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She +e } a ; fh py atc a4 " ai | ee } A Oa oonT B 1 (| a ae ea ( iy aE ty hon ‘a .) } en ne oe ab wae ah Poa Pe ity ll neken i eee bu so a wy : i, WOON AL an ar es a ud ae 7) ra Un) ee Reh | i : - “a ' } ee I 7. ay - a i" PW ik | a ae : che! - | ied i = / 7 rv a on iN - 4) Be foe a cnet Py ileel - i ae } r a) i a Oh 7 : 7 oy Mu ny ere ea a vi , 3 eer Pa wan ‘? tie! TA oe f ep 0 eh ‘wei te a ne wi : eu Y 4 a) -_ 1" ,; a) =; a Stay Cae at - efi } a - 7 ASN emcee erga | at ; if My 1 Oe 7 : ay 7 hon h oe es a _ ie it Pues Cy i Bas wis nt 1 pT eS eye mes A OrT: wy Aint ae ie tay eee 7 eae ne ye ie ; 7 ie re ue Dp ane é ; c.. ' Fin : by a i ae : : (ate ay : t : >y ae + il ‘A ay Ndi uy Ae bs iS Ae a = 7 7 ae f rave : i 7 : ‘Ae a : oy : e 7 Py rt) Ws , a, Al ath ie Ms 14 ii 7 h wih de bie 7 Mi =) q Pa : top ' be ” ne ‘ dg ri me . May yen ra De fl WA hee vs vs it ; 1! eee a ‘/ ay ie mie 7 oh ar MAG : ; ia e ae oe en a a i a Ve Me ae a ,j a ih. fd ae wo ot bt ecient mn vee " are hs Cs , inl iy - mad), hat ; ys ‘a Hy f 7 ; ‘| Me :' ; tre Pt é ‘ P; Sekt MIMI, |) CR Ol, a ie Mk poee . by hk Tsecreeern, 1 i Pann 5 va ae i Hts dit y Pee a en Mi gi *\ Mi, ‘a i. i te 7 ier D 7 , r y ; fe, poe : ie ry th an 4% . : ; : : : - 49 Minute Anatomy of the Eye of Rizneura . 3 c : ; : : : : - 50 Typhlops lumbricalis . 3 ; : : : : 6 c 5 : sa General Account of the Byes of Snakes : : . 2 : : : < : > 54 Eyes of Zyphlops vermicularis. 3 : : : : 3 ; ; : : - 55 Eyes of Zyphlops lumbricalis . . 5 5 : : : : : 9 ¢ 50) Conclusions as to the eyes of Blind Reptiles : : : 5 ; : : : : 59 Amphisbena . : : : : : ; : 5 c 4 . 3 59 Rhineura . : 3 : : : : : : ‘ 2 5 : 5 : o Le Typhlops . : : : : é c ; : 5 : ; : Go Eyes of Cyclostome polistotrema stot : . é : : ; : : : ; : = | 6 Fishes : : . : : : : - A 6 c : = Gz General Remarks on the bye of Fishes : © : 5 : c : c ; ; > 62 The eyes of Zygonectes notatus . : : A ; : : : - 64 Typhlogobius : The Point Loma Blind Fish andi its Relatives é 5 : : A - =) 65 The Eyes of the Blind Catfish, Ameturus nigrilabris . : : ¢ 5 ‘5 : 5 - 69 lil lv CONTENTS. Fishes — continued Pace The Amblyopsidz ‘ ; 3 ‘ ‘ 3 y iy 5 : Fi 7 “FO Relationships of the eae onside . * . ; : 5 : : ‘ : aD ee7fe) Distribution of the Amblyopside : : 3 5 : : 5 5 : : Ae Ai Amblyopsis speleus 5 ; : : c 5 ; 5 ; : ; c eyh! Lroglichthys rose . 5 - : c : 5 : ; 7 : : : 72 Typhlichthys : : 3 ; é 7 : : F 3 : : . 2 Typhlichthys subterraneus . : . : : : 3 : : : . Ye} Typhlichthys osborni . : : : : : d : 2 ; : - - 74 Typhlichthys wyandotte : : é : 3 3 : : 5 0 : ae 75 Chologaster cornutus . ; : ‘5 ; : é : : : c ; ot As Chologaster papilliferus ¢ : 7 c ' : : : 3 0 é 75 Chologaster agassizit . : : - 5 : : : : - : 5 7) 76 The color of the Amblyopside — . : : : , : 3 : : 5 : - 76 General habits of Amédlyopsis —. : : : : 5 ; : : C 5 a £6) Respiration . : : : : = : : : : : : . ts Feeding habits of Anbbohcs 3 : : . : : E ¢ ‘ : : 2) Sk Habits of Chologaster . ; 2 ; : : : ¢ : 5 c es é SEOs Reactions to Light. : : ; : : : . : ‘ : : : Sa 187 Breeding habits of Amblyopszs . : : 3 : : : < : eeeG2 Rivalry of Males and Secondary Sora Dieeencee ; : : : 5 : : : 3293 The Egg and General Development of Amblyopsis — . A : : A 5 é : 04 The Migration of the Anus : F A ‘ , c : ; F i ; é 32.595) The Tactile Organs. : : : : : : : : 5 . , : E - ‘96 The Ear of Amblyopsis 2 : ‘ : . : : : : : * 5 - Ico Does Amblyopsis “ hear”? : : : , : : - : : - : : - 102 The Brain of Amédlyopsis . : : : 5 2 : . : : ; i - 106 Conclusions on the Aniblyopatde : ; é : é é : : : 3 : + 109g The eyes of the Amblyopside . - 5 , , 7 : F : c 4 : A) Chologaster papilliferus : : 7 : co p : : 5 : = = /ELO Chologaster agassizit . : ; ¢ A 2 : a : = 2 3 c + E16 Chologaster cornutus . : ‘ é : : : 3 A : : - SeeLI7. Typhiichthys subterraneus . ‘ : : . a : ¢ E ‘ 5 - 120 Troglichthys ros@ f A 3 A é : : : 5 : é = 126 Amblyopsis speleus . : A 6 : 5 : . c “ 5 > 134 Summary of the Eyes of the anionic : : : é c = é on 14s Development and Later History of the Eye of Aabiiebses 5 é ; 2 : : : 2 LA7, Growth of the Eye from Time of its Appearance . : ; : 5 ; - é ~ 257 History of the Lens. E : : c : a : . : c : : - 158 History of the Scleral Cartilages . : : : : : : : ; : ; = 158 History of the Optic Nerve . : 5 c 5 : - 159 History of the Development, Maturity, a Deeeeeuan of the Eye : 3 - 160 Comparative Rate of Ontogenetic and Phylogenetic Degeneration of the parts of the Eye - 164 The Future of the Eye : : 5 4 - 166 Retardation and Cutting off of re ey of he Devedapmnent Gh the Eye : : : - 166 Causes of Retardation and Cessation in the Development of the Eye . é : : 2 167; The Eyes of Amédlyopsis and the law of Biogenesis —. ‘ : : A : : - 170 Conclusion . A 5 A E a é : a A173 General Summarial Aeedant of the eyes of the Aniblvopeide ; 3 é ; 4 : 2 175 Phyletic Degeneration of the eye of the Amblyopside . : 75 Results of Phyletic Degeneration on the Different Parts of the Eyes of the Acai aecides eel77, Ontogenetic Degeneration . F ; : ; : : . 180 Plan and Process of Phyletic Degcreatiens in ite. Ambion : : : : é . 180 The Cuban Blind Fishes . : : - : : : : : F : ; < : - 183 History of the Work . : : ; : 5 ‘ . : . ELS Zoological position of Stygicola and urea : : : : c : > 3 2187 Primary and Secondary Sexual Characters . : : 3 é 5 : 5 c Si iSy Distribution of Stygzcola and Lucifuga : : 3 : é : = : > - 188 CONTENTS. The Cuban Blind Fishes — continued Nature of the Habitat of Stygécola and Lucifuga . Abundance of S¢ygzcola and Lucifuga . Origin of the Cuban Blind Fishes Physical environment of Stygzcola and ba a and their Reactiong to it Biological environment of Stygzcola and Lucifuga General habits of Luczfuga and Stygicola Breeding habits of Luczfuga and Stygicola The Ovaries of Lucifuga and Sty, es The Eyes of Lucifuga The Eyes of Stygicola On the Ovary and Ova in iar aud Sygcole Conclusions in Regard to Luczfuga and Stygicola The causes of Individual and Phyletic Degeneration . Frontispiece. Plate A. Is NON ND mw moot NF OO ON AM HW NH O bw NHN NN ON Amn & WwW iS} xe) CPW AX EYES MISE .Or shi Adwes.: Entrance to Ariguanabo River, Cuba. Blind-fish rocks at base of Point Loma, San Diego, California Twin and Shawnee Caves Chologaster papilliferus, S, deler pies seiiaticaneda Splerbs ssnagr Zz iad Zypletrito Speleus ¢ Spelerpes longicauda ana Typhiomolge juhbune Rhineura floridana Eye of Zyphlops Pere Amblyopsis . Chologaster agassizit, ve Payne r0S@, penal Typhlichth Lys nee aneus Views of Améblyopsis, early stages : : 5 Tactile organs of Améblyopsis and Chebeeser Heads of Zygonectes notatus, Chologaster agassiztt, C: Rotazaster papilifors, Zyphichtys subterraneus, Troglichthys rose, and Amblyopsis speleus é é Photographs of the eyes of Amblyopsis and Troglichthys . Carboneria Beach near Matanzas. Cave of the Insurrectos, near the Gabonen : . Young of Lucifuga in Ashton Cave. Cave Isabella, showing roots . Stygicola. (Preserved specimens) . Living Stygicolas . Views of Lucifuga : - Sections of eye of Lucifuga . . Two sections through right eye er gee. ; . Sections of eye of Luczfuga, showing contents of lens, its aad ire ers Bf retina . Eyes of Luczfuga, showing pigment layer and retina and folding of sclera . Eyes of Lucifuga, showing differences in size and structure . . Sections through left and right eye-cavities of Lucifuga . Sections of eyes of Luczfuga, showing eae layer and cells and ebtace: aaah rectus muscles . Eye of old Luczfuga, Stoves SCG mass tend apes nebrone bout eye . Eye of Lucifuga . - . Eye of Stygicolas and Luc ee : . Eye of Stygicola . . Ovaries of Luctfuga and Stygicoa . Sections of ovaries : - Sections of ovaries FACING PAGE . Title 6 12 28 48 54 70 72 NN NNN NN WWW NRHN ew NNN NNN AGA N ~ KEY TO DESCRIPTION.OF PLATES AND TEXT FIGURES: 1. Pigment epithelium. pi. Densest pigmented section of the pigment epithe- lium, just below the nucleus. . Rods and cones. . Outer nuclear layer. Outer reticular layer. Horizontal cells. Inner nuclear layer. Spongioblastic layer. . Inner reticular layer. . Ganglionic layer. . Optic-fiber layer. a, 0, Ophthalmic artery, am, Ameloid bodies of the pigment epithelium. 4, Brille. bac. Rod. ct. p. Ciliary process. cj. Conjunctiva, cj. s. Conjunctival sac. chr, or cha, Choroid. chr. 1, Choroidal lymph. chr. f. Choroidal fissure. en. Cones. cn. nl, Cone nuclei. cor. or crn, Cornea. OD ONIONS U ND cps. or cpl. sng. Blood-corpuscles in normal vessels. cps. s. Stagnant blood-corpuscles. d. Dorsal aspect of eye. dr, Dermis. e. m. End member of cone, F. cj. Fornix conjunctiva. jr. ol, Olfactory pit. ha. or kyl. Hyaloid membrane. H. gl. Uarder’s gland. Iris. 1. Outer layer of iris. 2. Inner layer of iris. c. Interpolated cells. Left side of eye. Z a2 Zz dc ih 21, 22, 23 First, second, and third labial scale.’ vl dns. or 2. Lens. 2. c. Lens capsule. M. Miillerian nuclei. m. m. Middle member of cone. msc. or mu, Eye muscle. ni. Nucleus. nl. con. Cone nuclei. nl. f. Nuclear fragments. nl. g. Nuclei of the ganglionic cells, ni. 1. or ni.) Elongate nuclei of the pars ciliaris. nl, Muet. Miillerian nuclei. n. op, Optic nerve. n. s. Nasal scale. oc. Eye. o.¢c. Ocular scale, o. f. Orbital fat. o. Ss. Ocular scale. of, Otolith. p. Pupil. p. i. Palpabra inferior. p. Ss. Palpabra superior. ji. s. Pigment appearing in optic cavity with senes- cence. pt. sph. Pigment spheres. p. 4. Pigment layer. po. s. Preocular scale. pr. nl, Processes of the cone nuclei. pupl. Pupil. y. Right side or retina, ry. or rt. Retina. ro. Rostral. scl. Sclera. sel. c. Scleral cartilage. subo, or sb, orb, Suborbital. v. Ventral aspect. vit. Vitreous body. x. Flattened cells beneath pigmented layer, of doubt- ful significance, y. Flattened cells beneath inner nuclear layer, of doubtful significance, PREFACE. INTRODUCTORY. A cave is a unit of environment so well circumscribed and of such simplicity that we may know its contents, its elements, and its conditions nearly as well as the experimental zoologist knows the contents and conditions of his aquarium. These contents and conditions are of rare uniformity, changing but little from day to night, from season to season, or from decade to decade. The point of chief interest in the cave environment is the total absence of light in all parts except about its mouth. Probably no animals have a more intimate environmental adaptation than those inhabiting caves. This adaptation is largely of color and structure of eye, which modifications are surpassed only by the functional adapta- tion of the tactile apparatus of the blind forms. While no one has followed, and although we may not be able to follow in detail, the steps through which the cave animal has acquired this environmental adaptation, a knowledge of the present condition of their unchanging environment gives us a knowledge of what it has been during their entire period of development. We know, or can know, what the present stage of their adaptation is. Not in- frequently we know what the condition of the animal was at the start of its cave experiences and enough of the steps along its line of evolution (indicated by the degrees of adaptation reached by different members of the group) to enable us to form so clear a picture of its entire route of evolution that we may conjecture what elements of the environment caused the modifications, and by what process they were brought about. We have, in other words, a long experiment conducted by nature unrolled before us. I propose in this work to give an account of the cave as an environment ; to bring together in a revised form the papers on blind and cave vertebrate animals so far published by myself and my students, together with further observations on the species previously considered, to consider the habitat, mode of life, and the origin of the Cuban blind fishes, and to give an account of their eyes. My first experience with blind vertebrates was in 1886, when Superintendent Funk sent to Indiana University a living blind fish which had been taken from a well at Corydon, Indiana, and which proved to be a new species, T'yphlichthys wyandotte, the only representative of the genus so far taken north of the Ohio River. Later, when a stay in southern California came in prospect, a study of the blind fish, T'yphlogobius, living under rocks along the base of Point Loma, was one of the first definite plans formed. When, in 1890, I returned to Indiana and was once more within reach of the caves, the problem again came up. My laboratory is excellently located for the study of cave faunas, the series of caves to which Wyandotte, Marengo, Mammoth, vii viii PREFACE. Colossal, and Nickajack belong, beginning in or about the campus of Indiana University. But while seemingly ideally located, and in spite of the fact that numerous trips were made to Indiana caves, especially those from which blind fishes had been reported, no blind fishes were found till 1896. In May, 1896, I was again looking for blind fishes east of Mitchell, Indiana, this region being drained by underground streams. East of Mitchell several of these find their exit in caves of romantic beauty in the escarpment flanking the valley of White River (plate A). The roof over one of the streams has fallen in at two places, Dalton’s Spring and Twin Caves. At Dalton’s Spring the cave-stream runs above ground for about too yards when it again enters its subterranean course. Within sight of the lower opening of the “spring” I saw two blind fishes swimming in a quiet pool. I secured about 20 specimens and had found the stream which in its varying reaches has furnished me with an unlimited supply of specimens which have enabled me to give the complete history of the eye of this species, Amblyopsis speleus De Kay. More material has been obtained from this cave than from all others put together. In 1903 the State legislature of Indiana placed the land, about 182 acres, on which are the entrances and exits to this stream in the keeping of the trustees of Indiana University. While some litigation has arisen as to the ownership of the farm, it will probably be permanently preserved as a State park. ACKNOWLEDGMENTS. Through grants from the Elizabeth Thompson Science Fund and from the American Association for the Advancement of Science I have been able to visit the cave regions of southwestern Missouri, about San Marcos, Texas, Corydon, Indiana, and Mammoth Cave, Kentucky. In 1902, through a grant from the American Association for the Advancement of Science and assistance from various other sources, I was able to visit the blind-fish caves of Cuba. Subsequently the Carnegie Institution of Washington aided me in making additional investiga- tions in Cuba. The part of the present volume dealing with Stygicola and Lucifuga is my final report on the work carried on with this aid, and in it a detailed account of the Cuban work is given. Prof. S. A. Forbes kindly lent the drawing for figure A, plate 1. The draw- ings of sections of eyes were made under my direction by Mrs. E. R. Bieling in the laboratory of Prof. R. Wiedersheim, in the University of Freiburg, Germany, and I am indebted to Professor Wiedersheim for placing his laboratory at my disposal. I am under many obligations to various friends, both at home and in Cuba. Mr. Oscar Riddle, Dr. John Beede, Mr. John Haseman, Mr. Norman MclIndoo, and Mr. T. L. Hankinson acted as volunteer assistants on various Cuban trips, always working without remuneration and in part paying their own expenses. The late Prof. Jose 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, has been invaluable. His finca, the “Isabella,” is at the margin of the cave region of Cuba, and in the interval between our trips he ferreted out unsuspected caves, determined their rich- ness in blind fishes, and put himself at our disposal in guiding us to his various finds. Prof. D. W. Dennis of Earlham 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 T'yphlomolge. 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. (t) The bleached condition of animals living in the dark, an individual envi- ronmental adaptation, is transmissible and finally becomes hereditarily fixed. (See page 8o.) (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 AND, 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 to 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. Zyphlomolge 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; * 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 ‘ 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 been 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 Cafias 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 Lucifuga 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. Only the larger streams whose rapidly deepening channels have made the caves possible flow on the surface. ‘‘One may travel on horseback all day, through certain parts of Kentucky, without crossing a single running stream; all the rain water that falls being carried down through the sink holes into caverns below where are the gathering beds that feed the few large open streams of the region, of which Green River is an example. It is reported that there are 4,000 sink holes and 500 known caverns in Edmondson County (Kentucky) alone.” * ' For an account of the principal caves of North America see Hovey, Celebrated American Caverns, Cin- cinnati, 1882 and 1896; and Packard, The Cave Fauna of North America, Memoirs of the Nat. Acad. Sci. vol. 4, 1888. or 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: (1) 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. Glaciéres, 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." 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 ro° C.’ 1 A very extensive list and excellent account of glaciéres is given by Balch in his Glaciéres or Freezing Caverns, 1g00. Concerning the cause of glacitres, 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 . . . 0° to 24° Bottom, 1895 . . . 1.5° to 17.1° Surface, 1896 . . . 0° to 26° Bottom, 1896... 2° to 16° 6 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 may be very clear; after rain it may carry a large amount of sediment. In its low condition it may flow 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 go 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: Temperatures for air and water in Donaldson and Shawnee Caves. Maximum Maximum i | Maximum Tempera- | tempera- Tempera- | tempera- Minimum Tempera- | tempera- | Eee ; ture of air) ture of | ture of air} ture of “tempera- ture of air} ture of tempera- Time. _|in center of|water at its| Time. in center of|water at its! “(ure at Time. in center of|water at its) “ture at | Donaldson| exit from |} Donaldson} exit from ein Donaldson) exit from anne | . Cave. Shawnee Cave. Shawnee SFiS Cave. Shawnee place } Cave. Cave. bgt | | Cave. cae eves te = a | a aa = — a 4 == =f Se 1goo. 1907. 1907. | rulyeeerrs 12.7 12.4 Webstore iy 01.67 9:5," ||| Julyce.e.| “sortor || =eeced Sc August... 9 12.5 Feb.=-- 11.5 et. 3 8.9 || August...} 12.7 16.4 | 13.1 September) 13.2 12.6 March..| 11.5 12.6 9.9 September] 12.7 723 al LS | October - . iad |\pealene a | April.-.:|(< 2i55 I2.I | 10.2 |} October.-| 12.2 13.4 T203 November! 11 10.3 May.-..] 1.5 12.8 | -x1.6 November] 11.9 rey le ara December] 12.2 | Io. | June.... DIT. 15.1 12.5 || December} 11.7 L2.0 7.4 * The higher temperatures are caused by rains and last only a few hours after a heavy rain. During the first 10 days in September, 1907, the temperature of the water was 14.5, 15.6, 17.3, 16, 14.0, 14.6, 13-9, and 15.3 on successive days. During the last 10 days of the month it ranged from, 15? to 15.5°. ? From the 1st to the r5th the temperature was between 10.6 and 11.6. Currents in water and air differ materially in different caves and at times in the same cave. 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 crust of carbonate of lime and magnesium. In the blind-fish caves at Mitchell, Indiana, a small current of water flows during normal conditions. The stream becomes a raging torrent in high water. Currents in the air may be caused, (1) by the flow of water; (2) by the epigean air-currents; (3) by changes in the atmospheric pressure; and (4) by differences in temperature.’ ‘A detailed study of the currents of air and temperature of the water in the Mitchell Caves will be published within a year. PLATE A EIGENMANN Entrance to (Lower Twin Cave) and exit (Shawnee Cave) of underground river on the Indiana University Cave Farm. Twin Cave. Shawnee Cave. The two openings are at opposite ends of an underground tube about three-quarters of a mile long. Dunng winter the warmer air of the cave flows out of the upper opening. The moisture in the outflowing air congeals, forming the heavy frost seen on the shrubs above the opening. ; ja 2s. sed . ny - 7 7 - ; Seip a a — : 7 7 ioe 7 Vv. a uP 7 i - ; my - y ah ’ = a : i ve 2) > = - _ 7 o A! rut 3 - D ~ i 7 > : 7 an a : i ’ - 7 0 7 7 _ i ~~ = > : _ ‘a : ee =a * 164 Aare , -_ i 1 ae 7 - ft 7 ui vo . - a das | ed pe 7 7 7 igh? #) ’ a i f ‘eo . iy : - 7 i] - _ eo ae . : ! 4 » : “n ' _ = Hs _ 7 ® . - ‘ - ca A 47 7 7 a Oe I ; ; - a a) A Tea ” : Be ' a “a Ans : _ a a 7 . : _ § : 7 ip; aa 7 , 7 | - _ a ae en op 9) % . a - : i 7 err & MD yee Jin he a : i y ! eis. 7 : - - ' ar | | hp a ee r Sa, “4 ae) A — Ac! \ te Ste a - ae : fae t : -.5 D : a G Bs 4¥ " 7 oy) is = Whee, a a : ; 4g - : _ 7 ¥ o 7 _ AIR-CURRENTS IN CAVES. us 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 rg02 I was also at the entrance * 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; g" 40 a.m., 55,830 feet; and 12" 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 4o seconds, stopped r5 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 ; 5 30, p.m, 77,396 feet; 6" 20" p.m., 79,896 feet; February 19, to 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.? 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. 5 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. Asa 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 size is incomparably richer than the subterranean one.' 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). ' 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 Siluridze, 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 aquaticus), the long-tailed mole (Parascalops brewert), 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 phcebe 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 domammals. 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 10 BLIND VERTEBRATES AND THEIR EYES. burrow in the ground. Amniella, a small, legless, burrowing lizard of California, probably indicates their origin. ‘This lizard has well-developed eyes. It burrows in sand and gravel. I have frequently seen it cautiously thrust its head out of the ground for an instant as if to take a survey of the field. It evidently still uses its eyes. Amphisbeenians,' which are widely distributed over the warm parts of the globe, burrow in the ground or live in ant hills, and are partially or totally blind. The blind snakes, members of the Typhlopidz, have similar habits.” Many salamanders live in damp earth under logs or rocks. It is but natural, therefore, that they should be found in or about the entrances to caves, where sheltering rocks are not infrequent. Others are true cave animals. ‘Two of the salamanders in North America that habitually live in caves have apparently quite normal eyes. They are Spelerpes maculicauda found from Indiana and Kentucky to Missouri, and Spelerpes stejnegeri from southwestern Missouri. ‘Two others living in caves have quite degenerate eyes, T'yphlotriton speleus from caves in southwestern Missouri, and T'yphlomolge rathbuni from the caves of Texas. Pro- teus, the nearest relative of the latter, lives in the caves of Carniola. There are no blind epigean salamanders. Of Anwra there are no permanent residents in caves, nor are there any blind forms. A jumping animal would be sure to meet with dis- aster in a cave if it practiced its usual mode of progression. The classes of vertebrates furnishing the largest number of blind forms are the fish and fish-like vertebrates. Excluding the Branchiostoma, the Cyclostomes have for the most part degenerated eyes. Polistotrema stouti is quite blind. Benthabatis moresbyi Alcock is a blind Torpedinid Selachian from ‘Travancore, from a depth of 430 fathoms. Of the lowest teleosts the Siluride are represented by Gronias nigrilabris Cope, which occurs in a cave near Philadelphia.* The eyes of many other cat-fishes are not highly organized and but little used in detecting food.* Other cat-fishes are occasionally met in caves, but no others are permanent residents. The cave fishes of North America, par excellence, are the Amblyopside. All the members of this family, 8 in number, have degenerate eyes; 5 have mere ves- tiges; 6 permanently live in caves; 1 is known only from a spring and another from open streams. These will be considered in detail later. In Cuba 2 fishes belonging to a marine family, the Brotulidee, have become adapted to a cave life in fresh water. Both are blind. Many of their marine relatives are also blind. Along the coast from San Pedro, California, to Encenada, Lower California, but more particularly at the foot of Point Loma, a blind goby lives under rocks embedded in sand between high and low tide. 1“ All the members of the family are burrowers, and many live in ant nests. They bore narrow galleries in the earth, in which they are able to progress backwards as well as forwards. On the ground they progress on a straight line, by slight vertical undulations, not by lateral movements, as in other limbless reptiles; the tail of many species appears to be more or less prehensile. The food of these lizards consists of small insects and worms. * * * As many as 65 species are characterized in this account; 39 are American, of which only 2 (Chirotes and Rhineura) occur north of the Tropic of Cancer, and 4 (Amphisbena) in the West Indies.’’ — Boul- enger, Catalogue of Lizards, vol. 1, p. 430, 1885. 2 There are altogether about 100 species reaching, in the Americas, as far north as Cuba: Typhlops hum- bricalis, Yucatan; Typhlops microstomus, Mexico; and Typhlops tenuis, Guatemala and Mexico. 3'Two blind cat-fishes have recently been described from Brazil. 4 Herrick found that the cat-fishes detect their food, not by means of their eyes or olfactory organs, but by the touch and taste organs over the body and in the barbels. MARINE BLIND FISHES. 11 The fishes, blind or partly blind, living in the depth of the ocean bordering the American continents are Jpnops murrayi 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 from the type specimens from Lat. 2° 34’ N., 92° 6’ W., at a depth of 1,360 fathoms. Jpnops stands alone in a family and is the only vertebrate in which no eyes have been found. The Brotulidze 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 A phyonus mollis Goode and Bean, 955 fathoms, and Alexeterion parfaiti Vaillant, 2,736 meters. Other deep-sea blind fishes are A phyonus gelatinosus 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; 7'yphlonus 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 Eigenmann 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.! | The Amblyopside are a small family of fishes, 8 species 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 are 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 the accident of being carried into caves happened to 6 or 7 out of 8 of the Amblyopsidz, and to only 1 of all the other fishes about the caves. The absurdity of this supposition is self-evident. A comparison of the abysmal fauna with the pelagic and shore faunas would probably give us similar results. 1 A distinction ought possibly to be made between the aquatic cave animals that will be discussed under the “fourth process,” and non-aquatic forms. Non-aquatic cave animals are later immigrants of caves. ‘These must either be voluntary recruits from the twilight fauna about the entrance of the cave or they must have become other- wise adjusted to live in the dark. There is no difficulty in accounting for the presence of Myriopoda on this score nor for the other forms habitually found under bark and under rocks. Myriopods are,everywhere abundant in the caves of North America and they (if any animals) may have accidentally been carried into caves with sticks of wood or trunks of trees. EIGENMANN ae PLATE | MrsERBiehing del I Tib ah Lesh al iP x pated etre? tres Bie ag B Me A. Chologaster papilliferus. B, Bb. Spelerpes stejnegeri. 112 mm. Wilson’s Cave, Sarcoxie, Missouri. C, Cc. Spelerpes maculicauda. 130.5 mm. Wilson’s Cave, Sarcoxie, Missouri. D, Dd. Typhlotriton spelzeus. 134 mm. Marble Cave, Missouri. f Pair i i é ’ (yesr’ ' bests 4 ds ve j } ’ v8 a3 «1 i Litrp ety i tf ‘ ta : iH a sii "f ij at re) } in ivti rv. | ii ‘ } ' ; 1S § ¢ ‘ rite) day anit = ’ ] Te } ' f * ib) i ; Ly ei hovers ‘ mr * ‘ ’ “ i “i j bd | : ry st ‘ ' , ’ « ' ew atl) fl f tia Var i i ‘a sie ae Ta (yrerseser Pu cpl . A — 2 a | 1p ech : ron vob baa + ooh bo Sern ee g A ee r Th “Wa weenie -_ * ty ohitl| soceneoek taleatle Waow Vee? BNE | eR Se) 7 THOhS aT ra path ape? ov wit \ sfenrers 1 . tog ¢ bg - wh? “veers Hon ob a ar hs veri he Wife tie Smee ail Lett” “enti 3a r ‘ 4 P + 7 rs hy iwienrp cnitoilay wh ada wort 49 . ‘ ~ oe ' | = - eR) I ROTA A 4 tle preter) 2oveo) bit Sere aa ' 1 otal ) @iamin ty outa”, fuses ee er tdn ae el ic) ‘VED aX) TY UPRT IO oe yes ME Oy jedi OLA AU , > ae [ees ij (et ay bee ; fad y 5 ae +) «) pele amt ‘.. ‘ 4 » . 7 ‘ coo) qd teetion Showitd ont lo SOR ene ae, ST ee ‘ Pa : 4 Ps Par } 4 vero fi Wik 3 erp pet fey ee , bh dth we Te a a por ' Use we) polio Wea ther oy septiy any ta i \ +4 > , | Alvav ea. Te 2 rege ty Binwiin Seg SAG LOcrhgRuys { 4 d _a . : ’ be Jove i =v’ fo ye ota yee deb bie lado. par sea ; ad tn oe shila boll ; Yitod bag baw sede! Grow “ele ae nf) jad Ly tisnnay , , - > f - Py . é ‘ — , beater rity ie tiPh TM] Rh cline wt it tot 17a we ¥ - eo." | shtapaerrre Try OE bree veletartceeh: ter Seb teen OF wet ‘ : yore pittey neObavTy ’ . >. r > Po * 4 ¥ 7 yitbs P Rei bet ko ERs rrr a fod Miata Be ii aes Vata * * a a > n nit dant a) ay ave core orl boliige vies imlanies | » ilhypyties tye - oD teens ae iwi ii pifol ve! i yee . fica) sehue 3 ane gel she pee Bearer oe 14 ml ireipald : Vian croft) fines of que ie , Meareh iets vege ce ail Ute, Seis tent j Paeedon sed tay i',) als “t ES rete: ga: Henk ail : rt a coi theaphe Saecrp rae | in , juts wi : . sie twotad teeth adie Firat rity et} 7 ety b -efene boom’ pointy gitus ° |. F ‘ rete nwl? cal Canta ae Sees anieto nd are ee eet ' : ‘ tell ml remo see 28 4 sen nies ottisteae net iter : ; se"y trevi'A eueds welt: lead saree PD shar A Pn oa) ad _ a is i in] “h Levefing ath) ateatigt or pear cet tite herp Be ae - 5 “ : 7 he R TAs ery.) To MITYy he Ld = : ‘s a * a 4 7 tis gil eel ene ah acer Land — ; ‘ : ' i jo) 26> avs oul) Meyaraetad owl T- ay Th eR Spal nie ay od thet GN soe ¥ : hree? wh woe ewticaau a wetinel qedlbeveree tails COAT PIR ‘ 5 Tae tas ee bite ylibhvwel f supper atk > “ft i] yet) Gh? eb muleank) om tah : naa t | a < ie fee 0) idabainls toll ' su, a dtel i We ives « ' { ‘ - aan AP Di, 11? v1.4 Be he wwe. ell pci eyalven wth SANT i i . me (tunes ane Deere ; f Ph piriitras, | |} term i ree iede mal ‘ : d 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, xtmt, 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 maculicauda 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 maculicauda 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 ' In answer to the statement made by Eigenmann, Krause [Promethius, No. 457, p. 652, 1898] said: ‘‘ Nicht weil sie in dunklen Hoéhlen leben, seien ihre Gesichtsorgane verkiimmert, nicht die Entziehung des Lichtes habe diese Organe zuriickgehen lassen, sondern umgekehrt, weil sie sich schon in der Oberwelt dem Leben ohne Licht angepasst hatten, waren sie wohl vorbereitet gewesen, in den Hohlen, von volliger Dunkelheit umgeben, so — glanzend, kénnte man beinahe sagen —zu reiissiren. * * * Nun, wer’s glaubt, mag ja auch bei dem Glauben selig werden kénnen, dass die Héhlen gleichsam zum Tummelplatz und Elysium der Blinden aller Thierklassen erschaffen seien. Wir haben diese Sirenenklinge aus dem mystischen Dunkel der Gegner des Lichtes und der Entwickelungslehre schon 6fters gehért; sie stehen in Harmonie 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 shells 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." 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 the 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 fish 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 beneath 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 above. As the stream sank beneath the surface, naturally those fishes depending on light for food and courtship left it, and only those either negatively heliotropic or positively stereotropic remained. The blind aquatic fauna looked at from this standpoint is not a new acquisi- tion of the present cave stream, but a relict of the fauna of the river when it still flowed above ground. The cave and its fauna have developed hand in hand. The presence of the cave fishes and other aquatic cave dwellers do not so much need explanation (they were present long ago) as does the absence of all of the other forms that must have been present when the stream flowed in its epigean valley. The prime requisite for a candidate for underground existence is a nega- tive reaction to light, or positive stereotropism, or both. It must also be evident that a fish depending on its sight to procure its food can néver become a cave form. Sun-fishes, which are annually carried into the present fully developed caves, belong to this class of fishes. They are always 1 Shaler, 1875, considers that during the glacial epoch the conditions in the caves of Kentucky were such that the present fauna could not have existed 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 Amblyopside belong to the latter class. It is surprising that more cat-fishes have not established themselves in caves. Among the Amblyopside, even those with functional eyes depend on touch and vibrations for their food. Chologaster 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 7Typhlichthys, 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 Amblyopside 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 Spelerpes. Some of them, like the myrio- pods, may even be accidentally brought in with their food and habitat,’ 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. g 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. The cave salamander, Spelerpes maculicauda, 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 Spelerpes may have taken place with Amblyopsis, except that Spelerpes found its caves ready made, while Amblyopsis was present during their making. The ancestry of the Amblyopsidee 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. Garman’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 are 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 Amblyo psis 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: (1) That the cave fauna is in large part the result of the formation of the caves themselves, that environment and habitat developed pari passu. (2) That to this original fauna have been added and are being added species (such as Spelerpes maculicauda) which, because they are negatively heliotropic or positively stereotropic, are gradually becoming adapted to the deeper and deeper recesses of caves. (3) That to the fauna of the larger caves may also have been added animals which 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 CAYES. 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 62 mm.; Typhlichthys, 55 mm.; Troglichthys, 55 mm.; Amblyopsis, 135 mm.; Lucifuga, 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, (b) the entrance and point of inflow of the stream in a wet cave. That cave is best 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." 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, Spelerpes maculicauda, has well-developed eyes, that the Missouri salamander, T'yphlotriton, has degenerate eyes, and that the Texas salamander, T’yphlomolge, has very much more degenerate eyes. The degree of degeneration seems here coérdinate 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 EPIGEAN 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; flesh eaters, 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 see 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 seasonal 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- ferent groups of mammals are seen to keep curiously parallel, as may be noticed, for example, in the series of skulls figured by Kowalevsky, where we find similar changes occurring in such families as the pigs, deer, antelopes, horses, elephants, etc. Indeed, one may speak with propriety of a Puerco, or Wasatch, or White River type of skull, which will be found exemplified in widely separate orders. On some riffles of the San Juan River of Cuba I found a small fish that is very strikingly like other fishes inhabiting similar localities in the eastern United States. The former is a goby, a marine form, Philypnus dormitator, which has become adjusted to conditions found about the rifles of streams; the others are darters, Hadropterus, 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- cynine to our garpikes; our mullets are duplicated by Prochilodus; 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 Lucifuga and Stygicola, the Cuban blind fishes, from each other. On the surface the specimens of Tvoglichthys rose very closely resemble Typh- lichthys subterraneus from Mammoth Cave, differing slightly in the proportion and in the pectoral and caudal fins. ‘These fins are longer in rose. 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 subterraneus or rosé 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 ros@ is but about one-third the diameter of that of swbterraneus, 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." 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 Amblyopside possesses scleral cartilages. ‘The ancestry of rose is hence unknown. Amblyopsis has the scleral cartilages, and the eye of rose 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 rose. The epigean ancestry of Amblyopsis is also unknown. The ancestry of Typhlichthys being quite distinct from that of rose, 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; (6) 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. (11) 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. BEIND AND CAV EY VERTEBRATES AND DETR 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. (1) 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 EYES. THE CAVE RAT AND ITS EYES.’ The cave rat, Neotoma magister, ranges eastward to Suu 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., xrx, 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. 1, page 171, Darwin says that the eyes of Neotoma of Mammoth Cave are “lustrous and of large size; and these Fic. 1. (a) Eye of Mammoth Cave Rat. (6) Eye of Common Gray Rat. animals, as I am informed by Professor Silliman, after having been exposed for about a month to a graduated light, acquired a dim perception of objects.” The cave rat, Neotoma, is still abundant in Mammoth Cave. Its tracks are numerous, and in places little paths have been made by the rats where they run backward and forward along ledges of rock. Since, however, a track once made in a cave remains unchanged by wind or weather, the abundance of rats, as judged by their tracks, may be misleading. A number of traps were set in the rotunda. During three days one 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 with a light, the rat turned about and stared at the light. It then ran to a pile of rocks, but did not attempt to hide; instead, the rat ran to one end of the pile, then along the top back to where it had stood, then stopped and again stared at the light. : The histology of he eye is eende snsed pipes Dre Jib: Sitnateraaceeune from which figures 1 and 2 are taken. See Proc. Ind. Acad. Sci. for 1898, p. 255, 1899. THE CAVE RAT. Di 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. Fic. 2. Retinas of Neotoma and Common Gray Rat Compared. (a) Mammoth Cave Rat. (b) 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 1 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. bo C BLIND VERTEBRATES AND THEIR EYES. THE 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. Spelerpes maculicauda (Cope) (plate 1, 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). Speler pes stejnegeri Eigenmann (plate 1, fig. B) is found in the twilight regions of the caves of southwestern Missouri. Its eyes are also normal. Other species of Spelerpes' are sometimes found in caves. Typhlotriton speleus Stejneger (plate 1, fig. D) is restricted to the western caves of the Mississippi Valley. It has so far been found in Marble Cave and Fic. 3. (a) Head of Spelerpes maculicauda, 54mm. long. (6) Head of Typhlotriton speleus, 54 mm. long. (c) Head of Typhlomolge rathbunt, 47.5 mm. long. Rockhouse Cave, and smaller caves in the same neighborhood in southwestern Missouri. 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 Speler pes, but has not yet reached the degenerate condition of T'yphlomolge. Its eyes are apparently normal in the larva, but in the adult have undergone marked degen- eration. The eyelids are disappearing and the rods and cones are no longer present in the adult. ‘The eyes of this species will be dealt with below. Typhlomolge rathbuni Stejneger (plate 2, fig. B) is found in the underground streams near San Marcos, Texas. It has been taken from the artesian well at San Marcos and a surface well. It has also been noticed in one of the caves near that place, Ezel’s, in which the underground water can be reached. It is said to have come out of some artesian wells south of San Antonio. It is a peren- nibranch and spends all of its time in the water. Its remarkably long and slender legs are not able to support its body when out of the water. Figure 3 shows ‘ Bilineatus is frequently found about the caves of Bloomington, Indiana. EIGENMANN rye Be Ser ee REATE?: t e % i { re ie i sypstt. A, Aa. Spelerpes longicauda. 147.5 mm. Carlisle, Pennsylvania. B, Bb. Typhlomolge rathbuni. 88 mm. San Marcos, Texas. at : UFUMAWAIAR BYRD eAeaT Tai oy: ait’) itoand diei4 to et aliiann hep Vito salt ey whol vim wanes bina .ydrpa tet hp ola cnapery of intoiestl oa Oot of- Ail ine ott &:t% paw a oom é ut hWall slaw ‘lieing tthe Htavi-eorry ol) 2uetines Pre wile’ vise eens ah Se Tae a) Gene fi an sali Aoi i rafts « led oD | Sign yakpiane ong to de ' ail hae a it 4a ‘poceal TE? web vet scott ool oem oF . ' » de Teciiowaen conee aft gig enicveay yn vole aagglae- (fo enprisege pobinag: dew oft) O08. 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Bint ant ape rel iter ot col¥é hen) 16 alt bquali vileied ee _. ‘eho fvrett ote. ae byl a elt ban freed tye: : unit odtlu Obirks adtrhinds al evel se Shik ail ot net a ee 4 et eis pon ow Oded 2) 40 gale ee ii Sales oie mo vilagen tos) goed atabe See] ah epee B somierigirn sinvxttt 7 dined Saeed at ful wi ull sleule 9 ading tee ‘ mexyl) Ont) xem ah draepete cpl othe awals aaicqot bod s ose ia , oat 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: (1) 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 16 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, sufficiently 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 obstacles, 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 front 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, followed by a sudden snap and seizure. The salamander 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. THE EYES OF TYPHLOMOLGE RATHBUNI.* The U. S. Fish Commission, through Dr. B. W. Evermann, sent me four speci- mens of this salamander and a number of its eggs. ol Of these, one adult had been received in Washington, April 8, 1896, and three young, of different sizes, March 1, 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 the Eyes of Typhlomolge in Millimeters. | DIAMETER OF LEFT EYE. DIAMETER OF RIGHT EYE. LENGTH OF | DISTANCE —— —$— SPECIMEN: BETWEENVEVES: | Longitudinal. Transverse. Longitudinal. Transverse. —-- ~ | —— — — — — mm. | mm. 30 | 1.44 0.336 0.232 0.368 0.240 47 1.92 .432 .320 -432 «304 70 | 3.10 544 384 -608 308 oD Soe -496 -432 “544 384 go 4.00 +592 400 -592 -448 The eye of T'yphlomolge is, in many respects, much more degenerate than that of its European caverniculous relative, Proteus. In Proteus the six muscles are all present; in T'yphlomolge 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, 7.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 u above and below this point gives the following: thickness over the eye 73 p, 320 # above the middle of the eye 96 4, 320 » 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 associated 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 specimens: Cartilaginous elements are found in the sclera of but two eyes. In one individual, 90 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 Fic. 4. Outline Sketch of Part of Section of Head of Specimen of Typhlomolge rathbunt, oo mm. long, showing Position of Eye. the optic nerve and measures 96 » in thickness, 160 » vertically, and 204 antero- posteriorly. 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. Over the front of the eye there are a few denser strands, which may repre- sent the remains of the cornea. Over the sides of the eye of the largest individual the sclera measures from 4 » to nothing. About the entrance of the optic nerve it attains a thickness of 14 #, and contains many flat nuclei with a length up to 17 p. The choroid reaches a thickness of 20 » near the entrance of the optic nerve, and dwindles regularly from this point to the distal face of the eye. Blood-vessels are found in it next to the pigmented epithelium of the eye. Otherwise it is a mass of pigment interlarded with streaks of colorless tissue containing nuclei. Over the front of the eye, next to the epithelium, there are a number of colorless cells with large, granular nuclei. (b) Essentially the same conditions exist in younger specimens, but the parts are relatively thinner. The ophthalmic artery, extending approximately parallel with the optic nerve during its distal 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 about 8# 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 pave- d BS ag eS AE GG), Usee alr ot Leis of igh Eye Gt Speniaee okt yplomolze yo mus. long. ment epithelium. In places, however, processes of the cells extend in among the cells of the nuclear layers, for a distance of 4o # in some cases (fig. 52), to the inner reticular layer. In the individuals 70 to 90 mm. long, the pigment epithelium reaches 16 p in thickness. The only indication of a lens was found in the eye of a specimen 72 mm. long. In this a small lenticular group of cells lay in the opening of the pupil. It measured 24 x 4op (fig. 6). THE IRIS AND ORA SERRATA. Marked changes take place in the iris from the smallest to the largest individuals examined, so that = &"s- these must be dealt with seriatim. The smallest individual is 30 mm. long (fig. 5 @ and c). On the left side the pupil measures 22 m in diameter; the distance from the margin of the pupil to the ora serrata measures approximately roop. The epithelial part of this iris consists of an outer layer of dense pigment considerably (14 #) thicker than the pigment epithelium of the rest of the eye. At the pupil this pigment appears rolled into the inner surface of the iris, where it is continuous with the inner layer of cells, which consists of a layer of ordinary pigmentless epithelium 6 » thick, with the nuclei elongate and placed obliquely, and 24» in length. A few of these ordinarily pig- mentless cells show pigment. There is a distinct thickening of the iris at the margin of the pupil. The pigment cells lying on the inner face of this region are much less densely pigmented than those of the outer layer, and their nuclei are quite evident. The pupil is closed with colorless cells belonging to the choroid (fig. 7 a). Fic. 6. Lens of Specimen 72 mm. long. 34 BLIND VERTEBRATES AND THEIR EYES. Very marked changes have been brought about in the specimen 70 mm. long. The pupil is now an oblique channel and the lower margin of the iris overlaps the upper margin. On the left it is more nearly as in the younger stages, but wider (48 #). The free margin of the iris now reaches the enormous thickness of Fic. 7. (a) Right Eye of Specimen 70 mm. long. (b) Right Eye of Specimen 90 mm. long. 56 4 to 80 p. The pigmented epithelium has rolled in more, so that the elongated nuclei, free from pigment, are crowded together in the region of the ora serrata. The pupil is filled in part with pigment, evidently of choroidal origin (fig. 7 a). In the right eye of the specimen 90 mm. long the choroidal pigment has forced its way into the interior of the eye and forms a conical-shaped mass like a plug in EYES OF THE TEXAN CAVE SALAMANDER. 35 the iris and extends into the depth of the vitreous cavity. Apparently on the external half of the iris the pigmented layer has become rolled in and folded upon itself in the interior of the eye, giving rise to a pigment mass over 100 w thick. No such mass is present in the left eye. ‘The pigment on the inner or upper half of the iris is as in the younger stages. The choroidal pigment entering the eye is in solid, vermiform strands (fig. 7 0). THE RETINA. The retina of Typhlomolge is much simpler than that of Proteus. In the latter all the layers typical of the perfect retina are still distinguishable. In the former the outer reticular layer has entirely disappeared, and the layers between the rods and cones and the inner reticular layer form a mass of cells that are homogeneous as far as ordinary histological methods permit one to determine. There are no- where the slightest evidences of any rods or cones, either in the largest or smallest individual. The nuclei of the outer nuclear, the horizontal, and inner nuclear layers are alike. Miillerian fiber-nuclei have not been distinguished as such. This layer consists of about five series of nuclei and measures 44 p in thickness in the smallest (30 mm.), and 48 in the largest (90 mm.), specimen; it is between 32 and 48 p in the specimen 70 mm. long. The inner reticular layer is thin, but well defined. It is 6 » thick in the smallest specimen and 16 w in thespecimen 70 mm. long. In section the ganglionic layer forms a U-shaped mass of cells. In the larger specimens it is about 60 w thick and made up of from five to seven series of cells. The vitreous cavity is a widely flaring, trumpet-shaped structure, with its pointed end reaching to near the center of the eye (fig. 7 a). In the older specimens it is filled by fibers and cellular tissue, apparently continuous with the choroid ingrowth from the pupil (fig. 7 0). The optic nerve is 17 » in diameter in the 30 mm. specimen. In the largest spec- imen it is 24 » thick without its sheaths. At its passage through the pigmented layer of the retina it is contracted to a width of but 14m. Within this layer it expands to 28 p. After passing directly through the ganglionic layer it is distrib- uted to the cells of this layer, some of the fibers being bent at an acute angle to reach the cells near the entrance of the nerve into this layer. A large number of iso- lated pigment granules are found associated with the nuclei of the optic nerve within the eye from its entrance to the gan- glionic layer. ‘There is no sheath of pig- ment such as that found in Typhlogobius. Pigment cells are also occasionally present in the very center of the eye (fig. 7 a 2), and are presumably associated with the optic nerve. The sheath of the optic nerve consists of a direct continua- tion of the choroid layer, which is for a shorter distance pigmented, and of a continuation of the sclera (fig. 8). Blood-vessels do not enter the eye with the nerve, and none were with cer- tainty detected except in the largest individual, where they are closely associated with the choroidal mass of tissue that has grown into the eye through the pupil. Fic. 8. Exit of Optic Nerve of Eye shown in fig. 7 0. 36 BLIND VERTEBRATES AND THEIR EYES. THE EYES OF TYPHLOTRITON SPELAUS STEJNEGER.’ A single specimen of a salamander was discovered in Rockhouse Cave, Barrie County, Missouri, by Mr. F. A. Sampson in July, 1891. The specimen was described by Dr. Stejneger (Proc. U.S. Nat. Mus., vol. xv, p. 115), as T'yphlotriton speleus. His diagnosis reads as follows Vertebre opistoccelous; parasphenoid teeth; vomerine teeth; eyes concealed under the con- tinuous skin of the head; tongue attached in front and along the median line, free laterally and posteriorly; maxillar and mandibular teeth small and numerous; vomerine teeth in 2 strongly curved series; parasphenoid patches separate; nostrils very small; toes 5; 16 costal grooves, or 18 if counting the axillary and groin grooves; tail slightly compressed, not finned; toes nearly half-webbed; vomerine teeth in two V-shaped series with the curvatures directed forward; gular fold strong, very concave anteriorly; color uniformly pale. He further wrote, before he discovered T'yphlomolge in the underground streams of Texas: Although many of our salamanders are known to inhabit caves, this seems to be the only one, so far discovered, which, like some of the other animals exclusively living in caves, has become blind or nearly so. A preliminary note by Eigenmann and Denny (Proc. Ind. Acad. Sci. for 1898, p- 252, 1899) completes the list of papers dealing with this species. In the spring of 1897, I visited Rockhouse Cave and secured a number of larvee, which Dr. Stejneger pronounced the larvee of Typhlotriton. Later Mr. E. A. Schultze informed me that he had seen this salamander in the underground passage leading to Blondi’s Throne Room in Marble Cave, Stone County, Missouri. In September of 1898, I visited this cave and secured 4 adults and 3 larve of T'yphlotriton. A large number of the larvae were obtained from Rock- house Cave a few days later. Those from the latter cave were found under loose stones and gravel in the rivulet at the mouth of the cave. ‘They had been exposed to the light. It is scarcely supposable that those from Marble Cave had ever been subjected to light. In the caves both larve and adults are found under stones, the old ones in and out of the water. Occasionally one is seen lying on the bottom of a pool. In the aquarium the larve creep into or under anything available; a glass tube serves as a “hiding” place. The rubber tube admitting water to the aquarium is sometimes occupied by several during a temporary cessation of the flow of water. A wire screen sloping from the bottom of the aquarium formed the most popular collecting place for the larve. They collected beneath this, though it offered no protection from the light. From these observations it seems probable that stereo- tropism rather than negative heliotropism accounts for the presence of this species in the caves, and that this reaction has been retained after the long stay of the species in caves necessary to account for the changes in its eyes. The eyes of the larvae when examined from the surface appear perfectly normal, but they are little used in distinguishing objects. When hungry they will strike at a stick held in the hand as they would at food. A stick lying undisturbed at the bottom of the aquarium is not molested. They strike at a worm when pote es by it, or when it approaches close enough for its motion to be perceived. 1 By Carl H. Eigenmann and Winfield Auguees Denny. See Biol. Bull. ITI. p. 33, 1900. EYES OF TYPHLOTRITON FROM MISSOURI. 37 In the larvee up to go mm. long the skin passes over the eye without forming a free orbital rim and the eye does not protrude beyond the general contour of the head. In the adult from 97 mm. on, the eye forms a beadlike projection. There are in the adult distinct lids. These are closed over the eye, covering it entirely, the slit being much too small for the eye. The lower lid is free from pigment, but Fic. 9. (a) Diagrammatic Representation of Eye of Typhlotriton drawn to scale. (b) 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 corneal 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 T yphlotriton : : an, Length from re ength o: - A : optic nerve ertical Locality. specimen! Size of pupil. | Length of eye. fcitrontios diameters lens. Rockhouse Cave .......- 54 0.432 1.30 0.80 Rockhouse Cave .......- 78 0.640 1.50 1.20 1.248 Marble Cave .......-..- Si TW ingePoseueeae M00) e-eyi|ccrs scree cee 1.28 Sections of the adult and larva from Marble Cave were made in the usual manner. ‘The six normal eye muscles are present in 7'yphlotriton. ‘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 18 to 4op 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. 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 36m thick, 60 » wide, and not more than 4o 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. Fic. 10. (a) Section of Retina, exclusive of Pigment Cells, of Larva 35mm. long. (6) Tangential Section through Rods and Cones about on Level with Innermost Extent of Pigment (seen on Right) showing Relative Sizes and Abundance of Rodsand Cones. (c) Section of Ret- ina of Larva 48 mm. long. (d) Section of Retina of Larva 90 mm. long. (e) Tangential Section showing Rods and Cones at about Inner Limit of Pigment (seenon Left). (/) Section of Retina of Adult 106 mm. long. (g) Tangential Section at about Inner Limit of Pigment. (hk) Section of Retina of Adult 97 mm. long. The average thickness of the cornea is 40. In the adult it is covered by a layer of stratified epithelium, 25 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 0). 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 size is given in the table on page 4o. The layers of the retina are well developed in the larva. The retina of the larva differs from that of an Amblystoma larva in the greater thickness of its gangli- onic layer. This layer is, in the young larva of Typhlotriton, composed of 5 or 6 layers of cells. This thickness may in part be an artifact, since the retine examined are shrunken 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 T'yphlotriton is apparently normal in all of its histological details. The relative thickness in the different sizes of the larva may be gathered from figures ro a tod and from the comparative table at the end of this chapter. Figures 10 a tof are drawn with the same magnification and show the relative thickness of the different layers in the retine of the larvee 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. 10 f.) 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 larve, 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. 10 # 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 w. ‘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. Thecones 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 Fic. rr. (a) Only Cone found in Eyes of Adults. greatly degenerated or practically disappeared in the Goo ieerence ta Shere of Quiros 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 100 mm. long. It is shown in figure tra. 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 to 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. 10 f). 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 11 } the upper part of the nucleus is very much elon- gated. ‘This form is of frequent occurrence. In figure 11 ¢ 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 Miil- 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 Typhlotriton. Length of Specimen. 35 | 48 62 90 07 103 106 mm mm. mm mm. mm. mm mm. Vertical diameter of eye.......--.-- | 810 800 ae g6o 800 1170 From front of lens to back of eye...- | 600 672 = 720 720 720 1134 Outer nuclear layer with the rods... .- 76 42 112 36 28 28 Outer reticular layers<....2/.-22-5 5. I 2 i: ae 3 He Inner nuclear layer. 22. 2.10o> 25. « 76 72 80 50 48 72 72 Inner reticular layer! =. 22s ~sc02 Ae 16 | 20 16 24 8 8 13 Ganghonic layer &.< 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 qui vive. 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. It can be 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 ampulle 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 ampulle anterioris, which extends to the anterior ampulla; the ramulus ampulla externe, 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 lagenze, 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 lagen, 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. In Amblyopsis 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 Kreidl’s 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 heteroclitus. We 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 1 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 4o 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 ro tests, and from the too 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 roo. 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 ears in different fishes may function differently. In fact, Parker found no evidence of hearing in the smooth dog-fish (J/ustelus canis) when he subjected it to the same experiments as the killifish. Bigelow used Parker’s methods of experi- menting and reéxamined the gold-fish. He concludes that the gold-fish hears.' 1 Since writing the above Kérner in Lucae’s Festschrift, 1905, reviewed the evidence advanced to show that fishes can hear, and concludes 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 toa 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 speleus. 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 by 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 (4 x 5 x 8 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 roo and 512 per second, which gave negative results, I used a large fork 12.5 inches in length vibrating roo 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 todart 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. (b) 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 Eigenmann 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 1 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, onthe other hand, they lay with the body quiet and with the pectoral fins moving slowly when the sounding board was caused to vibrate, they responded by stopping the fin movements. Ten fishes were each subjected to ro tests, and out of roo 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 too 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 speleus 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. mm. mm p. ct Campostoma anomalum. . . 5 2.8 56 Eupomotis gibbosus...... 5 a7 74 Percina.caprodes .:......: 6.4 ak6 54 Amblyopsis speleeus...... 2 4 12 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. |—————_- oo === | | No, }|____ —— — | | Length of body. | Length of brain. Per cent. | Length of body. | Length of brain. Per cent. | | mm. | mm. ; I) Sw ee al “mm. mm. - I g2 5-5 6 I | 88 | 8.5 | 9.6 | 2 80 5-3 6.6 | 2 103 9 8.7 | 3 go 5-5 6 3 72 7-5 | 10 | 88 5.8 6.6 4 68 7 10 5 80 5-2 6.5 5 58 6.3 i fe) 6 100 6 i) 9.8 av a 6.3 ay The result shows the brain of Amblyopsis to be only two-thirds as long as that of Campostoma. ‘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. The 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). (s) 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 Amblyopsis 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 6). 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. Db ~14 Pics G3 Gross section through Middle of Opie Lobes of Ambisopsts speteus Specimen 77 mm, Tong. Res EMTs Gi ger alMsEe Stes nnerNL ance 6: gractuiaieal nets 7 oy optics ak Seaant 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 0). The fifth layer is reduced to diagonal fibers. The descending fibers which join the 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 0). 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 where 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. Ww Nd aan + Il. 7 18. IQ. 20. 2I. 22. CONCLUSIONS. 109 CONCLUSIONS ON THE AMBLYOPSIDA. . Amblyopsis speleus is found from Mammoth Cave north to Michigan. It is the only blind species occurring on both sides of the Ohio. No direct comparison of specimens from south and north of the Ohio has been made. . There are 3 species of Typhlichthys occurring in 3 different localities, one of them north of the Ohio. . Troglichthys is confined to the caves of southwestern Missouri. . The 3 species of Chologaster are found in 3 disconnected areas. . The color pattern of Chologaster is controlled by the underlying musculature. . 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. . Respiration is probably in part carried on through the skin. Amblyopsis is a bottom and pelagic (ubiquitous) feeder on living, moving animals. . Chologaster does not depend upon its eyes for detecting and securing prey, or for avoiding a rod held in the hand. Amblyopsis is negatively phototactic. It seeks the dark regardless of the direction or wave length of the rays of light. . In well-lighted, open pools Amblyopsis hides under rocks during daylight. . 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. . Amblyopsis probably breeds during the entire year, but more individuals carry developing eggs between March and May. . 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 larve are about 10 mm. long. The eggs hatch in about a month, having a length of about 5 mm. . 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. 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 ro mm. longer, the anus has moved forward to between the bases of the pectorals. In mature specimens it lies anterior to this point. The heads of the Amblyopside are provided with tactile ridges, rows of tactile organs regularly and definitely arranged. These fishes are not keener in perceiving vibrations than other fishes. They may have greater power of discrimination between vibrations. The ear of the Amblyopsis is normally developed. These fishes do not “hear” in the ordinary sense of the word. 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. 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; (d) the granular layer is thinner than normal and the ependyma is thinner and has fewer cells; (e) the optic tracts are wanting. 110 BLIND VERTEBRATES AND THEIR EYES. THE EYES OF THE AMBLYOPSIDA. The Amblyopsidz offer exceptional facilities for the study of the degeneration of eyes. They furnish gradations in habits from permanent epigean species to species that have for ages been established in caves. The eyes of the following are considered : r. Chologaster cornutus Agassiz. Locally abundant in the lowland streams and swamps in the South Atlantic states from Virginia to Florida. Maximum length about 55 mm. 2. Chologaster agassizii Putnam. Found in the underground streams of Kentucky and Tennessee. It is rare. Maximum length 62 mm. 3. Chologaster papilliferus Forbes. Found under stones in the springs of Southwestern Illinois, in Union and Jackson counties. Maximum length 55 mm. 4. Amblyopsis speleus De Kay. Widely distributed in the caves east of the Mississippi both north and south of the Ohio River. Maximum length 135 mm. 5. Typhlichthys subterraneus Girard. Found with the latter species in the caves east of the Mississippi, but confined as far as known to the south side of the Ohio River. 6. Troglichthys rose Eigenmann. Found in the caves west of the Mississippi River. Maxi- mum length 55 mm. The first two species mentioned live, as far as known, altogether in terranean streams; the others, altogether in subterranean streams. Chologaster has well- developed eyes, the others mere vestiges. We have thus two epigean species with well-developed eyes, one subterranean species with well-developed eyes, and three subterranean species with greatly degenerate eyes. ‘The three latter species are descended from three distinct terranean ancestors. Amblyopsts is the only member of the family possessing ventral fins, and Troglichthys has scleral cartilages which are not found in the other members except Amblyopsis. It must be apparent that an experiment on a vast scale has been conducted by nature, leaving us but to read the results. Moreover the experiment is one in evolution without the assistance or intervention of natural selection. CHOLOGASTER PAPILLIFERUS. The only account of the eyes of Chologaster papilliferus Forbes, aside from the measurements in the description of the species, is a note by Wright. Professor Wright obtained his specimen from Prof. S. A. Forbes, and therefore had C. papil- liferus. He announced that the pigment is absent in the pigmentary layer of the retina of this species. But this condition was unquestionably either accidental or due to the reagents employed. Chromic acid partly or wholly removes the pig- ment, leaving the cells in good condition. The vertical diameter of the eye in a specimen 39 mm. long is 640 ; in a specimen 55 mm. long (the largest secured), 960 w. The distance from the point of entrance of the optic nerve to the front of the cornea is 560 p and goo p, respec- tively, in the two specimens. The distance from the point of entrance of the optic nerve to the front of the epidermis over the eye is 600 » in the smaller specimens, the lens about 360pin diameter. For further measurements see the table, page 120. The eye is small when compared with that of other fishes of the same size, and especially so when compared with the eyes of Zygonectes. It is located high up on the side of the head, its upper surface being nearly on a level with the top of the head. It is directed outward and forward. In a specimen 35 mm. long it is 1.44 mm. from the tip of the snout and 0.88 mm. long. The distance between the eyes is 1.60 mm. EIGENMANN PLATE 9 Heads seen from above and showing the relative sizes of the eyes of: A. Zygonectes notatus; D. Typhlichthys subterraneus, about 35 mm. long; B. Chologaster agassizii, 4] mm. long; E. Troglichthys rosze, 38 mm. long; C. Chologaster papilliferus, 35 mm. long; F. Amblyopsis spelaeus, 35 mm. long. The dermis over the eye is thinner than elsewhere and devoid of pigment. EYES OF CHOLOGASTER PAPILLIFERUS. epidermis passes directly over the eye without any free orbital rim. totally absent over it (fig. 35 a). Fic. 35. fier! 9) (a) Section through Lower Left Half of Iris of Chologaster papilliferus, seen from in front. t, iris; ¢c, cornea; ep, epidermis; d, dermis; sub. 0., suborbital. ©} Section of Right Half of Head of Chologaster papilliferus. (c) Section through Retina at Entrance of Optic Nerve. (d) Inner Surface of Retina nearly tangential at Entrance of Optic Nerve. "3 Vertical Section of Pigment Cells of Retina, depigmented with Chromic Acid. (J) 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. It is much thinner, 24 » in specimen 39 mm. long, than elsewhere about the side of the head (so to 60 ») and consists solely of epithelial cells; those at the base are columnar, those at the free end of the epidermis are flat. All the other elements of the epidermis — goblet cells and mucous cells, very abundant all about the eye — are 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 0). 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 T’roglichthys. 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 rose, 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 Typhlogobius, who found no cellular structure in the pigmented mass in Typhlogobius and identified this pigmented mass as the choroid gland. A choroid gland or the rete mirabile is not found. -9 90 . “Qe ge be ty <3, e Oro & % é °, ee éo0%*, ; 99 . @ oe *% g e ee eo er 3 og ® ty om ba) oot re) 4 23 %e KC xe) ° eo, ee 90°0 00 QUO OOD O09%o eo ° £900900% % Do G0 ; a 8— - OD 090 00 90 90H 1-72 OE IXO CprO one OE. ee o f = 9---@ -. @® 2) p S ‘7 p Aes 7 0 fas) ig a Q oO 2 e wl Fic. go. (a) Section Tangential to Ganglionic Layer, showing Distribution of Ganglionic Nuclei, 0. On Left, 4-7, Row of Nuclei of Inner Nuclear Layer, 2 mm. and 4. (b) 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. (c) 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- The dioptric arrangements in this eye and the cones are better developed and the layers in general are better differentiated than in JT. 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 terraneus. a matter of measurements, the following tables are added : 120 BLIND VERTEBRATES AND THEIR EYES. Measurements of the Eyes of Chologaster in Groups. [Treated alike.] eaeeal | epee aie a Sie cera ae a. Eye dissected out and measured directly : - ‘i ‘i q 1 C. cornutus Agassiz, 32 mm. long ......-- g60 1,120 752 ey 544 As TAM: arrester nis cas, || e200 1,300 688 33 coe C. papilliferus Forbes, 32 mm. long ...... 832 888 816 320 | 390 (720 without lens) C. agassizii Putnam, 39 mm. long........ 720 800 560 304 | 336 b. Head mounted in balsam, the eyes measured Medio-lateral from above : without the lens C. papilliferus Forbes, 35 mm. long ...... ae 880 640 (612 without lens) (688 with cornea) C. agassizii Putnam, 41 mm. long....... Aste 720 486 (576 with cornea) c. Heads sectioned in paraffine: Medio-lateral | C. cornutus Agassiz, 27 mm. long....... 720-800 Sri 672 | sot 480 C. papilliferus Forbes, 39 mm. long .. .. -. 640 805 560 with cornea |... 260 C, agassizii Putnam, 38 mm. long....-.-- 530 738 about 520 with cornea i acct 290 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. 43mm. | 29-39 mm. 55 mm. 38 mm. 62mm. be Mw 0 M a Mh BipMentsc mance ce cise cman enna eee 47 CEG 64.5 102 48.5 74 Outen niicleany=-Aeacccscteis sete 4 4.5 E335 13 14 Io Quter-reticulars. © .22-2s5.5 cd . , 1 Sq oe [ . ory) csr ‘ 5 ie | / a) ae ' akan ON . . a 2 ‘ a \ . < lic. 44. (a) Cross-section of Part ‘of Head of Troglichthys, 25 mm. long, showing Position and Proportions of Eye. (b) Head of Troglichthys from above, showing Relative Positions of Tactile Organs and Eyes. (c) Part of Same Head, showing Eyes with their Peculiar Pigmentation and Distribution of Pigment Cells in Surrounding Tissues. The 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 best be made out from horizontal sections. In cross-sections it is very difficult to identify or follow them. The best-developed rectus was found in a specimen 35 mm. long. It is com- posed of a number of normal fibers forming a bundle 20 p in thickness, and from its origin to its insertion it is 256 # long. ‘The remarkable peculiarity of this muscle is that 100 p of this is a tendon 4 pin thickness (fig. 46 6, msc.r.). Thetendon spreads into a cone-shaped mass of fibers attached to the proximal face of the eye. ‘Traces of two muscles were made out connected with the right eye 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 9 # 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 p in length and is attached to the eye by a tendon of equal length. The rectus in the same individual is 208 p 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 p» in diameter the length of one of the cartilages reaches 160 (fig. 45 a). They have not kept pace in their reduction with the reduction of the Fic. 45. (aandb) Two Cross-sections of Eye of Specimen preserved in Alcohol, 38 mm. long. Sec- tions show Variable Extent of Pigment, Choroidal (ch.) Pigment, and Scleral Cartilages. Extent of latter represented by dotted lines in figure a. 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, scl.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, subo.) 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 subterraneus, and in the species of Chologaster, which in other respects resemble T'yphlichthys in all but the develop- ment of the eye and the color. It is quite evident that Troglichthys 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 T'yphlichthys subterraneus. 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 Tyroglichthys rose 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. Fic. 46. Two Horizontal Sections through Eye, showing Extent of Scleral (sc/.) Cartilages and Tendons of Oblique (a, msc.) and Rectus Muscles (}, msc.r). Fig. @ represents section just above Fig. 6, from an individual 34 mm. long. Drawn under magnification of 560 diameters. The eye of Tvoglichthys 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. rose (9, fig. 47) I found but three of these cells. Both in size and in structure the eye of 7. rose is the most rudimentary of vertebrate eyes so far known, except that of Jpnops 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 (1 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 scl.e / 007G.0 0g eat } yh Fic. 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 7. subterraneus 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 7. ros@ 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. Fic. 48. Cross-sections through Right and Left Eye of an Individual 25 mm. long. Sections bc pass through Lens. Fig. a isa Composite from 3 Sections. Fig. b represents one Section, but the “‘Lens"’ is from the Next Section. The lens is the only structure of the eye concerning which Kohl has not made any mistake.’ It is a small group of cells closely crowded together and about ro or 12 # in diameter (figs. 48 a’ and b, 1). 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 considered 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 Amblyopsis, that the lens was inclosed by the closing of the pupil over the 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. ‘ Considering the history of the lens in Amblyopsis, I am not sure now whether Kohl was or was not mistaken about these cells. PLATE 10 EIGENMANN 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 100 mm. long. D. Anterior face of transverse section of left eye of fish 123 mm. long. E. Transverse section of left eye of fish 130 mm. long. No definite structures are distinguishable aside from scleral cartilage. F. Transverse section of right eye of fish from which E was taken. é G. Cross-section of right eye of fish 105 mm. long, showing large vesicle formed by pigment epithelium and remainder of retina as small nodule on its distal face. Ag Eye of Troglichthys rosze showing large scleral cartilages and different layers of the eye. THE EYES OF TROGLICHTHYS. 133 The Ketina: The elements of the retina proper, z.¢c. 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. (x) 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, 6, 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 J. 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 Chologaster, 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 Troglichthys rose is three cells in the most highly developed eye found (fig. 47 and plate to, fig. 1). 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 Chologaster 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 ganglicnic 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 » 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 4o 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 ro. The inner reticular layer, including the optic fiber layers, is about 4o in all directions, reaching a proximo-distal length of 70. The lens measures from ro 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 eye of Amblyopsis 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 the 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 (1) a thin mem- brane, the sclera; (2) a layer of pigment cells, the choroid, which were most abundant about the anterior 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 Amblyopsis 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 papilliferus. ‘The position of the eye can be determined from the surface in older individuals by certain tactile ridges, being Fic. 49. (a) Section of Right Half of Head of Chologaster, through Eye. (b) Section of Right Half of Head of Amblyopsis, 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. T he 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 Amblyopsis, drawn with the same magnification, but from different sized individuals (figs. 49 a, 6). Beneath the dermis (black in the figures) a thick layer of connective tissue has developed in Amblyopsis. 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 papilliferus are 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 Tvoglichthys by the cartilaginous masses forming a hood over the front of the eye. ‘These cartilages (scl., fig. 49) are present in front of the Amblyopsis eye, and it can readily be seen that they have nothing to do with the suborbital bones (sub. 0). 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 be drawn from those treated by Golgi’s silver method and stained at times with hemalum or Biondi-Ehrlich’s 3-color stain.’ In one individual the upper rectus and upper oblique muscles are inserted together 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. The origin of 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 origin. It takes its origin in the projecting angle of a cartilage above and in advance of the origin of the lower oblique. In the inner part of the orbit a small muscle *Golgi’s method did not give the desired results for nervous structures, but by staining with the above methods 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 hemalum 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 rose, 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 Tvoglichthys 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. msc. Fic. so. Section through the Eye of Amblyopsis speleus 75 mm. long, killed with Chromic Acid and stained with Biondi-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 found in the eye represented by figure 53. Blood-vessels are present in the choroid. They are apparently as great in relative capacity as in Chologaster. In an individual with the vascular system injected, a vessel, o.or 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. 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- ment 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. Se In general the pigmentation of the retina Fic. 51. From Amblvopsis.o5 mm. long killed in Picric varies inversely as the pigmentation of Indies Carmine, -Faurce made with Bauschand Lomb the choroid. In other individuals the eye (a) 8 sion ae Re an Sh are 1G ith f f t f 1] fi a ection oO! ight Eye. oroida roove with one o! o Scleral Cartilages in front of Eye. Nuclear Layers orms a compac mass OF Cells ( cols 53)- thinner than usual. Densely Pigmented Segments of AN) anticipate somewhat, the vitreous Pigment Cells form a Conspicuous Layer just below Pi t Nuclei. 1 7 1 (b) Next Section after 51 a, showing Group of Elongate cavity with the hyaloid membrane and ee 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. Pigmented 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- 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, Fic. 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 of Granular 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 angular indentation on the surface. ‘These occupy the relative position of the cone bodies, 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 hemalum are quite dark and give the appearance of a large centro- some. These I 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 flat pavement cells, forming a large vesicle (plate 10, figs. Dand G). In others the THE EYES OF AMBLYOPSIS. 141 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 Fic.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 serrata in Chologaster. These im font ¢ Cone orem yeaividual the Largest, Strand. passes are much more regularly present in Rauiar'Vnver and the Central Ganglion Mass. 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, qe 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 142 BLIND VERTEBRATES AND THEIR EYES. in many sections of Chologaster and the method of the closing of the pupil in Typh- lomolge 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 win 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 Fic. 54. From an Individual 35 mm. long killed in Perenyi’s Fluid and stained with Mayer's Hemalum. ; j (a) Outer Nuclear Layer in Center, Choroidal infolding on Left. Lower Part of figure passes through Choroidal Fissure Area and Pigment Cells are here undifferentiated, quite different from those of the Dorsal Part of Same Section. (6) Further Forward and shows Strands of Optic Nerve (”. 0p.) and Elongated Nuclei of Inner Layer of Iris irregularly arranged (nl. 1.). Choroid and Sclera can not be separated from each other except where Latter is differential as Cartilage, in front of Eye. (c,d) Surface and Deeper Focus of Section passing through Iris and Central Ganglionic Cells. In fig. d Irideal Structure with Elongated Inner Nuclei is well shown. F (e) Passes nee Center of Eye. Choroidal Fissure Epithelium seen below and Irregular Mass of Section through Elongated Irideal cells (ml. 1.). (f) Passes through Optic Nerve and Pupil of Same Eye as fig. e. Figs. a to d are from Left Eye, e and f from Right Eye. All under Lenses 2 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 Layer. — This layer is always well developed ; occasionally a few nuclei extend partially in from the outer nuclear layer. It is frequently thicker on the dorsal half of the eye (fig. 54 f) than on the ventral half, but sometimes the reverse. In figure 50 the ventral half is but o.or2 mm. Nuclei have but once been THE EYES OF AMBLYOPSIS. 143 found in this layer, and I have not been able to identify Miillerian nuclei as such either in this or the nuclear layers. The ganglionic layer forms a compact mass of nuclei, somewhat funnel-shaped, with the narrow end toward the exit of the nerve (9 in figs. 50-54 e). I have found from 60 to 125 nuclei in this mass. At the wide end of the funnel this mass of cells is directly continuous with the cells of the nuclear layers. The cells in this intermediate layer are of the large type, and as they give off fibers to the optic nerve, they may be classed as ganglionic or possibly as cells belonging to the spongioblasts. Optic Nerve and Lens. — The optic nerve is always evident in the eye itself except in very old individuals. It passes as a compact thread through the pigmented layer into the ganglionic layer. Here it breaks up into smaller bundles, the fibers of which pass in part to the cells within the ganglionic core, while the greater part pass to the large cells at the outer rim where the ganglionic cells pass over into the cells of the granular layers. The fact that these large cells give off the greater part of the optic fibers suggests whether or not these cells are really the ganglionic cells, while the cells forming the core are such cells as are seen at the entrance of the optic nerve in Chologaster (z in fig. 35 c) and there form a plug around which the optic fibers pass directly to the ganglionic cells. The bundles of fibers passing to the anterior cells never pass through the mass of core cells but at one side of this mass. In the right eye of an individual 25 mm. long they pass out in front of the mass; in the left eye of the same individual, behind them. Outside the eye itself the matter of following the optic nerve becomes a much varying task. In very young, and up to 25 mm., there is no difficulty in tracing the optic nerve to the brain. In newly freed individuals (about two months old) the optic nerve passes nearly obliquely down and in, while in an individual 25 mm. long it passes horizontally back and in toward the foramen for the optic nerve. In the latter individual the nerve leaves the eye, not as might be expected at the posterior inner face, but at the anterior inner, making a sharp turn as it leaves the eye. Its compact nature is entirely lost after leaving the eye, forming a loose bundle several times as thick as the optic nerve within the eye. It is here surrounded by a very thin film of pigment, which in its turn is surrounded by layers of fibrous tissue. In individuals much more than 25 mm. long it is usually no longer possible to follow the nerve to the brain. It can be followed some distance, but usually dis- appears before reaching the optic foramen. In but one instance did I succeed in following it into the brain cavity in an adult specimen. ‘The structures surround- ing the optic nerve are as variable as those surrounding the eye. In one case it is surrounded by various layers of pigment, while in others scarcely any pigment is found with it. The most highly differentiated lens ' was found in an individual 130 mm. long, i.e., a very old one. The lens in this case consists of a few nuclei about which there are concentric layers of a homogeneous tissue (fig. 54). In other individuals structures approaching this condition were found (fig. 55 q), in one a large cell, in another a cell with concentrically arranged lamelle. The lens, in an individ- ual 25 mm. long, could not be found at all, and in another 35 mm. long could Tt is certain that this is not the lens. The name “ secondary lens” may be applied to it. Similar structures are found occasionally in Rhineura and Lucifuga. 144 BLIND VERTEBRATES AND THEIR EYES. not be determined with certainty. The relative development of the lens is not dependent on age. The lens described by Wyman was undoubtedly one of the scleral cartilages, for these cartilages are frequently nodular in this species and one usually lies in front of the eye. The supposition of Wyman that one of the scleral cartilages is the lens need not be criticised too severely. The structures described above as the lens are con- sidered such, more because they could not be identified as anything else, and be- cause nothing else that could with certainty be considered a lens could be found 4 YE EG: BS Rh Sundae Sees of Lert eye of Paiittial (ae ones ug alu eae Nig Gee enameaedys STG aside from these structures, rather than on any direct evidence. The development of the eye would indeed lead one to suppose that the lens is actually placed entirely outside the optic cup, and in that case none of the structures here described can be the lens. With as much variation as is found in all the structures it is not improb- able that the lens may, in some individuals, be found within the optic cup, and in others outside of it. The progressive ontogenetic degeneration of the eye after maturity will be given in the section dealing with its ontogenetic history. Measurements of the Eye of Amblyopsis in p Pigment Layer Length of Diameter of Diameter of = Nuclear Granular Ganglionic Fish Eye, Axial Eye, Vertical. Laver. Laver. Layer Posterior. Anterior. mm 25 160 160 28 re 10 24 | 12 | ee 60 | 144 108 26 20 16 24 6 | 99 108 28 12 12 2 75 56 142 50 4 12 24 12 85 204 108 16 4 12 22 | 108 200 142 56 8 fe) 54 108 ? 84 52 4 132X120 pAverapes! > Ui) b..: Sieve lll | menemences 39 83 13 24 12 SUMMARY OF AMBLYOPSID#. 145 SUMMARY OF THE EYES OF THE AMBLYOPSIDA. 1. There are at least 8 species of “‘blind fishes,” Amblyopside, 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 w. — Chologaster b. Eye in adult more than 1 mm. in longitudinal diameter. Lens over 0.5 mm. in diameter. Retina very simple, its maximum thickness 83.5 m in the old; the outer and inner nuclear layers consisting of a single series a cells each; the ganglionic layer of isolated! cells. Maximum thickness of the outer Wales: layer 5 #; of the inner layer Sy lies ‘ ‘ ; é cornutus bb. Eye in adult less than 1 mm. in longitudinal diameter. Lens less than o.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 to mw thick, the latter at least 18 yp. c. Pigment epithelium 65 thick in the middle-aged, 102 in the old. papilliferus cc. Pigment 49 m thick in the middle-aged, 74 in the old; 24-30 per cent thinner than in papilliferus. Eye smaller : : agassizit aa. The eye a vestige, not functional; vitreous body and lens mere Seite 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 w. Eye probably connected with brain throughout life : . Typhlichthys dd. 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 ee Maximum diameter of eye about 200 pm : Amblyo psis 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 @ . — Troglichthys 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. g. The retina in Chologaster is the first structure that was simplified. to. 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 Amblyopsis, its pigmented epithelium having been most profoundly affected. 12. The eye muscles have disappeared in T'yphlichthys. 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, 7.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 examined 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 speleus 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: Ie 2. 3: ba | to. Is there any evidence for or . What parts of the eye degen- erate first ? / pea . What is the comparative rate / \ ( ») oe a / Do the rudiments of the eye appear as early as usual or later? How much does the eye grow from the time of its appearance ? When does each part of the eye reach its maximum (q) in size, (b) in mor- phogenic development, (c) in histogenic development ? . When does the eye as a whole reach its maximum development ? . Are there evidences of a slowing down of the rate of the developmental processes: (a) cell division, (0) cell arrangement, (c) cell differentiation ? . 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 ? . 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 ? of the ontogenetic degenera- ’ tive modifications of the vari- ous parts of the eye, and how yam does their rate compare with Pe the rate of phylogenetic de- b generation implied by the oc structure of the adult eye \ of Amblyopsis and the dif- r Pa eo ferent stages of degeneration reached by other members of the family? against the dictum of Sedg- 5 OF (a) Outline of Head of Embryo between 1.3 and 1.5 mm. long. wick that structures which (b) Outline of Brain and Optic Thickening in Mounted Embryo 1.6 re mm. long, with 4 Protovertebr (2.30 p.m., May 5). have disappeared from the (c) Outline of Brain and Optic Thickening in Living Embryo 1.92 ‘ 3 ® Fath long: wie 10 yy Uae ie ean ay 5). = Jutline of Brain and Optic Vesicle of Living Embryo 2.4 mm. cme a ee oa are | a long with 10 Prstovertebes (12 p.m., Mayr e)! tained in the em ryo only 1 the organ was of use to the larva after it had ceased to be of use to the adult ? EARLIEST STAGES TO A LENGTH OF THREE MILLIMETERS. HO) 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, 1gor, will be described."| 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. 148 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 11a.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. 56a). 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 (oc.) has a length of 80 pw and projects 36 » 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 are 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 protovertebre 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 and has a longitudinal extent of too p (fig. 56 c). The greatest diameter — measured from the anterior inner angle of the eye to the posterior outer — was 116 ps. 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 g 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 to protovertebre had been formed. The optic lobe was still broadly united with of those shown in figs. 56 d and oe (eed a.m., May 6). was lar gely rep! ese nted m the (b) Outline of Brain and Optic Vesicle of Living Embryo 2.4mm. long, x. 4: lS ig By ey with 12 or 13 ByolceerieGce (nai: May 6). i lobe extending bac k. I here (c) Horizontal Section through Left Eye of Embryo about 2.44 mm. oO : long, 2 Sections Ventrad of one represented in fig. 56 d. was no cavity as yet in the (d) Horizontal Section through Head of Same Individual, showing Optic i ae Vesicle (11 a.m., May 6). nervous system. A little later (e) Outline of Brain and Optic Vesicle of Embryo 1.68 mm. long, with 5 = Protovertebre from Living Specimen. the canal of the central nervous system made its appearance, for at 12 p. m. it was well formed. ‘There was probably some fluctuation as to the rate of growth in length and the degree of differentiation DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 149 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 d). 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 protovertebree had been formed (fig. 57 0). 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. 57 -C,a): 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. Fic. 58. Horizontal Sections through Optic Stalk (fig. 2) and Optic Vesicle (fig. b) 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 11, 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, ro a. m.) was taken from a living specimen, showing 5 protovertebrae. Sections demonstrated that at the stage repre- sented by figure 57 ¢ the neural tube was still a solid structure. The distance from edge of eye to edge of eye measured 164 p. About a day later the larve 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, c) 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 Fic. 50. (a) Horizontal Section of Head of Embryo 2.5 mm. long, two Sides at Different Levels. (b) Left Eye of Same Embryo as that from which fig. 59 @ 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. (ce) Transverse Section of Optic Vesicle and beginning of Eye of a Cymatogaster 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 Cymatogaster' larva about half as long. (Com- pare figs. 59 c, d.) There is no indication of a differentiation of an iris. The secondary cup is a shallow, bowl-shaped structure, the depression being entirely filled by the thicken- ing of the skin which is giving rise to the lens (figs. 59 6 and d). FOUR-MILLIMETER STAGES. In specimens 4.4 mm. long the eye had become a deeper cup than it was during the 3-mm. stage. The lens, which no longer fills the entire cavity, has become ‘ Cymatogaster is a teleost with large and well-developed eves. Figures 60 a, 6 (Cymatogaster) should be compared with figures 60 d, e (Amblyopsis). DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 151 a spherical mass of cells, solid in some cases (fig. 60 d) but with a cavity in others. It is still connected with the skin. In one case the lens was a vesicle with a distinct epithelium bounding the cavity (fig. 60 e). In the other cases there seemed to be no regularity in the arrangement of the lens cells. The pigmented layer has become very thin compared with the thickness of the rest of the retina. Its thickness increases toward the margin of the cup. The retina is very thick, with about 5 layers of nuclei; these are crowded except at the free margin of the retina, which is free from nuclei. There is no histological differ- ence between the different cells of the retina unless there is an appreciable elonga- tion in the cells at the margin of the cup. Optic fibers are not yet developed. Fic. 60. (a) Transverse Section of Eye of Cymatogaster larva, 3-2 mm. long. (b) Transverse Section of Eye of Cymatogaster larva, 4.5 mm. long. (c) Transverse Section of Eye of Amblyopsis embryo, 4.4 mm. long. (d) Section of Right Eye of Larva, 4.4 mm. long. Nuclei all drawn without a change of focus. (e) Vertical Section of Eye of another Larva, 4.4 mm. long. FIVE-MILLIMETER STAGES. The embryo is hatched at the beginning of this period. The least differentiated eye of this stage is represented in vertical section in figures 61 aand 6. The second- ary vesicle has become more definitely formed. The vitreous cavity is reduced in size and the retina has become distinctly thicker, but shows as yet no differentiation into different layers. 12 BLIND VERTEBRATES AND THEIR EYES. In a larva 5 mm. long the eye is still in contact with the epidermis on one side and the incipient dura mater on the other. The epidermis is distinctly thinner over the eye, reaching an extreme thinness of 16 as compared with a thickness of 4o p at a distance of 100 p below the eye and of 0.24 p at 100 w above the eye. The lens lies directly beneath the skin. In this particular eye (fig. 62 a) it isan ellipsoid, 30 » by 38 p» (36 by 28 in another eye). It is entirely separated from the skin and takes ona deeper stain. The cells of the lens are not very regularly grouped, but apparently they are arranged about a median point or space. The lens lies entirely outside of the eye in contact with the outer face of the dorsal part of the iris. ‘The eye proper is a subspherical solid mass with only a shallow depression below the lens representing the vitreous cavity and choroid fissure. In the eye more particularly described here the depression is filled largely with blood corpuscles (fig. 62 a, cpl.sng.). ‘The pigmented layer is not more than 4 m thick, and is very sparingly pigmented over the posterior face of the eye. At the iris and the lower margin of the choroid fissure it is continuous with the inner layers of the retina through cells whose nuclei are distinctly elongate. ‘The retina proper, from the pigmented layer to the vit- reous cavity, is 64 thick. Fic. 61. Two Vertical Sections of Eye of Individual about 5 mm.long. Fig.a taken through Lens, Vitreous Cavity, and Choroid Fissure. Fig. 6, Second Section Proximal to that from which fig. 61 a was drawn and passes through Innermost Part of Vitreous Body. Layers of Retina have not yet begun to be differentiated. It is differentiated into a nuclear layer (the outer and inner together) and the ganglionic layer, separated by the incomplete inner reticular layer. The ganglionic layer is composed of two sorts of cells. Those nearer the vitreous cavity have much more distinct nucleoli than those nearer the reticular layer. Cell multiplication is still going on. The optic nerve is well developed, forming a solid strand of fibers, 12 # in diam- eter, readily traceable to the brain. The muscles are represented by strands of cells closely crowded. No striation is evident. SIX-MILLIMETER STAGES. In embryos 6 mm. long the cells giving rise to the oblique muscles and those for at least 2 of the recti can be distinguished. Scleral cartilages are not yet formed. In 3 of the specimens sectioned there was no indication of a lens. In others it was 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 with 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 ¢). 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 ~S SCR CX+1724{D strl.exsin. SLreLin. \ shen. “Stopt. -etapig. S- 72.0f0. Fic. 62. (a) Anterior Face of Transverse Section of Left Eye of Larva 5 mm. 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. (b) 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 Ganglionic, Inner Reticular, and Nuclear Layers. (d) Anterior Face of Transverse Section through Right Eye of Larva, 7.5 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, 7. 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 » 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 (/ms. 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 lens in such eyes as still show this structure. The pigment layer is pigmented over the dorsal part of the eye. In vertical sections 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 where this 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 fibers to the optic nerve. Fibers have not been definitely traced to the cells of the same series occupying the proximal or middle position. The optic nerve reaches a thickness of 20 » and breaks up into bundles a short distance within the eye. These bundles radiate, forming an incomplete funnel-shaped structure. The incomplete inner reticular 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 larve 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. cplsng. Kk vit. 1: ~ i ~~1.0pt. eth pig a ) 2 Bae SEONG, SNe __ Slrel.in. 22:50 00L eX. 27 ae < fo, fie) stop? 0b Gere SN SSS ee Cd OpUQO ORES c ic SLIEL UN, — aa Fic. 63. (a) Horizontal Section 10 » Dorsal to that given in fig. 60 e, and showing Iris and Vitreous Cavity. ) Outline of Lens of Same Eye as that shown in figs. 60 e and 61 a but at a Level Dorsal of fig. 61 a. (c) Region between Eye and Epidermis of Larva 7.5 mm. long, showing Degenerating Lens. (d) Lens of Larva about 7 mm. long. (e) Vertical Section near Center of Right Eye of Fish 9.5 mm. long. (f) Anterior Face of Transverse Section through Eye of Fish 9.5 mm. long. (g) Horizontal Section through Left Eye of Fish 9.5 mm. long. NINE TO TEN-MILLIMETER STAGES. In larve 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 #; antero-posterior, 98 /; vertical, 106 p (figs. 63 e, 64 6). 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 f). 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 100 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 to 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 /). 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 part 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- tinct conical area free from cells and abundantly supplied with fibers (fig. 63 g). It is possible that this represents the optic-fiber layer. The optic nerve is well developed, but its fibers seem to go to their respective cells directly without first going to this apparent optic-fiber layer. The outer nuclear layers measure about 20 #, the inner reticular about 8 p, and the ganglionic layer about 32 p in thickness. The changes taking place between 10 and 25 mm. are insignificant. DEVELOPMENT OF THE EYE OF AMBLYOPSIS 1S 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 1 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 o' Sapna chet ; Axial diame- Niam gases if eae ie | Length of ee ee Vertical ter from coe Diameter. ee oode direction of the sections. embryos. measured. | diameter. diameter. sea rOeP ue nerve. NIN Gsas cop SHeeEdesae 1.6 I 80 aa 30) faiiee ) ceis-r =p LVING Seu toons id Soo See 1.76 I TOOss t|| Geer AS ay || ee a: VN Thieves ere ee eo 2 3 135 ae AO PMS \peesre INN Os saeco dems ce gaat 2.5 2 190 55% LOOM Wl tec-em ATIVE 2s eerste Ba see 2.8 I I vie) ae COO a BeeMRNSGos Alive . nes eae ae 4 I 200 150 100 Sissy Alive S536 Seton 5 7* 144 134 16 to 48 Sagittal Se OSs eS ne 6 T 136 88 Lape || Peasstes PEranSVerses ese se = 6 T apes 70 100 Horizontal -2-=2-2--. 6 I 130 SAG Sovand 168 |) === Mounted entire...-.... 6.5 to 7 | T 160 160 Se hh, aaeee iransversee-==-t 5.5 to7 | 3 360 120 99 16 to 36 or none 5 Horizontal .....-...-. 6.5 to 7 | 3 152 So 115 18 to 500r none 17 SHG lbopaseespaseee 9 to 9.5 I 108 oe 13) ee |||) eects Transverse... -2- 2... 9 to 0.5 I 108 106 OCR will) Wane fepta-%s I Mionzontalls. .s2.cc2.-1- 9 to 9.5 I 114 Saye @f | Seales 12 SEigsitiall 6 cqqcieseacdce fe) I 120 112 Sh5 gIraAMSVeETSE sto) -f-at-t- 1-1-1 fe) 2 sac 108 moe) |} ewig 12 Mounted entire....-.. | 10 I 135 130 ste ape eesees Horizontal .....-----. 2 I 120 re L295 0e esa E pir ansVverseye eee 25 I 160 TOO Mae | Rs fetaaie EVorizontalincttr-eteereers: || 35 I 192 So Wi || — WABigne 60 to 1c8 = son 115 TSO) Ma Bite-= * The following gives the individual measurements of the eyes of the seven specimens whose average is here noted : No. 1.| No. 2.| No. 3.| No. 4.| No. 5. | No. 6.| No. 7. Longitudinal diameter .. . 176 160 136 172 160 160 128 Vertical diameter ........ 144 128 112 160 144 128 128 Wenlspeeeeeeciecmineseiet)= 48 4050 3e0 16 Be see sees 158 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 6). 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 ¢, 60 d, 60 e, 63 ¢ 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 many eyes, certainly in all in which a lens could be detected in later stages, the lens becomes separated from the skin (fig. 60 e). 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. larvee it could no longer be found (fig. 62 b). In 7-mm. larve exactly half the eyes were without a lens (figs. 63 8, c, d), and in g to 1o-mm. larvee 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 Pie O4 eat Same Fish as tat face Of the eye and the other on the posterior face caudad from which fig. 63 g was . ° . . fe) ee Gale op Keak of the optic nerve. The earliest Stages at which carti- Eye of Another Fish of Jages were noticed were 9.5 to 10 mm. (figs. 63 g, 64 4, b) long. In one fish 10 mm. long there were in the right eye about ro 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 front 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 1o 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 eye caudad of the exit of the optic nerve (plate ro, 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. -;- crt.sel, GS 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 », with a maximum diameter of the eye of 12 p. 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 mp long by 40 » 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 (ro mm. stage), the proximal ones not till much later (25 to 30 mm. stage). The cartilages do not reach their maximum size till later." 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. Ina specimen go mm. in length a globular cartilage 62 m 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 ro5 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 » 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 ro, fig. D). In the left eye of the largest fish a single large cartilage 64 ~ by 96 p in sec- tion occupies the region to one side of the distal face (plate ro, fig. D). In the right eye (plate 10, fig. F) the distal cartilage measured 48 p by 160 in section, and two smaller proximal ones were also present, one of them 24 # by 32 » 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. larve its fibers were 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 1o, 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 medullated, 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 ro 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 100 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 Period. — 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 cavity are of about equal thickness. A little later the secondary optic vesicle is formed by the thickening of the skin over the eye and the consequent cupping of the distal face of 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 time 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 with the reservation that no adult ancestor is supposed to have had eyes like these embryonic stages. 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, 2). 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. 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 exit of the optic nerve, which should, according to this view, be at the proxi- mal 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- FO en oot eee ee aelerce eect” doubtedly found along 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 » 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. Miillerian 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 the eyes of specimens more than 7 mm. long. The nuclei of the retina in the ro-mm. stage are all granular and measure 4 to 5 pm in diameter. -St.gn, --st.ret.in, ~st.nlin, The Third Period. — ‘This extends from the time the fish has reached a length of ro mm. till marked senescent changes begin, which take place when the fish approaches too 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 histogenesis than to the process of degeneration which has already DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 163 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 1 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 too 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.’ 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 to, 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 ros mm. long whose pigmented epithelium forms a vesicle 320 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 to, 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.’ 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. 1In 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 », the larger ones measure 3.5 to 5 w. 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 ro, fig. £). 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 @ in section, with a large cartilage to one side of its distal half, 48 by 160 m in section, and two smaller proximal ones, one of which measures 24 by 32 min 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 afew 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- End of | End of End of | Bepinnin®- of : | tnecon ills 1 ton, | TRgnRte” | histogenesis. | degeneration Pere mm. mm. = mm. mm. mm. mm. 1D) dieadanoce ace 1.5 5-7 Io Before 25 25 Beyond 130 Choroid fissure. - - - ane = = = 10-130 Pigmented layer. - - 2 iy 235 10 too or before Beyond 130 Cones ........-.-| Rarely and then = P ? ? after 10 5-7 — Io Before 25 Beyond 130 Outer nuclear .... 4.4-5 Outer reticular... . Never = = = _— — Horizontal cells. . . Never = ad — _— — Inner nuclear. .. .- 4-4-5 ae —= bo) Before 25 130 mm. and beyond Ganglionic ....... 4-4-5 = > Io Before 25 | 130mm. and beyond Optic fiber layer OF NEFVE si. wis 4-4-5 == ia 5 25 100 Scleral cartilages - . g-10 fs te 75 — — Tees! os cemieet ae 2. 5 5 == 3 6-10 Corneal epithelium | 5 ? _ _ 7 10 ? I do not know. — Does not take place. COMPARATIVE RATE OF ONTOGENETIC AND PHYLOGENETIC DEGENERATION OF THE PARTS OF THE EYE, On pages 134 ef seg. an outline of the probable phylogenetic history of the eye of Amblyopsis is given. In the preceding chapter the rate of ontogenetic degen- eration and its extent has been found to vary in different parts of the eye. It has also been found that certain parts begin to degenerate earlier than others. We shall now attempt to discuss briefly the ratio between the rates and extent of onto- genetic degeneration and the rate and degree of phylogenetic degeneration implied by the structure of the eye. The discussion is somewhat intangible, but certain definite results can be obtained by it. DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 165 In order to compare the ratio between the ontogenetic and phylogenetic rates of degeneration, it is necessary to use some stage in the development of the eye as the point which phylogenetic degeneration has reached. For such a point we shall use the optimum reached by various parts of the eye during their develop- ment. It is certain that the phylogenetic stage is below this optimum, that some of the degeneration in individual eyes is due to phylogeny, but since we do not know how much of the descent from the optimum is due to heredity and how much to the peculiarities of the environment and the resulting functionless life of the parts during the life of the individual, it will be best to take the optimum as above indicated. All phylogenetic time is taken as a unit, although some parts of the eye have been degenerating longer than others. The ontogenetic degeneration leads from the optimum to the vanishing point for most parts of the eye. Ontogenetically the lens degenerates very rapidly, reaching its vanishing point from its optimum during the period in which the fish grows not more than 5 mm. in length. The rate of its phylogenetic degeneration must have been proportionately rapid, for at its optimum in Amblyopsis it is minute and its cells are undifferentiated. In the epigean relatives of Amblyopsis the lens is one of the parts least affected, so that it must have degenerated very rapidly in its later phylogenetic history, after the fish had entered the caves. At its best the vitreous body is so inappreciable in amount that T have not been able to consider its ontogenetic degeneration. Its phylogeny has approached the vanishing point toward which most parts of the eye are heading. The retina may be considered in its extent and in the degree of the histogenic differentiation of its parts. In the matter of its extent or size there is little change from its optimum until its disintegration in old age. Its ontogenetic changes are slight. Its optimum is comparable with that of the lens and indicates a rapid and great reduction from the lowest retina of epigean relatives. The ontogenetic and phylogenetic rates of degeneration in the extent of the retina differ greatly, the former having come practically to a standstill. In its histogenic differentiation the retina is not comparable with the lens, for it rises above the embryonic phases. In fact, in its histogenic differentiation the retina rises far above the requirements of the case, and the most highly developed eye of Amblyopsis approaches the lowest of its epigean relatives. Over any given area it is doubtful whether the ganglionic and inner reticular layers are more degenerate or as degenerate as the same parts in the eyes of Chologaster cornutus. It is certain that in their highest development the parts between the inner reticular and the pig- mented layers are below the lowest point reached by the corresponding parts in the epigean species mentioned. The same is true of the pigmented epithelium. The simplification of the structure of the retina from its maximum to its mini- mum in ontogeny is of greater extent than its simplification from the lowest differ- entiated retina found in epigean species to the maximum found in Amblyopsis. From the foregoing we may conclude that there is no constant ratio between the extent and degree of ontogenetic and phylogenetic degeneration, and that the observed rate of ontogenetic degeneration is not necessarily proportionate to the rate of phylogenetic degeneration inferred from the degree of degeneration of the eye at its optimum. 166 BLIND VERTEBRATES AND THEIR EYES. THE FUTURE OF THE EYE. There can be no doubt that the phylogenetic fate of the eye, exclusive of con- nective tissue, sheaths, sclera, etc., is total disappearance. The most degenerate ontogenetic eye indicates as much. There are no relatives of Amblyopsis that have reached this condition, but Troglichthys has an eye distinctly more degenerate than that of Amblyopsis. It may offer a clew as to whether any of the ontogenetically degenerate eyes, such as are found in old specimens of Amblyopsis, are prophetic of the condition through which the eye will pass in its route to the vanishing point. The most highly developed eye found in any specimens of Troglichthys (plate 10, fig. H) is comparable in a general way with the eyes of the old of Amblyopsis. The pigmented epithelium is larger than the requirements of the eye in both cases, and the scleral cartilages are disproportionately developed in both cases. The ganglionic cells extending through the center of the eye of the younger Amblyopsis are absent in both cases. Only 3 cells have been found in this region in all the eyes of Troglichthys examined. When we attempt a closer comparison, our efforts fail. We may conclude that if Troglichthys indicates one of the steps through which the eye of Amblyopsis will pass to its annihilation, the degenerative phases seen in the oldest specimens of Amblyopsis indicate only ina general way the phylogenetic path over which the eye will pass in the future. RETARDATION AND CUTTING OFF OF LATE STAGES OF THE DEVELOPMENT OF THESEYE. In my first paper on the Eyes of Blind Vertebrates (Roux’ Arch. vit. p. 596, 1899) I said: Cessation of development takes place only in so far as the number of cells are concerned. The number of cell generations produced being continually smaller results in an organ as a con- sequence also smaller. In this sense we have a cessation of development (cell division, not morphogenic development) in ever earlier stages. That there is an actual retardation of devel- opment is evident from Amblyopsis and Typhlichthys in which the eye has not reached its final form when the fish are 35 mm. long. Tam convinced now that this statement did not go far enough. There is, indeed, a gradual retardation in all processes of development which frequently terminates in a complete arrest of development before the final stages of normal eyes are reached. This is especially true of the lens. In discussing the changes it will be best to keep separate the three groups of processes concerned in development. The proof of the limiting of the number of cell divisions mentioned has been brought out in the chapters on the development. It has also been seen that the rate of division is very much retarded. In the retina it stops altogether at the time the fish has reached 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 after it is cut off from the skin. The lens is at this time relatively as well developed asthe 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- genctically older structure, suffers a similar stoppage. There is no evidence, then, DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 167 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 Carassius and Amblyopsis 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 ro mm. Histogenesis is carried surprisingly far in many degenerate eyes. In Rhineura, 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." 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 Amblyo p- 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 Amblyopsidz 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- opside 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. Chologaster agassizii, a member of the Amblyopside, 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 effect of light on the total development is to colonize the adults in an outdoor pool where the young can be reared, from the fertilization on, in normally lighted waters. The lack 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 before. 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 all 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. Ihave 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." 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 Siluride, which is certainly in many instances more elaborate than that of Amblyopsts, 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.’ The causes operating in ontogeny and phylogeny that have led to the limited power of development and differentiation T have fully considered in the concluding chapter, which was also published in the Popular Science Monthly.* 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 verlisst 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, Haeckel’s law, or the law of biogenesis, has been frequently called into question. Its general tenets are: (1) 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. No 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. I do deny that the phylogeny can so control the ontogeny as to make the latter into a record of the ‘The same conditions are found in Lucifuga. ?See the next chapter. DEVELOPMENT OF THE EYE OF AMBLYOPSIS. WAL former — even into an imperfect record of it. * * * Vestiges, and these only, can give any em- bryological clew to past history which could not be equally well made out from comparative anatomy. Zittel finds cases in paleontology both in support of and against this first propo- sition : All know that it (development of Antedon) does not in the remotest manner agree with the facts of paleontology. * * * No observations of embryology would warrant our imagining the former existence of graptolites or stromatophores. No stage in the development of any living brachiopod informs us that numerous spine-bearing genera lived in Paleozoic and Mesozoic times. * * * The beautiful researches of Hyatt, Wiirtemberger, and Branco have shown that all ammonites and ceratites pass through a goniatite stage, and that the inner whorls of an ammonite constantly re- semble, in form, ornament, and suture-line, the adult condition of some previously existing genus or other. : Smith finds that ‘‘the development of Placenticeras shows that it is possible to decipher the race history of an animal in its individual ontogeny.” But it is not the intention to review the numerous expressions of opinion pro and con which have appeared on this subject in recent years. A full discussion of the literature to 1897 has been given by Keibel. The eye of Amblyopsis presents, however, such an excellent opportunity to test an opinion vaguely expressed by Balfour in his “ Embryology,” and carefully and clearly stated by Sedgwick and reiterated by Cunningham in his “ Sexual Dimor- phism ” and in other places, that the facts presented in the foregoing pages may be reéxamined in their relation to this point. Balfour says: Abbreviations take place because direct development is always simpler, and therefore more advantageous; and, owing to the fact of the foetus not being required to lead an independent exist- ence till birth, and of its being in the mean time nourished by food-yolk, or directly by the parent, there are no physiological causes to prevent the characters of any stage of the development which are of functional importance during a free, but not during a fetal, existence from disappearing from the developmental history. * * * In spite of the liability of larvae to acquire secondary characters, there is a powerful counterbalancing influence tending toward the preservation of ancestral char- acters in that larve are necessarily compelled at all stages of their growth to retain in a functional state such systems of organs, at any rate, as are essential for a free and independent existence. It thus comes about that, in spite of the many causes tending to produce secondary changes in larve, there is always a better chance of larve repeating, in an unabbreviated form, their ancestral history than is the case with embryos which undergo their development within the egg. The most concrete critique of the law of biogenesis has been offered by Sedgwick. After rejecting the second proposition by showing that, while in many cases the adults differ more from each other than the young, in other cases the embryos differ more from each other than the adults, he takes up the main question stated in the first proposition by a consideration of ‘The Significance of Ancestral Rudiments in Embryonic Development.” It is, indeed, around this phase of the subject that the discussion has centered. His views are best given by a series of excerpts from his paper. Thus Sedgwick states that * *& %* The tendency in embryonic development is to directness and abbreviation and to the omission of ancestral stages of structure, and that variations do not merely affect the not-early period of life where they are of immediate functional importance to the animal, but, on the contrary, that they are inherent in the germ and affect more or less profoundly the whole development. The evidence is of this kind: 1. Organs which we know have only recently disappeared are not developed at all in the embryo. For instance, the teeth of birds, the fore limbs of snakes, reduced AZ BLIND VERTEBRATES AND THEIR EYES. toes of bird’s foot (and probably of horse’s foot), the reduced fingers of a bird’s hand. * * * 2. Organs which have (presumably) recently become reduced or enlarged in the adult are also reduced or enlarged in the embryo. * * * 3. Organs which have been recently acquired may appear at the very earliest possible stage. * * * The latter arrangement [‘‘ancestral organs have disappeared without leaving a trace’’] seems to be the rule, the former the exception. I think it can be shown that the retention of ancestral organs by the larve [embryos ?] after they have been lost by the adult is due to the absorption of a larval or immature free stage into em- bryonic life. A larval character thus absorbed into the embryonic life, its disappearance is no longer a matter of importance to the organism, because, the embryo being protected from the struggle for existence, the pressure of rudimentary functionless organs is unimportant to it. Characters which disappear during free life disappear also in the embryo, but characters which, though lost by the adult, are retained in the larva may ultimately be absorbed into the embryonic phase and leave their traces in embryonic development. To put the matter in another and more general way. The only functionless ancestral structures which are preserved in development are those which at some time or another have been of use to the organism during its development after they have ceased to be so in the adult. * * * But another explanation is possible, which is that organs which are becoming functionless, and disappearing at all stages, may in some case disappear unevenly, that is to say, they may remain at one stage after they have totally disappeared at another. The question seems to me not quite so simple as imagined by Sedgwick. De- generate organs may or may not be better developed in the young than in the adult. (1) They are better developed in the young if they are still functional in the young after they have become functionless in the adult. (2) They may be better developed in the young, if they were of use to the young, after they ceased to be of use to the adult. (3) They may be well developed in the young after complete disappearance in the adult if the material is used for other purposes in later life. (4) They are better developed in the young if their presence is essential to pro- vide the necessary stimulus to bring about or to inhibit cell movements or cell dif- ferentiation in the development of other organs. (5) They are supposed to be no better developed in the young than in the adult, if they ceased to be of use to the young when they lost their use in the adult. The material entering into the formation of the eyes is not used for the building up of other organs, and it is uncertain whether the eyes positively or negatively influ- ence the development of other organs, so that a discussion of numbers 3 and 4 of the above possibilities is not profitable. Inasmuch as both young and adult live perma- nently in total darkness, and the eye of the young can not be functional under the present mode of existence, the first possibility is also eliminated from the discussion. In Amblyopsis, which carries its young in its gill cavity, we are undoubtedly dealing with an animal in which the eyes are useless in the young as well as in the adult and in which they became totally useless in the young at the same time that they became totally useless in the adult, that is, at the time the species took up permanent quarters in the caves. Do the eyes in this case repeat the phylogenetic history of the eye, or have the eyes in the embryo degenerated in proportion to their degeneration in theadult? In this form the question is whether a perfect or better eye is produced to be finally metamorphosed into the condition found in the adult, or whether development of the eye is direct. We have seen in the preceding pages that the foundations of the eye are nor- mally laid, but that the superstructure, instead of continuing the plan with new CONCLUSIONS ON THE EYE OF THE AMBLYOPSIS. 173 material, completes it out of the material provided for the foundations, and that in fact not even all of this (lens) material enters into the structure of the adult eye. The development of the foundations of the eye are phylogenetic, the stages beyond the foundations are direct to the present adult condition of the eyes from which they are now ontogenetically degenerating to the vanishing point. CONCLUSIONS. The study of the development and its related questions shows: 1. The eye of Amblyopsis appears at the same stage of growth as in fishes developing normal eyes. 2. The eye grows but little after its appearance. 3. All the developmental processes are retarded and some give out prematurely. The most important of the latter is the cell division and the accompanying growth that provides the material for the eye. 4. The lens appears at the normal time and in the normal way, but its cells never divide and never lose their embryonic character. 5. The lens is the first part of the eye to show degenerative steps and it disappears entirely before the fish has reached a length of 1 mm. 6. The optic nerve appears shortly before the fish reaches 5 mm. in length. It does not increase in size with the growth of the fish and possibly never develops normal nerve fibers. 7. The nerve does not increase in size with growth of the fish. 8. The optic nerve gradually loses its compact form, becomes flocculent, dwindles, and can not be followed by the time the fish has reached 50 mm. in length. In the eye it retains its compact form for a much longer time, but disap- pears here also in old age. g. The scleral cartilages appear when the fish is to mm. long; they grow very slowly — possibly till old age. They do not degenerate at the same rate as other parts of the eye, if they degenerate at all. to. The history of the eye may be divided into four periods: (a2) 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 development except that cell division is retarded and there is very little growth. (b) 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 nor- mal embryonic stage reached in the first period to the highest stage reached by the Amblyopsis eye. (c) 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 characterized 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 degenerative processes taking place during this period. (d) 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. 174 BLIND VERTEBRATES AND THEIR EYES. - 11. For a summary of the origin, development, and degeneration of the eye and its parts see table, page 164. 12. There is no constant ratio between the extent and degree of ontogenetic and phylogenetic degeneration. The observed rate of ontogenetic degeneration is not necessarily proportionate to the rate of phylogenetic degeneration inferred from the degree of degeneration of the eye at its optimum. 13. If Troglichthys indicates one of the steps through which the eye of Ambly- opsis will pass to annihilation, the degenerative phases seen in the oldest specimens of Amblyopsis indicate only in a general way the phylogenetic path over which the eye will pass in the future. 14. Some late stages of development are omitted by the giving out of develop- mental processes. Some of the processes giving out are cell division, resulting in the minuteness of the eye and the histogenic changes which differentiate the cones and the outer reticular layer. 15. There being no causes operative or inhibitive either within the fish or in the environment that are not also operative or inhibitive in Chologaster agassizit, which lives in caves and develops well-formed eyes, it is evident that the causes controlling the development are hereditarily established in the egg by an accumu- lation of such degenerative changes as are still notable in the later history of the eye of the adult. 16. The foundations of the eye are normally laid, but the superstructure, instead of continuing the plan with additional material, completes it out of the material provided for the foundations. The development of the foundation of the eye is phylogenetic, the stages beyond the foundations are direct. SUMMARIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID. ite or GENERAL SUMMARIAL ACCOUNT OF THE EYES OF THE AMBLYOPSIDA. As in all organs no longer of use or hindrance, and therefore no longer under the control of selection, the individual variations in the structure of the eye of Amblyopsis, Troglichthys, and Typhlichthys are very great.’ There is also a marked change in the eye with age. It is therefore necessary to distinguish be- tween individual variations and stages in ontogenetic and phylogenetic degeneration. The eye of each species has a general structure which is typical for the species. The individual variations have been sufficiently described under the respective species. PHYLETIC DEGENERATION OF THE EYE OF THE AMBLYOPSID2. The steps in degeneration in the Amblyopsidz are indicated in figure 66. The most highly developed eye is that of Chologaster papilliferus. The parts of this eye are well proportioned, but the eye as a whole is small, measuring less than Imm. ina specimen 55 mm. long. The proportions of this eye are symmetrically reduced if it has been derived from a fish eye of the average size. The retina is much simpler than in Zygonectes. The simplifications in the retina have taken place between the outer nuclear and the ganglionic layers. The pigment layer has not been materially affected. These facts are exactly opposed to the supposition of Kohl that the retina and the optic nerve are the last to be affected, and that the vitreous body and the lens cease to develop early. In Chologaster papilliferus (b) the latter parts are normal, while the retina is simplified. That the retina is affected first is proved beyond cavil by cornutus (a). ‘The vitreous body and the lens are here larger than in papilliferus, but the retina is very greatly simplified. Cornutus, it must be borne in mind, lives in the open. The eye of Chologaster agassizii (c) differs from that of papilliferus largely in size. There is little difference in the retinas except the pigmented layer, which is about 26 per cent thinner in agassizii than in papilliferus. If we bear in mind that no two of the eyes represented here are members of a phyletic series, we may be permitted to state that from an eye like that of cornutus, but possessing scleral cartilages, both the eyes of Amblyopsis and Troglichthys have been derived, and that the eye of Amblyopsis represents one of the stages through which the eye of Troglichthys passed. The eye of Amblyopsis (h) is the eye of Chologaster cornutus minus a vitreous body with the pupil closed and with a minute lens or more probably none at all. The nuclear layers have gone a step farther in their degeneration than in cornutus, but the greatest modification has taken place in the dioptric apparatus. In Troglichthys (i) even the mass of ganglionic cells present in the center of the eye as the result of the collapsing after the removal of the vitreous body has vanished. ‘The pigmented epithelium, and in fact all the other layers, are repre- sented by mere fragments. The eye of Typhlichthys (g) has degenerated along a different line. ‘There is an almost total loss of the lens and vitreous body in an eye like that of papzlliferus 1! This is also true of the eye of Lucifuga and Stygicola. 176 BLIND VERTEBRATES AND THEIR EYES. without an intervening stage like that of cornutus, and the pigment layer has lost its pigment, whereas in Amblyopsis it was retained. The reduction in size from the normal fish eye went hand in hand with the @aenceesqeea ao go — ee6 9650 09/9 05— ——> meh eeee Es |oEO On Fic. 66. Diagrams of Eyes of all Species of Amblyopsidw and Typhlomolge, d, ¢, 8 h,and 7 drawn under same magnification. (a) Chologast» cornutus, (b) Chologaster papilliferus, (c) Chologaster agasstzit, drawn to scale; (d) Retina of Chologaster cornutus; (e) Retina_of Chologaster papilliferus; (f) Eye of Typhlo- molge under lower magnification than d-/; (g) Eye of Typhlichthys sublerraneus; (hk) Eye of Amblyopsts speleus; (i) Eye ot Troglichihys rose. simplification of the retina. There was at first chiefly a reduction in the number of many times duplicated parts. Even after the condition in Chologaster pa pillif- erus was reached 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 AMBLYOPSIDA. 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- gobius, 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 Amblyopside, so that, should the lens disappear, and Ritter found one specimen in which it was gone, the type of eye found in Troglichthys would be reached without passing through a stage found in 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 AMBLYOPSID2. The different structures of the eye may now be taken up in detail. (a) The eye muscles are normally developed in Chologaster. 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 Chologaster 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 1738 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. (e) 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 agassizit. 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 fact that the cones are very sparingly developed, if at all, in this species. In Amblyo psis, in which the degeneration of the retina has gone farther, but in which the cones 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 ros@ 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. (f) 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,to the undefined layer in T'yphlichthys and the merging of the nuclear layers in Amblyopsis, and their occasional total absence in rose. ‘The single cones disappear first, the cones long before their nuclei. (g) The outer reticular layer naturally meets with the same fate as the outer nuclear layer. It is well developed in papilliferus and agassizit, evident in Cholo- SUMMARIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID. 179 gaster cornutus, developed in spots in T’yphlichthys, and no longer distinguishable in the other species. (h) The layers of horizontal cells are represented in papilliferus by occasional cells; they are rarer in cornutus and beyond these have not been determined with certainty. (¢) The inner nuclear layer of bipolar and spongioblastic cells is well developed in C. papilliferus and C. agassizit. 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. (7) 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 papilliferus and agassizti 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 , a ees vitreous cavity the cells have been brought together ss of each Layer of Retina in x, Zygonectes nolatus; 2 and 3, Chologaster cornutus, 27 mm. again into a continuous layer in Typhlichthys, — ‘ox and 43 mim. long; 4, Chologaster papilli- Jerus, 29 to 39 mm. jong, and 5, 55 mm. long; 6, Chologaster agassizii, 38 mm. long and 7 although there are much fewer cells than in cornutus Syn egasie, agnsetei, 38 mm. Jong and 7-02 even. The next step is the formation of asolidcore =‘ 178°) of ganglionic cells, and the final step the elimination of this central core in T'roglich- ithys, leaving but a few cells over the anterior face of the retina. () Miillerian 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 Typhlichthys. 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. 180 BLIND VERTEBRATES AND THEIR EYES. ONTOGENETIC DEGENERATION. The simplification of the eye in cornutus 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 AMBLYOPSID-. 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 prevent 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 has 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 which 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&. 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 i 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 Amblyopside 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, Tam 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- genetic 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. _ : Sed bib vera Say bhaas Pt vit any op lebias ae Deiat 6. Ava si? th OEE eh eR aie Piet 2 Gohan Podt epei bE. Bumalinniy & -) tert ads Ed bier eae ah.6p) CANE G@iaeed® trv gits Dy A en e ~ RE 44 ++ tag | (be ep Opes a fPeguurle: + j (4144 4 Gy he Piel hes Wie reels pes ' te Pips 9019)! ame ire ee Pitan bint 4 a PR uvelies Salih Pt) 2 pS Grieg) hub tug 1 ¢ evel lieve ; THE CUBAN BLIND FISHES . - - oe - 7 . : , v ; Lae a 4 a | a 7 i ie ml a | ‘ 4 iene) 1 oe - * ‘ ’ 7 : > 7 a 7 fab CUBAN BEIND: FISHES? 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. 95-114, 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 experience’ 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. * The specimens were numbered as they were collected, r, 2, etc., and when referred to are given by their serial number. * On the viviparous fishes of the Pacific coast of North America, Bull. U. S. Fish Com., 1892, pp. 381-478, 27 plates, 1894. 185 186 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 Cafas. 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 1 a few fishes were collected and sent me by Mr. F. Martinez, of Canfas. 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 1 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 1, 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 1 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. Iam 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 McIndoo, 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: 1. The Blind Fish of Cuba. Science, N. S., xv1, Pp. 347. 2. Eigenmann, C. H. ‘The fresh-water fishes of western Cuba. Bull. U.S. Comm. Fish and Fisheries, 1902, pp. 211-236, plates 19-21. The water supply of Havana. Science, N. S., xvi, pp. 281-282. Aug. 28, 1903. In search of Blind Fish in Cuba. World To-day, v, pp. 1129-1136. Auf der Suche nach blinden Fischen in Cuba. Die Umschau, vu, pp. 365-367. Hay, W. P. Ona small collection of crustaceans from the island of Cuba. Proc. U.S. Nat. Mus., XXvI, PP. 429-435, Feb. 2, 1903. 7. Lane, H. H. The ovarian structures of the viviparous blind fishes, Lucifuga and Stygicola. Biological Bulletin, v1, pp. 38-54, 1903. aun Sw PLATE 11 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. ee > 2 ah 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. g. Pike, F. H. The degenerate eyes in the Cuban cave shrimp, Palemonetes eigenmanni Hay. Biological Bulletin, x1, pp. 267-276, 1906. ; to. Payne, F. The eyes of Amphisbena punctata (Bell), a blind lizard from Cuba. Biol. Bull., x1, pp. 60- 70, plates I and 1, July 1906. 11. Weckel, A. L. The fresh-water Amphipoda of North America. Proc, U.S. Nat. Mus., xxxm. De- scribing a new Amphipod, Gammarus cecus, from the Modesta Cave, Cuba. pp. 47-49, 1907. 12. Haseman, J. D., and McIndoo, Norman N. On some fishes of Western Cuba. Proc. Acad. Nat. Sci. Phil., 1906. ZOOLOGICAL POSITION OF LUCIFUGA AND STYGICOLA. Lucifuga and Stygicola are members of the Brotulide, of which Jordan and Evermann say: ‘These fishes are closely related to the Zoarcide. In spite of various external resemblances to the Gadide, 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 marginatus (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 alge 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 Lucifuga and Stygicola differ from each other in their dentition. Stygicola has teeth on the palatines; Lucifuga has none. In Stygicola 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 bya 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. g1.4; A. 74; and for the males D. 91.1; A. 73.3; or the average for the two, DE Oie2h. A. 73.6, 188 BLIND VERTEBRATES AND THEIR EYES. Of Lucifuga * 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 Lucifuga 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. Stygicola is known to be distributed from Jovellanos and Alacranes on the east to Canas. Lucifuga is confined to the region from Guira de Melena 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 Lucifuga 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 larve 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 Lucifuga 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 ro 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 11, 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 half being covered with shrubs and trees. All of these zones occupy per- haps too yards. They are followed by the level, practically treeless, meadow, ? The following account 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 were not considered, because differences 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 at the end of a year after birth the young are about 50 mm. long. In obtaining the average size of the sexes only those specimens over 50 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 mm. long, 23 were males and 22 females, or 100 females for every 104.5 males. Counting the fin rays of the first 43 specimens 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. 67.5. HABITAT OF STYGICOLA. 189 perhaps 0.75 mile wide and less than ro feet above sea-level. It is such a beach as is shown in figure A, plate 11, 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 rr, 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 too 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 ro feet deep. It is evidently situated along the line of an original fissure in the coralline rock such as is shown in plate 11, 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. Fic. 68. Diagrams of Cave of the Insurrectos and the Carboneria Well (fig. B, plate 11) taken from X. 3, Depression about Mouth of Cave; 2, Dry Cave; 1, 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, 7. e., at Matanzas and at Alacranes, or Alfonso x1r. 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 11, 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 190 BLIND VERTEBRATES AND THEIR EYES. water is encountered in a crescent-shaped pool. ‘The caves extend down for an indeterminable distance below the water-level. The surface of the water in the caves is near sea-level. Light penetrates to all the recesses of these caves, one of which is called Cueva dos Insurrectos from the fact that a company of Cubans was quartered in it during the Revolution. Figure B, plate 11, gives a glimpse down the Cave of the Insurrectos from the entrance X in figure 68 to the pool of water at the bottom, at a vertical distance of 83 feet. ‘These caves are inhabited by Stygicola, 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 about 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. About 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 of 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 Cafas 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 Cafias 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 from this place, where the thin limestone roof of the underground channel has given way, the stream can be seen. (See frontispiece.) lor reasons to be mentioned at once the streams are inaccessible. In August of 1904 a very heavy rain caused a small torrent to run in the road leading south from Canas for a distance of about a mile to the Finca Rosa, where the water spread out over a depression of several acres, so shallow that the depression was ''The elevation of the railroad track is 62.92 m. EIGENMANN PLATE 12 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.'’ 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. Xvili, 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 rs 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.” The difference in the nature of the sink holes of * 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 1 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. ? 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 “MM” 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 eS ae MMT IAA Fic. 68a. Diagram of the Kentucky Cave Region, after Shaler. A. Sandstone and limestone showing ordinary topography. B. Sink holes. C. Domes below large sink holes. D. Upper line of caverns first formed. . 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 1o-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 Cafas differ from those of the Carboneria and Alacranes. They are cistern-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 surface to indicate their presence. There is absolutely no general surface indi- cation that one is in a cave country in traveling through it, and it is not until standing at the very brink of one that the presence of a ‘‘cave’’ may be suspected. All of the caves in the Cafias region are modifications of the Modesta type. They are dome-shaped rooms (fig. 69) whose roofs are in different stages of dilapidation and collapse. They have a circular doughnut or crescent-shaped pool of water at the bottom. In most cases the roof is very thin; that is, the dome is just beneath the surface, the room being high. More rarely the roof is thick and the cave correspondingly low. In one case the roof is intact and a narrow tunnel slopes down to the cave from the side. In several cases a vertical shaft leads down at the edge of the cave, in other cases asmaller or larger open- ing or openings occur near the middle of the dome, while not infrequently more than half of the roof has fallen, forming a slope down one side, while at the opposite side the overhanging walls still stand (fig. 70). The latter is the Ashton type found in several of the caves on the Finca Ashton. In all CAVES OF CUBA. 193 ae Sal Peel 3S —— | al | qa = | Le I 1 | Sa | ~ ——= a ie : = =e | a ae Fic. 69. Diagram of Modesta Cave. the caves visited there was a pool of water. (There are said to be dry caves, but we had no time to visit them.) In one case the pool forms a simple sheet of water ; Fic. 70. Diagram of Ashton. Hypothetical Outline of Cave before Fall of Right Part of Roof is indicated by Dotted Lines, 194 BLIND VERTEBRATES AND THEIR EYES. very frequently there is an island in the water beneath the opening in the roof, and in the Ashton type the water has become restricted to a crescent at the base of the wall still standing. It is possible that the Carboneria and Alacranes caves belong to the latter type of caves. Almost invariably one or more trees (Ficus) stand over the cave and send long roots down through the cave to the water below, where they break up into number- less rootlets (plate 12, fig. A). The roots were very useful in gaining access to the bottom of some of the caves. During my earlier trips, access was gained to most of the dome-shaped caves by climbing down the roots or a bamboo pole. In the later trips the roots were still the most effective ladders to some of the caves, but I substituted a portable rope ladder for the slippery bamboo pole. | fe} AY) a Finca Rosa (a) Finca Isabella 7 \ @Well O Tranquilidad O .@) (@) Isabella . O Modesta O x Frias X Modesta, I. x I l ; — Ss fe =x tne Modesta Hawey \ ae Scale O O Well® Finca\Frias e eis \ ODrago Fic. 71. Partial North and South Section through Cave Region about Cafias, Cuba. Entire area has subterranean drainage. Road from Canas becomes a stream in heavy rains and sinks within the area inclosed in circle. Caves marked with small circles were located by traverse readings, those marked x were located by guess. There are caves south of area mapped, but land slopes to ocean so that water is found very near surface. There are many others in area covered that are not indicated on this map. The density of the caves may be gathered from the accompanying sketch of a sec- tion extending south from the station Cafias on the Western Railroad but not quite to the southern edge of the cave region (fig. 71). The caves marked with a cipher (o) were located by traverse readings by Dr. Joshua William Beede, of the Geo- logical Department of Indiana University, who volunteered his services on one of the trips. ‘The caves marked with a star (x) were ‘‘discovered” on a subsequent visit and located by estimate. Numerous ‘wells’ and other caves are not indicated, but from the number located an idea of the abundance of the caves can be formed. They are about as numerous as sink holes in the cave regions of Indiana and Kentucky. ‘There are caves south of the area mapped, but the land slopes to the ocean 15 miles to the south, so that water is found very near the surface. ELEVATIONS OF CAVES. 195 An attempt was made to determine the relation of the water in the various caves to a general level of groundwater and to ocean-level. An aneroid barometer was used for this purpose, but although it was of latest pattern and its vernier read to 1 foot, Iam afraid that the readings are approximations only, because allowance for barometric changes could not readily be made. Barometer readings along the line of the Western Railroad compared with the elevations determined by the engineers of the line may give us an approximation to the dependence that may be placed on the respective readings. Barometer readings. Engi- Barometer readings. Engi- Stations. = ‘ =a ley ay Stations. ce ; cae cee orgs f ee ’ | Average.| tions. f eae f ae » | Average.| _ tions. Gnristinaze- see 9} SIBe Soe 9 Gabnel te o----=- 77 89 83 76.65 IPINOS? safe cs/e 21, =)=1= 156 164 160 162.16 Guira sefatels(aeyerere 47 63 55 52.77 Arroyo Narranjo..| 222 227 224.5 | 210.83 || Alquizar........ 47 63 55 57-33 Calabazar........ 156 160 158 T4205 9) ||Wapame’ =. o= <= 86 ata Hee 99.22 Rancho Boyeros. - 209 215 211 2O2 8S. ||(Canasy.-mc.ccnje