eyes is less densely pig- mented than that of the shriveled eyes. In one eye conditions normal to a fish eye are more nearly retained. I am not able to say that one part of the retina undergoes a more rapid degeneration than another. They all reach the vanishing point with extreme old age. In an old individual (94 mm.) the eye of one side consisted of a few vacuoles surrounded by nucleated fibrous tissue (plate 24, fig. D). It is impossible to deter- mine to what these parts of the eye belonged. There are also scattered pigment granules and cells, while near this eye are a few pigment masses. The eye of the other side is better preserved and represented in plate 23. In one eye, which is shriveled to very small dimensions, a peculiar lenslike structure occupies most of the interior. Such lenslike structures I found in Amblyopsis and erroneously considered them the lens. In Rhineura it is distinctly seen that the structure fills an invaginated pocket of the pigment layer. A census of a series of eyes of individuals from the time of birth to old age gives us the following statistics concerning the lens, the vitreous space (that is, between retina and iris), and the aqueous space (between iris and cornea) : EYE OF LUCIFUGA. 219 Statistics of Lens, showing Vitreous Space (between Retina and Iris) and Aqueous Space (between Iris and Cornea). LENS. VITREOUS SPACE. AQUEOUS SPACE. Left eye. Right eye. Left eye. Right eye. Left eye. Right eye. mm. 25 (66) 25 (i°3) 24 67 25 I05 27 no 28 64 28 in 38 62 43 61 44 l°4 S3 82 53 95 54 i°9 57 75 65 76 69 56 So S3 84 51 93 42 94 29 Collapsed empty capsule Very large Large Large Large Large Large Very small Right ? ? .... Very small o Very small 0 o Large .... Small .... o .... o o o Large Large Large Moderate o o o Large o .... 0 .... o .... Very large o .... o .... o .... ? .... 0 .... o .... o .... o .... Large Large Large Moderate o ? ? o o o o 0 Large o o o o 0 Large, entirely filling vitreous cavity Small, filled by lens Large, empty . . capsule * Filmy capsule . . Nearly empty . . capsule Capsule with pig- ment Small, collapsed ? Capsule with pig- ment ? Large, empty Left very large . . . ? p Empty vesicle . . . ? * In the left eye the lens is not distinguishable, but is probably represented by a collapsed capsule in part filling the vitreous cavity. 220 BLIND VERTEBRATES AND THEIR EYES. THE EYES OF STYGICOLA. The account of the eyes of Stygicola is based (i) on two young born October 20, each about 20 mm. long; (2) on the mother of the above, 92 mm. long; (3) on various other older fishes, from 60 to 135 mm. long.1 The early stages of the development and the history of the eye between 20 and 60 mm. is not known. On October 30 I obtained a Stygicola at Alacranes. She gave birth to two young on the evening of October 31, at Canas. They were born tail foremost. The ovary of this specimen contained eggs 0.88 mm. in diameter, or nearly ripe. The 2 young are referred to as 125 a and 125 b. The head of 125 b, seen from above, is represented in plate 25, figure B, and the eyes are represented by plate 25, figure c. The eyes of the one born at Canas (125 b) were symmetrical, nearly of the same size. The eyes moved, and as far as I could judge were as readily movable as the eyes of other young fishes. The eyes were silvery, the argentea being apparently well developed. The iris was well distinguished, the pupil too large for the lens, having a downward directed notch continuous with the choroid fissure which is still visible as a pigmentless streak. While small, there was nothing in the general appearance of the eye that would lead one to conclude it might not be functional. The eyes are so placed in relation to the brain that a line tangent to their pos- terior faces would be tangent to the anterior face of the optic lobes. This condi- tion corresponds very well to the position in Lucifuga of equal size. Table of Measurements. Current No. 125 a 1256 i 126 125 "7 Size. 20 mm. 20 mm. 60 mm. 88 mm. 92 mm. 135 mm. Side. / r r ; r ; r / r / r I Vertical diameter... M 4I31 284 "3° V-1 6.7 tS-8 18 9 "•7 45 M 312 251 67 74 4.5 18 24 12.6 24 20 566' 191 128 78 1 1 13 26 II 9 M 560 198 122 73 II 18 20 9 93 C 95 1 60 p- 170 170 M 320 3M." 'S3 45 ii n 250 210" 3° 5° 1 1 H 1" 198 256" H "3 140 M 13° no Pupil 1 .CMS 1'i^nu'ttt ' MiliT nuclear (Iranular 58 56 Ganglionic Hyaloid 1!!. ii »l \r ,' 1 ill . ,i > 3* outer margin of si_k-r;il cartilage, unless otherwise stated. i ii pigment to outside of pigment. • 1 ol.i! (hkkntss uf retina 67 MI as compared with 237 n in Zygonecles. « These eyes lie 0.5 mm. lielow the surface. a This eye lies 0.3 mm. below the surface. About the left eye of the second young (125 a) there was a large accumulation of blood, which in section is seen to be in the choroid layer and mixed with the orbital fat. Measurements of the eyes of the young, as well as of the mother, arc shown in the above table, and see also plate 25, G. * ""' 1*1 ••' "l>taiiie m • X : • // i; r% , "' ins ~n,op. 1 Mrs.ERBielm« del Eyes of Stygicola and Lucifuga. A. Lucifugas 20 mm. long and ready to be liberated. See plate 1 5 D for others from the same ovary. B. Dorsal view of head of a young Stygicola, about 20 mm. long. C. (a) right and (A) left eyes of a young Stygicola 20 mm. long (No. 1 254). In A details of marking of iris and ball are shown. D. (a) and (A) right and left eyes of the only other individual from the same ovary (No. I25lu-s arc undoubtedly descended from oviparous forms, and viviparity is probably a comparatively recent acquirement, though most probably attained bi-lore tin- change of habitat from the sea to the underground streams of Cuba. Some ;it least of the marine members of the Brotulidae are also viviparous. The production of tin- ninny "nests," each with its hundreds or even thousands of young ova, is a reminiscence of the oviparous condition, when it was necessary for OVARY AND OVA IN LUCIFUGA AND STYGICOLA. 231 the preservation of the species that a multitude of young be produced, as in the case of the oviparous fishes. The condition of viviparity, providing as it does for the greater safety of the young during the most critical period of their development, and their habitat in caves where the number of enemies is prob- ably greatly less than in the sea enable these species to maintain themselves by the production of fewer offspring. SUMMARY 1. The ovary in Lucifuga and Stygicola consists of a mass of stroma containing the ova and covered with epithelium ; the whole structure is V-shaped and is con- tained within the ovisac; the latter is continued to the urogenital pore as the oviduct. 2. The epithelium, lining the ovisac and covering the ovary proper, is unique in that it frequently contains numerous blood capillaries. 3. The sinuses within the stroma are filled with lymph and adipose tissue. 4. Lucifuga and Stygicola are viviparous blind fishes which give birth to but few young, 2 to 15 so far as yet observed. 5. The young are not developed in separate sacs, but lie within the lumen of the ovisac, gradually compressing the ovarian stroma as they develop. 6. The ova arise in "nests "or masses of several hundred each. The smallest observed have a diameter of 5 to 10 /A. 7. One ovum from each "nest" is developed to maturity; the other ova of the "nest" undergo rapid degeneration and are ultimately absorbed into the substance of the large ovum. 8. In those "nests" in which none attains maturity, all the ova undergo a slow, pigmented degeneration in situ. 9. The destruction of so many ova at an early stage is an adaptation to the vivipa- rous habit. 10. Viviparity is probably a comparatively recent acquirement of these fishes, though attained before these genera left the sea for the fresh-water cave streams. 232 BLIND VERTEBRATES AND THEIR EYES. CONCLUSIONS IN REGARD TO LUCIFUGA AND STYGICOLA. 1. Lucifuga and Stygicola are two marine fishes that have remained in the cracks and caves of the coral beaches which they inhabited, as these caves were elevated and became filled with and enlarged by fresh water. They have become entirely adjusted to a fresh-water environment. 2. Stygicola is known from both the north and south slopes from Alacranes to Matanzas and Alfonso XII. Lucifuga is known only from the south slope west of Havana. 3. The caves in which the fishes were found are all well lighted, but are always connected with dark underground channels. Each cave has only a limited supply of fishes that may be replenished from the underground channels. 4. Lucifuga and Stygicola are negatively heliotropic. They are adjusted to withstand but slight temperature changes. They feed on crustaceans and odonata larva?. 5. Both species are viviparous, giving birth to 2 to 15 young about 25 mm. long. Both probably breed throughout the year. Spermatozoa are transferred long before the ripening of the eggs. Lucifuga breeds probably most abundantly through March and May in shallow places. Its young are abundant near the surface. Stygicola breeds in unknown places and its young are not seen near the surface. 6. The eye decreases in size progressively from birth to extreme old age con- comitantly with the appearance of masses of pigment cells in the orbital fat. 7. The eye varies greatly in different individuals of the same size — from 260 to 425 /A in length, in brothers and sisters in the same ovary. 8. The ontogenetic degeneration results either in the shriveling of the entire structure or the great distention of the pigmented layer. One process may be found on one side, the other on the other side of the same individual. 9. The eye muscles are all present in the young, but undergo a variable amount of degeneration with age, disappearing entirely in very old of Stygicola. 10. The sclera is self-determining in both Lucifuga and Stygicola. In Luci- fu^ii the cartilages at the time of birth are too large for the eye, forming a shield over tin- face of the eye. In Stygicola it forms a ring about the middle of the eye. After 1 lirth they very rapidly degenerate and disappear entirely by the time Lucifuga has reached less than half its maximum length. In Stygicola it remains longer. 11. There is evidence that there is an early disturbance of the vascular system of the eye resulting in the formation of large blood lakes about the eye. 12. The lens has begun to degenerate before birth. Its contents liquefy, the cap- sule shrivels, and finally disappears at a length of about 40 mm. 13. It has not been determined when the histogenesis of the retina ends and its degeneration begins. The most highly developed retina was found in an unborn young <>1 I.nrifuga 20 mm. long. In this retina the outer nuclear, outer reticular, inner nuclear, inner reticular, and ganglionic layers are more or less distinctly represented. EIGENMANN PLATE 27 Ovaries of Lucifuga and Stygicola. A. Dorsal aspect. Round, opaque, white dots are larger ova seen through stroma and ovisac. B. Ventral aspect. X 2 diameters. PHOTOGRAPHS BY PROF C. H. EIQENMANN. EIGENMANN PLATE 28 * • - ' -- y,--^. Si~-~':* \ ' ~,>V?££.&~ N >.'*'* .t-.rt!.*£$*&v y : ;.^ •:.L,.&2* . f : '."M&£* '-Mi .llf. Of '. '•.^••f "£•• • '• '^K? j ;%^fe^^,- *' "SSH^^-V; /."' F jr.c A. "Nest" of small ova (s. c.), each about 1 0 M in diameter, one larger ovum (/.o.) 50 A1 in diameter. Whole nest contained within special arrangement of stromal cells (si. c.). X about 300 diameters. B. Developing ovum (m. o.), surrounded by rapidly atrophying small ova (a.o.); /., lumen of ovisac; n., " germinative spot"; si., stroma. Diameter of large ovum, 120/t. X 500 diameters. C. Cross-section of one horn of ovary. /., lumen of ovisac; o., ova; oV. s., ovisac; st., stroma. Guide line to si. crosses place of attachment of stroma to median wall of hom of ovisac. X about 50 diameters. D. "Nest of yellow cells." Diameter of individual "yellow cells" (y. c.) about 15 pst. c., arrangement of stromal cells around yellow cells, as around "nest" of small ova (si. c., in fig. A.) X 200 diameters. E. Few "yellow cells" more highly magnified to show pigment-granules and general appearance of slow degeneration. Nuclei can not be distinguished in photo- graph, though distinct enough in section. X about 800 diameters. F. 3 adjacent " nests " of ova (n.), each with developing ovum ; si. c , stromal cells which separate " nests " and likewise developing ova from one another. X 2 1 0 diameters. EIGENMANN PLATC 29 Sections of Ovaries. A. Cross-section through horns of pregnant ovary. B. Cross-section through middle part of pregnant ovary. Ovisac collapsed when fetuses were removed. C. Cross-section through posterior part of pregnant ovary. D. Cross-section of non-pregnant ovary with stroma in two lobes, one dorsal, other ventral. E. Part of cross-section of non-pregnant ovary. F. Part of epithelial covering of non-pregnant ovary showing capillaries (cpj). THE CAUSES OF INDIVIDUAL AND PHYLETIC DEGENERATION THE CAUSES OF INDIVIDUAL AND PHYLETIC DEGENERATION. It may now be profitable to take up the causes leading to the small degree of degeneration found in Chologaster, the degeneration of the eye in Amblyopsis, Typhlichthys, and Troglichthys to a mere vestige, together with the total disap- pearance of some of the accessory structures of the eye, as the muscles, in some of the species. In the outset of this consideration we must guard against the almost universal supposition that animals depending on their eyes for food are or have been colonizing caves, or that the blind forms are the results of catastrophes that have happened to eyed forms depending on their eyesight for their existence. This idea, so prevalent, vitiates nearly everything that has been written on the degeneration of the eyes of cave animals. The degeneration of organs ontogenetically and phylogenetically has received a variety of explanations. (1) The organ diminishes with disuse (ontogenetic degeneration, Lamarck, Roux, Packard) and the effect of this disuse appears to some extent in the next gen- eration (phylogenetic degeneration, Lamarck, Roux, Packard). (2) Through a condition of panmixia the general average maintained by selection is reduced to the birth mean in one generation (ontogenetic, Romanes, Lankester, Lloyd Morgan, Weismann) to the greatest possible degeneration in succeeding generations (phylogenetic, Weismann), or but little below the birth average of^the first generation (Weismann's later view, Romanes, Morgan, Lankester). (3) Through natural selection (reversed) (the struggle of persons) the organ may be caused to degenerate either (a) by the migration of persons with highly developed eyes from the colony living in the dark (Lankester), or (b) through economy of weight and nutriment or liability to injury (phylogenetic purely, Darwin, Romanes). (4) Through the struggle of parts (a) for room an unused organ in the individual may be crowded (ontogenetic, Roux), (b) for food, this may lead to the development of the used organ as against the disused through a compensation of growth (Goethe, St. Hilair, Roux) ; this ontogenetic result becomes phylogenetic through transmis- sion of the acquired character (Roux), or is in its very nature phyloblastic (Kohl). (5) Through the struggle between soma and germ to produce the maximum efficiency of the former with the minimum expenditure of the latter (ontogenetic and phylogenetic, Lendenfeld). (6) Through germinal selection, the struggle of the representatives of organs in the germ (ontogenetic and phylogenetic, Weismann). (7) To these special considerations should be added the recently suggested gen- eral process of mutation. The idea of ontogenetic degeneration is intimately bound up with the idea of phylogenetic degeneration. Logically we ought to consider first the causes of indi- vidual degeneration and then the processes or causes that led to the transmission 235 236 BLIND VERTEBRATES AND THEIR EYES. of this. Practically it is impossible to do so, because many of the explanations are general. Only number (4) of the above maybe taken in the ontogenctic sense purely, though it was certainly also meant to explain phylogenetic degeneration. In many of the explanations of particular cases of degeneration more than one of the above principles are invoked, though only one was meant to be used. In most cases, how- ever, the discussions of degeneration have been in general terms, without direct bearing on any specific instance of degeneration in all its details. It must be evident that such discussions can only by accident lead to right results. pfei*By the Lamarckian ontogenetic degeneration is considered the result of lack of use and consequent diminished blood supply. The results of the diminution caused by the lack of use during one generation are transmitted in some degree to the next generation, which thus starts at a lower level. A continuation of the same con- ditions leads finally to the great reduction and ultimate disappearance of an organ. No one, so far as the author knows, has attempted, or, perhaps better, suc- ceeded, in accounting with this factor in detail for the degeneration of the eye. Packard's explanations are evidently a mixture of Lamarckism and Darwinism. Packard says, "When a number, few or many, of normal seeing animals enter a totally dark cave or stream, some may become blind sooner than others, some hav- ing the eyes slightly modified by disuse, while others" may have in addition physi- cal or functional defects, especially in the optic nerves and ganglia. " The result of the union of such individuals and of adaptation to their stygian life would be broods of young, some with vision unimpaired, others with a tendency to blindness, while in others there would be noticed the first steps in degeneration of nervous power and nervous tissue." Packard evidently had invertebrates in mind. He clearly admits the cessation of selection or panmixia which is implied by his supposition that those born with defects may breed with the others. He supposes that the blind fauna may have arisen in but few or several generations, a supposition that may be applicable to invertebrates, but certainly may not be applied to the vertebrates. At first those becoming so modified that they can do without the use of their eyes would greatly preponderate over those " congenially blind." "So all the while, the process of adaptation going on, the antennae and other tactile organs increasing in length and in the delicacy of structures, while the eyes were meanwhile diminishing in strength of vision and their nervous force giving out, after a few generations, perhaps only two or three, the number of congenitally blind would increase, and eventually they would, in their turn, preponderate in numbers." Packard seems here to admit the principle of degeneration as the result of compensation of growth, the nervous force of the eye giving out with the increase of the tactile and olfactory organs. It is somewhat doubtful in what sense the term " congenitally blind " is used, but it probably means born blind as the result of transmitted disuse rather than blind as the result of fortuitous variation. The effects of disuse are thus supposed through their transmission to have given rise to generations of blind animals. The continued degeneration is not discussed. In 1873, 1874, and 1890, Romanes, in a series of articles in "Nature" and later in "Darwin and after Darwin," n, page 291 el scq., maintained that the beginning «f degeneration is due to cessation of selection, and continued degeneration to the reversal of selection and final failing of the power of heredity. Selection he supposed to be reversed because the organ no longer of use "is absorbing nutriment, causing CAUSES OF DEGENERATION. 237 weight, occupying space, and so on, uselessly. Hence, even if it be not also a source of actual danger, economy of growth will determine a reversal of selection against an organ which is now not only useless, but deleterious." This process will con- tinue until the organ has reached " so minute a size that its presence is no longer a source of detriment to the organism, the cessation of selection will carry the reduc- tion a small degree further; and then the organ will remain as a 'rudiment." Since, however, we can not consider that the force of heredity is everlasting, it will eventually fail and the organ dwindle still further and disappear. This failure of heredity, Morgan (" Animal Life," page 793) is unable to distinguish from the effect of disuse without which " the reduction of organs is difficult to explain." The principles involved in this explanation are panmixia natural selection, and, according to Morgan, disuse transmission. Weismann ("Nature," 1886, and " Essays," vol. n, i) contended that cessation alone, or panmixia as he terms it, is sufficient to account for all degeneration. He later gave up this view for his theory of germinal selection, of which more later. Roux, starting with the then generally accepted view that acquired characters are transmitted, attempted chiefly to explain degeneration in the individual. Degeneration is looked upon as the result of a struggle among the parts for (a) room and (b) food. He emphasizes the fact that a reduced functional activity continued for a long period reduces the functional possibility of an organ (page 176). The diminished use not only brings about this simple atrophy, but also the reduction, by stronger neighbors, to such a volume as is still of advantage to the animal. Disused organs that are not in the struggle for room may maintain them- selves a long time. The struggle among parts for food, which implies the principle of compensation of growth of Goethe, need not take place through the withdrawal of blood, but may take place through the more active osmotic selection by the stronger organ of food that would otherwise go to the weaker. Without doubting that both these principles are active agents in degeneration, it may be seriously doubted whether they were effective in the degeneration of the eyes in question. Certainly there can be no question of a struggle for room, for the position and room formerly occupied by the eye is now filled with fat which can not have been operative against the eye. The presence of this large fat-mass in the former location of the eye, the large reserve fat-mass in the body, the uni- formly good condition of the fish, and the low vitality which enables them to live for months without visible food, all argue against the possibility that the struggle for food between parts was an active agent in the degeneration of the eyes. Kohl considers that "Der Grund, und direkter oder indirekter Anlass zum Eintreten der Entwickelungshemmung ist Lichtmangel." The method of the direct operation of the lack of light he conceived to be as follows : The ancestry of blind animals lived where the light was uninterrupted and they had developed eyes. They got into an environment where the light was shut off more or less. The first generations retained their fully developed eyes without, however, being able to put them to full use. In consequence during phylogeny other organs became highly developed to compensate for the disuse of the eye. (Through natural selec- tion?) Thus touch organs (Myxine, Siphonops) or the auditory organs (Talpa and possibly Typhlichthys) became more highly developed. The eye was unneces- sarily highly developed. A process of degeneration (Riickbiklung) began, which _'.;s BLIND VERTEBRATES AND THEIR EYES. was never very extensive. Much more potent in placing the eye in harmony with its environment was the fact that every succeeding generation developed its eye less. This process of Hemmung of the eye did not begin until the developmental force began to go to the development of the compensating organs. On account of the loss of this developmental force the eye was unable to reach, in successive genera- tions, the former grade. The degeneration is thus explained as the result of a struggle of parts, although this term is nowhere used, acting through the princi- ple of compensation. The same objections may be offered to this explanation of Kohl as to all his theoretical discussions ; they are based on the assumption of conditions and processes that have no existence. The high development of " compensating " organs is not primarily the result of the loss of the eye, but the high development of the former organs permitted the disuse and later degeneration of the later. His whole process is a phylogenetic one without a preceding onto- genetic one, though on this point he does not seem to be very clear himself, for on one page we are told that degeneration leads to retardation, and on another that degeneration is a consequence of retardation. Lendenfeld endeavors to apply Roux's Kampf der Theile with reversed selec- tion to explain the conclusions reached by Kohl on the processes and causes of degeneration. The struggle is represented to take place between the germ and soma, the former endeavoring to keep the latter at the lowest efficient point as weapon for the germ. If a series of individuals gets into the dark, the organs of vision are of no advantage, and reversed selection will bring about their degenera- tion. The saving in ontogeny appears first as a retardation and then a cessation of development. Weismann later accepted the view of Romanes, Morgan, and Lankester of the inadequacy of panmixia to explain the whole phenomena of degeneration, and in his " Germinal Selection " rejects the idea of reversed selection and suggests a new explanation for what Romanes attributed to the failure of heredity and the Lamarckians to disuse transmission. The struggle of the parts, of Roux, has been crowded back by him to the representatives of these parts in the germ. "The phenomena observed in the stunting, or degeneration, of parts rendered useless show distinctly that ordinary selection, which operates by the removal of entire persons, personal selection, as I prefer to call it, can not be the only cause of degeneration ; for in most cases of degeneration it can not be assumed that slight individual vacillations in the size of the organ in question has possessed selective value. ( )n the contrary, we see such retrogressions affected apparently in the shape of a continuous evolutionary process determined by internal causes, in the case of which there can be no question whatever of selection of persons or of a survival of the fittest, that is of individuals with the smallest rudiments." The gradual diminution, continuing for thousands and thousands of years and cul- minating in its final and absolute effacemcnt, can only be accomplished by ger- minal selection. Germinal selection as applied to degeneration is the formal explanation of Romanes' failure of the hereditary force and the establishment of disuse effects in the heredity through the struggle of parts for food. "Powerful determinants will absorb nutriment more rapidly than weaker determinants. The latter, accordingly, will grow more slowly and will produce weaker determinants than the former." If an organ is rendered useless, the size of this organ is no longer CAUSES OF DEGENERATION. 239 an element in personal selection. This alone would result in a slight degeneration. Minus variations are, however, supposed to rest "on the weaker determinants of the germ, such as absorb nutriment less powerfully than the rest. This will enable the stronger determinants to deprive them even of the full quantum of food cor- responding to their weakened capacity of assimilation and their descendants will be weakened still more. Inasmuch now as no weeding out of the weaker deter- minants of the hind leg (eye) by personal selection takes place on our hypothesis, inevitably the average strength of this determinant must slowly but constantly diminish, that is, the hind leg (eye) must grow smaller and smaller until it finally disappears altogether." "Panmixia is the indispensable precondition of the whole process; for owing to the fact that persons with weak determinants are just as capable of life as those with strong, solely by this means is a further weakening effected in the following generations." This theory presupposes the complex structure of the germplasm formulated by Weismann. But granting Weismann the necessary structure of the germplasm, can germinal selection accomplish what is claimed for it? I think not. Grant- ing that variation occurs about a mean, would not all the effects claimed for minus variations be counteracted by positive variations? Eye determinants, that on account of their strength secure more than their fair share of food and thereby produce eyes that are as far above the mean as the others are below, may leave descendent determinants that are still stronger than their ancestry. It is evident that a large, really extravagant development of the eye in such a fish as Chologaster would not effect the removal of the individual by personal selection, still less so in Amblyopsis, which not only lives in comparative abundance, but has lived for 20 months in confinement without visible food. It seems that all the admitted objec- tions to degeneration by panmixia apply with equal force to germinal selection. This, however, would be changed were the effect of disuse admitted to affect the determinants, and this it seems Weismann has unconsciously admitted. So far we have considered germinal selection in the abstract only. In the concrete we find that degeneration is not a horizontal process affecting all the parts of an organ alike as Weismann presupposes, not even a process in the reverse order of phyletic development, but the more vital, most worked parts degenerate first with disuse and panmixia, the passive structures remain longest. The rate of degeneration is proportional to the past activity of the parts and the statement that "passively functioning parts, that is, parts which are not alterable during the individual life by function, by the same laws also degenerate when they become useless" is not applicable to the eyes. As one example of the unequal degeneration we need only call attention to the scleral cartilages and the rest of the eye of Troglichthys rosa'.* All are agreed that natural selection alone is insufficient to explain all, if any, of the processes of degeneration. All either consciously or not admit the principle of panmixia, and all are now agreed that this process alone can not produce exten- sive degeneration. All are agreed that the important point is degeneration beyond the point reached by panmixia, the establishment of the degenerating process, what- ever it may be, in the germ, or in other words, breaking of the power of heredity. It is in the explanation of the latter that important differences of opinion exist. 1 1 must again guard against cross-counter conclusions. In the Brotulidie the passive cartilages are among the first things to go. 240 BLIND VERTEBRATES AND THEIR EYES. Weismann attempts to explain the degeneration beyond the point which pan- mixia can reach by a process which not only is insufficient, if all his premises are granted, to produce the desired result without the help of use transmission, but has as its result a horizontal degeneration which does not occur in the eyes. Romanes supposed degeneration, beyond the point which may be reached by panmixia, to be the result of personal selection and the failure of the hereditary force. The former is not applicable to the species in question and is denied by such an ardent Darwinian as Weismann to be applicable at all in accounting for degeneration. Moreover the process as explained by Romanes would result in a horizontal degeneration which has no existence in fact. The second assumption, the failure of hereditary force, is not distinguishable, as Morgan has pointed out, from the effect of use transmission. The struggle of parts in the organism has not affected the eye through the lack of room, since the space formerly occupied by the eye is now filled by fat and not by an actively functioning organ. It is not affected by the struggle for food, for stored food occupies the former eye space. It could only be affected by the more active selection of specific parts of food by some actively functioning organ. It is possible that this has in fact affected the degeneration of the eye. The theory explains degeneration in the individual and implies that the effect in the individual should be transmitted to the next generation. This second fact seems but the explanation of the working of the Lamarckian factor. Mutation can produce definitely directed evolution such as we find in the degenerating eye only when each step, each successive mutation, has an advantage over the mother or sister lines. I do not think that any one after familiarizing himself with the variation of the eye and its insignificance will maintain that this minute organ is now or has been for many generations of selective value. If it is not of selective value, mutation is as powerless to account for its condition as is natural selection of favorable variations. The eyes of the two sides vary so much, independent of each other, that we are forced to conclude that there has been no check on their variation for a long period. The only answer to the objection that the eyes are not the result of personal selection is that they may be so correlated with another organ inversely propor- tionate to it, that the selection of individuals with this other organ in favorable condition carries with it the selection of individuals with the eye in decreasing!}' imperfect condition. No such organ is available. The Lamarckian view, that through disuse the organ is diminished during the life of the individual, in part at least on account of the diminution of the amount of blood going to a resting organ, and that this effect is transmitted to succeeding generations, not only would theoretically account for unlimited progressive degenera- tion, but is the only view so far examined that does not on the face of it present serious objections. Is this theory applicable in detail to the conditions found in the Amblyopsidae? Before going farther, objections may be raised against the universal assumption that the cessation of use and the consequent panmixia was a sudden process. This assumes that the caves were peopled by a catastrophe. But it is absolutely certain that the caves were not so peopled, that the cessation of use was gradual and the cessation of selection must also have been a gradual pro- CAUSES OF DEGENERATION. 241 cess. There must have been ever widening bounds within which the variation of the eye would not subject the possessor to elimination. Chologaster is in a stage of panmixia as far as the eye is concerned. It is true the eye is still functional, but that the fish can do without its use is evident by its general habit and by the fact that it sometimes lives in caves. The present conditions have apparently existed for many generations, as long as the present habits have existed, and yet the eye still maintains a higher degree of structure than reversed selection, if operative, would lead us to expect, and a lower degree than the birth mean of fishes depending on their eyes — the condi- tion that the state of panmixia alone would lead us to expect. There is a staying quality about the eye with the degeneration, and this can only be explained by the degree of use to which the eye is subjected. The results in Chologaster are due to panmixia and the limited degree of use to which the eye is put. Chologaster agassizii shows the rapid diminution of the eye with total disuse. The difference in the conditions between Chologaster and Amblyopsis, Typh- lichthys and Troglichthys is that in the former the eyes are still in use, except when living in caves ; in the latter they have not been in a position to be used for hundreds of generations. The transition between conditions of possible use and absolute disuse may have been rapid with each individual after permanently enter- ing a cave. Panmixia, as regards the minute eye, continued. Reversed selection was inoperative, for economy can not have affected the eye for reasons already stated. Simply the loss of the force of heredity, unless this was caused by disuse or the process of germinal selection, can not have brought about the conditions, because some parts have been affected more than others. Considering the parts most affected and the parts least affected, the degree of use is the only cause capable of explaining the conditions. Those parts most active during use are the ones reduced most, viz., the muscles, the retina, optic nerve, and dioptric appliances, the lens and vitreous parts. Those organs occupy- ing a more passive position, the scleral cartilages, have been much less affected and the bony orbit least. The lens is one of the latest organs affected, and not at all during use, possibly because during use it would continually be in use. It disappears most rapidly after the beginning of absolute disuse both ontogenetically and phylogenetically. All indications point to use and disuse as the effective agent in molding the eye. The process does not, however, give results with mathe- matical precision. In Typhlichthys snbtcrraneus the pigmented layer is affected differently from that of Amblyopsis. The variable development of the eye muscles in different species would offer another objection if we did not know of the variable condition of these structures in different individuals. Chilton has objected to the application of the Lamarckian factor to explain degeneration on account of the variable effects of degeneration in various invertebrates. But such differences in the reaction are still less explicable by any of the other theories. MB1. WHO! 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