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S 7 ! ii - - . oe : 4 ; ey 7 i 7 ' ar ee > I - wv * a ' iu ae a i em: a "oa 7 Tous - - wis ia : ’ : —_ «= , : : 5 4 BF ie : a (= ‘ie ait ; 7 7 i oe iy ; i, t ; - ” 7 * oO 1 etvnes. a a . i, = uy a — - / p a v a ; t a, - a I v : he a " if Np ye ” —- o ie! Le ti” os war : - » = ie aA ae - iF ; mi | - ¥ a - a } 7 +8 - 0 . 7 : ye og a. - - Cit i 5 7 it a i 7 ry = 7 7 7 1 y ee = ae ey fi J : eee he ¢ a bie = : ivi y aos ot aed sy : af * >. ; i “i vs . an OCT ama yur te Dye. ae ae ‘vier i i es ike ao Aha id ~¥e Pe. 7 mh - 0 oe iA ma a 7 Pe vue: U @, fo - ; : a Dy . a -% “ Dy ie i "t : ~ vs oe” : ae 2 at cme 0 ‘tae i 7 an % i ‘ me e Pat) Bey: te! 5 iad a ee ' " al Py ie aa ae Te ; Pay one \ e / fe) fy ha aMemoirs of the Museum of Comparative Zodlogy 2G I | AT HARVARD COLLEGE. ae Ses F O Wore xXexex, No: 2: 7: ae rsh REPORTS ON AN EXPLORATION OFF THE WEST COASTS OF MEXICO, CENTRAL AND SOUTH AMERICA, AND OFF THE GALAPAGOS ISLANDS, IN CHARGE OF ALEXANDER AGASSIZ, BY THE U. S. FISH COMMISSION STEAMER ‘ ALBATROSS,” DURING 1891, LIEUT. COMMANDER Z. L. TANNER, U. 8. N., COMMANDING. XXXII. THE RADIATING ORGANS OF THE DEEP SEA FISHES. By ROBERT VON LENDENFELD. ‘ha WITH AN APPENDIX ON THE STRUCTURE OF THE BUD-LIKE ORGANS OF MALTHOPSIS SPINULOSA GARMAN. By EMANUEL TROJAN. WITH ELEVEN PLATES, AND A CHART OF THE ROUTE. [Published by Permission of Marsaati McDonap and Grorce M. Bowers, U. 8. Commissioners of Fish and Fisheries. ] CAMBRIDGE, U.S. A. : Printey for the Museum, August, 1905. Dion! OF FISiER TABLE OF CONTENTS. PAGE PAGE fT eiINCRODUCTION 2.55 2. . = . 169 (2) Tue Fisnes with Rapratine Dis ae 4 2. DESCRIPTION OF THE RADI- Is¢s 197 Bassozetus nasus (PI. 4, Figs. 14—- 17) 197 eC RG RNS! hn 2 x ATO Leucicorus lusciosus (Pl. 8, Figs. (1) Tue Fishes with OceLtiar Rapi- 10-13)... . < 2 oe 198 ATING ORGANS. . - 170 Halosaurus radiatus (PL. Ie Fies. Argqyropelecus lychnus om. 6, Figs. 52-57) . . : 5 199 ZA 20)! es ce 8 eo oe YAU Mamnuuseane EL 5, Higa 18-20) 202 Sternoptyx obscura (PI. 6, Figs. Ipnops agassizii (Pl. 1, Figs. 8-5) 202 26,27) - - - - see 1 (3) Tue Fisnes wita Tusurar Ra- Lychnopoles racniects e Te MT Na On Gisele _ 203 Ripe at 04) > oa a ais pee ne (11, Figs. Cyclothone neclinidens (EF 6, Figs. Pl Figs. 6-9) . 203 28-30). . = . 180 Bee Pay Bipee 2). Oa ee Myctophum anol ate pnaien (PL Ds 3. CONCLUSIONS .... . . 205 Figs. 21-23) . . ech ae 182 PAS EPA TIRE sh) s. eeot ae, 207 Chauliodus barbatus (Pl. 8 gs. 38-40; PL 9, Figs. eae . . 183 APPENDIX. Idiacanthus antrostomus (Pl. 8, Tue StrRucTURE OF THE BUuD-LIKE Figs. 41,42)... keh OrGans OF MALTHOPSIS SPINU- Stomias hewagonatus (Pl. 10, Hee Losa. By E. Troyan . . . 209 AiO) rtmcnts Wer shes 6) ase LOZ LITERATURE s «6 © 3 2 2 ss « 213 THEA DIATING ORGANS OF THE DEEP SEA FISHES. i? INTRODUCTION: Tue fishes collected by the “ Albatross” in 1891 have been carefully described by Mr. Samuel Garman (99). A considerable number of them possess radiating, or, as they were formerly termed, phosphorescent organs. Mr. Alexander Agassiz was so kind as to place many of these at my disposal and to permit me to study the minute structure of their radiating parts. For this valuable material and his subsequent furtherance of the work, I express my sincerest thanks. The results of the examination of these organs are detailed in this paper. The fishes sent to me belong to the following species : — Argyropelecus lychnus Garman. Lychnopoles argenteolus Garman. Bassozetus nasus Garman. Macrurus canus Garman. Chauliodus barbatus Garman. Malthopsis spinulosa Garman. Cyclothone acclinidens Garman. Mixonus caudalis Garman. Halosaurus radiatus Garman. Myectophum aurolaternatum Garman. Idiacanthus antrostomus Gilbert. Sternoptyx obscura Garman. Ipnops agassizii Garman. Stomias hexagonatus Garman. Leuciocorus lusciosus Garman. All, with the exception of Mixonus caudalis, were sutticiently preserved in alcohol to allow of the examination of their radiating organs. Since Garman did not pay particular attention to these organs, his state- ments concerning their arrangement and number are not always suflicient. This applies to the figures even more than to the descriptions. For this reason the entire fishes are here figured again. The minute structure of the organs was studied by means of paraffin and celloidin sections. Various stains were employed. Van Gieson’s haematoxylin-picric acid-fuchsin gave the best results. Most of the sections 170 THE RADIATING ORGANS OF THE DEEP SEA FISHES. photographs, and drawings were made by Mr. E. Trojan while work- ing in my laboratory. I must express my thanks for the great pains Mr. Trojan has taken with this work ; and for his paper on the structure of the bud-like organs of Madhopsis spinulosa which I add as an appendix to this memoir. Three types of radiating organs are represented by the fishes ex- amined: — ocellar radiating organs, radiating discs, and tubular radiating organs. 2. DESCRIPTION OF THE RADIATING ORGANS. d) THE FISHES WITH OCELLAR RADIATING ORGANS. Argyropelecus lychnus Garman. Plate 6, Figs. 24, 25. This species has been described by Garman (’99, p. 254, Plate J, Fig. 1, lb). The radiating organs are of considerable size, possess exceedingly brilliant reflectors and are surrounded by a layer of dark pigment, so that they are very conspicuous. There are on each side | anteorbital (Plate 6, Fig. 25, ao), 1 postorbital (po), 2 opercular (op), 6 branchiostegal (br), 6 pectoral (pe), 12 ventrothoracic (vt), 2 anterior lateral (al), 6 medial lateral (ml), 4 ventromedial (ve), 6 anal (a), and 4 ventrocaudal (ve), radiating organs. Besides these there also exists, according to Garman (’99, p. 234), a frontal. The axis of all the organs lies very obliquely and nearly parallel to the surface in such a way that it is directed vertically downwards. The anteor- bitals, the postorbitals, and the opercular are simple. ‘The others are joined in longitudinal rows in which they le side by side. The right and left ventrothoracic organs form a double row which appears as a medial crest, protruding ventrally. All the organs with the exception of the anteorbitals, in which the sheaths are absent, are similar in structure, and size, and the differences in their appearance are due chiefly to the different degrees of coalescence. This is nd/ in the three cephalic organs mentioned, and greatest in the ventrothoracics. The organs are 2 to 3 mm. long and 1 mm. ora little more broad. Each radiating organ (Fig. 24) is composed of a slender elongated ellipsoidal proximal (A), and a narrow funnel-shaped distal part (D). These parts are THE RADIATING ORGANS OF THE DEEP SEA FISHES. iia separated by a clearly marked constriction (C). The proximal oval part has in the single organs a circular transverse section throughout; in the joined organs it is not so regular. The distal funnel shaped part is in life probably a rotation paraboloid, but on account of shrinkage appears somewhat irreg- ular in the sections (Fig. 24). In accordance with the smallness of the angle enclosed between the axis of the organ and the surface of the fish, the funnel abuts very obliquely on the latter; it is long on the inner, and very short on the outer face, a very oblique truncated cone with bulging parabolic sides. The whole organ with the exception of the funnel mouth, which is closed by a transparent membrane, is surrounded by a thin layer of cells containing a dark brown pigment. These form an outer pigment sheath (p). Inside this lies another sheath composed of colorless, highly refractive fibres. This (r) is exceedingly thick at the proximal end of the inner, oval part of the organ and gradually thins out from here to the margin of the funnel. It is most highly developed in the joined ventrothoracic organs, where it forms a continuous mass. The fibres composing this layer are parallel, pretty stout, lie close together, and extend longitudinally from the proximal apex of the elliptical part to the distal margin of the funnel. In life these fibres probably follow the curvature of the layer which they form, but are not otherwise bent. In sections (Fig. 24) they often form zigzag lines in the distal thin part of the reflecting layer (B). This also is probably attributable to shrinkage. In the tissue occupying the proximal and central part of the interior space surrounded by the fibrous layer three regions can be distinguished : an inner (g), a middle (1), and an outer (s). The limits between these regions are well defined. The inner region extends to the constriction between the oval and conic part of the organ and there ends with a slightly convex sur- 3 face vertical to the axis. In the joined organs, particularly in the ventro- thoracics, the inner regions of adjacent organs coalesce to form extensive cellular masses surrounded by common fibre and pigment sheaths. The tissue occupying the inner region (2) has in all sections, whatever their direction may be, an alveolar or reticulate appearance ; in the alveoles or meshes residual masses of protoplasm and small spherical nuclei, both readily staining with haematoxylin, are clearly visible. Very rarely a slight indication of an arrangement of the meshes of this network in rows is met with. The meshes or alveoles are about 20 wide. Occasionally one sees 172 THE RADIATING ORGANS OF THE DEEP SEA FISHES. a capillary vessel traversing this tissue but these appear to be rare. The residual protoplasmatic masses and the nuclei appear attached to the strands of the network, or the walls of the alveoles. The structure of this tissue is pretty much the same throughout the whole region it occupies. The middle region (1) occupies the lower portion of the conic part of the organ. Its limit towards the inner region is, as we have seen, concave and smooth. The outer limit is not well defined, the middle region gradually passing into the outer, which is composed of small digitate papillae lying close together. The middle region is composed of cells, which, in longi- tudinal, axial sections (Plate 6, Fig. 24) appear transversely arranged and mostly long, slender, and spindle-shaped. On the whole this tissue takes stains, particularly haematoxylin, somewhat less readily than the parts below and above it. It is traversed by numerous capillary vessels. Whether the cells here in the proximal part of the middle region are really spindle-shaped or lamellar, overlying each other like the cells in the epithelium of the human skin, could not be decided. Distally, towards the papillae, the arrangement of these cells becomes less regular and the cells themselves become smaller, the nuclei lying much closer together. The papillae forming the outer region are composed of similar cells; these are often much curved, but also on the whole arranged paratangentially. This outer region is poorer in blood vessels than the inner. The distal part of the cone beyond the outer region contains only a few doubtful traces of cells and chiefly consists of a gelatinous substance, which takes haematoxylin and some other stains pretty readily and which in some sections exhibits a very strongly marked stratification, the layers being slightly oblique to the outer surface. The structure of the radiating organs in the genus Argyropelecus was first examined in A. hemigymnus by Ussow (79, pp. 103-104, Plate 1, Fig. 5). According to his description these organs of A. hemigymnus are similar in appearance and arrangement to those of A. lychnus described above. Ussow states that the internal tissue of the inner region is composed of glandular cells supported by a scaffolding (Geriist) of connective tissue. Leydig (’81, pp. 26-39, Plate 1, Fig. 5; Plate 1, Figs. 13, 14; Plate 3, Figs. 19, 20, Plate 4, Figs. 22-27; Plate 5, Figs. 28-31; Plate 6, Fig. 35) has de- scribed these organs in the same species. So far as their arrangement and appearance are concerned, this description accords with that of Ussow. The anteorbital organ is innervated by branches of the nervus trigeminus, which THE RADIATING ORGANS OF THE DEEP SEA FISHES. ene enter it at various points. The stoutest nerve branch enters the organ at the constriction between the oval and conic parts. The cells composing the inner region are described as arranged in columns. Leydig suggests (1. ¢., p-. 38) that the fish may swim on its side like a pleuronectid, and not verti- ‘ally. He supports this view by stating that the pigment is not equally developed on the right and left side and by quoting a statement of Valenci- ennes to the effect that the similar Sternoplya hermanni “ flotait renversé sur le cote.” Garman (1. c.) does not mention the occurrence of any difference in the coloration of the two sides in A. lychnus, nor have I noticed it. However the case may be with A. hemigymnus I do not think it likely that A, lychiwus swims on its side. In a previous paper (87, pp. 303-307) L have described the radiating organs of A. hemegymius and other fishes, in which they have a similar structure. The fibrous layer in the organs of A. hei- gymnus was found to contain parallel and longitudinally arranged fibres of considerable length; so there is no difference between these species and A. dychuus in this respect. The statement there made that the inner region is composed of tubes dined with glandular cells does not apply to A. lychnus, where such tubes could not be made out; there is hardly even a trace of the columnar arrangement described by Leydig. Brandes (99, p. 470), who also described A. hemigymnus, says that the anteorbital organs are connected with muscles and can be moved about at will so as to throw their radiation in any direction the fish may choose. This statement has been contradicted by Handrick (01, p. 54). The fibre sheath consists of long and slender tissue cells containing guanin and lime. The tissue of the inner region is, according to Brandes ('99, p. 484), composed of roundish gland cells and is without efferent ducts. These cells produce a secretion which, according to Brandes, disintegrates chemically and thereby emits light. The light passes the middle region, considered by him as a biconcave lens, and illuminates the gelatinous mass which occupies the outer region, and which is backed by the inner face of the distal part of the reflecting fibre layer. ‘Thus the luminosity would be visible chiefly from points at the side of the fish. Chiarini (°00, pp. 7-10, Figs. 1, 4, 5) has likewise studied the organs in A. hemigymnus. He reproduces a figure (1) of an axial longitudinal section of one of them, which—apart from its being upside-down — closely corre- sponds to the figure of a similar section of the anterior lateral organ in A. lychuus here given (Plate 6, Fig. 24). According to Chiarini the pigment 174 THE RADIATING ORGANS OF THE DEEP SEA FISHES. layer of the organs of A. hemigymnus is not quite continuous, but inter- rupted by longitudinal lines free from pigment. The fibre layer is ex- ceedingly thick. The internal tissue has no special covering membrane and is in immediate contact with the fibre layer. The inner region of the internal tissue is composed of polygonal cells. Each contains granules and one or two nuclei. These cells are not regularly arranged; there is, however, an indication of a concentric stratification near the outer limit (towards the middle region). The cells of this inner region are supported by reticulate connective tissue. The middle region is composed of cells with a polygonal contour, arranged transversely. In the outer region the cells are arranged more or less longitudinally. Also in this middle region a supporting reticulation of connective tissue is met with. The distal portion of the cone is occupied by gelatinous tissue. Handrick (01) has published a detailed account of the nervous system and the radiating organs of the same species. He appears to have been unacquainted with Chiarini’s paper cited above. Handrick (01, p 58, 59) carefully worked out the innervation for most of the radiating organs (01, pp. 58, 59). The anteorbital is as Leydig had previously stated (see p. 173) —innervated by the trigemius, the postorbital, opercular, and branchio- stegal by the facialis, the others by spinal nerves. The radiating organs are not movable relative to the body (versus Brandes, see p. 175) but the con- cavity of the sheath can be changed by muscular action (’01, p. 54). A few very elongated nuclei belonging to the fibre cells were observed in the in- ner sheath. The tissue of the inner region, the “ Leuchtkérper,” is very minutely deseribed (01, p. 55). It is not in direct contact with the fibre sheath (as stated by Chiarini), but separated from it by a special membra- nous layer of connective tissue. From this, strands of connective tissue extend inward. These are much branched and anastomizing form a net- work along which nerves and blood vessels extend, and in the meshes of which large round or polyedrie cells lie. The columnar arrangement de- scribed by Leydig (81, p. 31, see p. 173) cannot be made out. Handrick COL, p. 56) gives a detailed description of the internal structure of these cells and distinguishes two varieties of them. According to him they al- ways have only one nucleus, not sometimes two as stated by Chiarini (see above). The tissue of the middle region is, according to Handrick (’01, p. 57), also supported by a reticulation of connective tissue. He considers the cells forming its proximal portion as spindle shaped, those forming the distal por- THE RADIATING ORGANS OF THE DEEP SEA FISHES. 175 tion as * polygonal,” meaning perhaps polyedric. The latter are, as it has been described above of A. dychuus, also in A. hemigymnus, smaller than the former. Handrick found blood vessels but no nerves in the middle region. The ex- tent of the gelatinous tissue occupying the distal part of the cone is dif ferent in the different organs; it is absent altogether in the anteorbitals. In this region a loose network of connective tissue with large nuclei ex- tends, the interstices being occupied by a gelatinous substance (O01, p. 58). According to this, the latest paper on A. hemigymnus, the radiating organs of this fish seem to have very much the same structure as those of A. dychuus. Sternoptyx obscura Garman. Plate 6, Figs. 26, 27. This species was described by Garman (’99, p. 232, Plate 53, Fig. 1). The radiating organs are, as in Argyropelecus lychuus, very conspicuous. There are on each side 2 opercular (Plate 6, Fig. 26 op), 5 branchiostegal (br), 10 ventrothoraic (vt), 5 ventral (ve), 3 anterior lateral (al), 3 poste- rior lateral (pl), 3 anal (a) and 4 ventrocaudal organs (ve). As in Argyrope- lecus lychnus all these groups form rows in which they lie close together. The 20 ventrothoracic organs of the two sides are joined to form a medial crest protruding from the ventral side of the fish. The inner regions of the internal parts of these organs are joined and form a continuous mass. The axes of all the organs enclose very small angles with the outer surface and thus le nearly paratangentially. The 2 opercular of each side are oblique, directed downwards and backwards, all the others more or less vertically downwards. Besides these organs observed by me, Garman (99, p. 255) mentions a silvery area below the eye which he thinks may be a rudimentary suborbital radiating organ. The structure of the organs is apparently the same as in Argyropelecus lychuus. In some of them, however, chiefly in the ventrothoracic organs (Plate 6, Fig. 27), the proximal part of the pigment sheath (p) is thicker and the middle and outer region together are much less extensive than in that fish, These appear here (1 and s) as a rather thin, oblique, lenticular body separating the extensive gelatinous outer mass (gl) from the cellular inner region (g). Traces of stratification could also be detected in the outer region. 176 THE RADIATING ORGANS OF THE DEEP SEA FISHES. I have described (’87, p. 303, Plate 70, Figs. 15-24) the radiating organs of Sternoplyx diaphana. In that species extensive radiating organs are met with on the lips, and I think that such may also be present in 8. obscura ; but as that portion of the specimen at my disposal is somewhat injured I cannot express myself with certainty on this point. The glandular tubes as figured (87, Plate 70, Fig. 20) in the inner coalesced regions of the ventrothoracic organs cannot be made out in S. obscura; in this fish they appear to consist, as in Argyropelecus, of an alveolar network containing large, roundish cells. Lychnopoles argenteolus Garman. Plate 7, Figs. 31-37. This species has been described by Garman (99, p. 244, Plate 55, Figs. 4, 4a). The radiating organs appear as spots, 700 » in diameter, with a silvery lustre, and are pretty conspicuous. There are on each side | ante- orbital (Plate 7, Figs. 31, 32 ao), 1 mandibular (Figs. 32, 33 ma), 11 bran- chiostegal (Fig. 33 br) 9 guttural (Figs. 33 g), 2 opercular (Figs. 31, 32 op), 14 ventrothoracic (Figs. 31, 32 vt)— Garman (’99, p. 245) counts 15 of these — 19 ventromedial (Figs. 51, 52 ve), 22 anal (Figs. 51, 32 a), 11 anterior lateral (Figs. 31, 32 al), 10 mediolateral (Figs. 31, 32 ee and 22 posterior lateral (Figs. 31, 32 pl) radiating organs. Garman (99, p. 245) states that there are besides these two parallel rows of radiating Aa on the sides of the body. In the partly injured specimen at my disposal these could not be made out. With the exception of the anteorbital organ which is somewhat peculiar and which will be described below, all the radiating organs mentioned have the same external and internal structure. ‘They consist, as in Argyro- pelecus and Sternoptyx, of two parts, a proximal and a distal. The proxi- mal part (Plate 7, Fig. 37 A) is approximately spherical and forms about three quarters of a sphere. The constriction (C) between the proximal and distal part is well defined but shallow, so that the “neck” of the organ has a considerable breadth. The distal part (D) appears as a broad and some- what short rotation paraboloid, obliquely cut off where it abuts on the surface of the body. The blood vessels in the subcutaneous connective tissue surrounding the organ (b) are more numerous close to the pigment sheath than elsewhere. The pigment sheath (Plate 7, Figs. 35, 37 p.) is stout, THE RADIATING ORGANS OF THE DEEP SEA FISHES. irerg dense, and of uniform thickness in the inner spherical part and the proxi- mal portion of the outer parabolic part. Towards the surface it thins out. The reflecting layer (Figs. 55, 37 r) is about as thick as the pigment layer and composed of longitudinally arranged fibrous cells. It does not extend to the surface and terminates some distance within the margin of the pigment layer. On the inner side of the reflecting layer a conspicuous connective-tissue membrane (Figs. 55, 37 c) about as thick as the two other layers, is met with. Distally this membrane passes into the gelatinous portion of the outer region of the organ. ie) Three regions (Plate 7, Fig. 57 g, 1, s), can be distinguished in the tissue forming the internal part of this organ in Lychnopoles as in Argyropelecus and Sternoptyx; in Lychnopoles, however, these are built on a somewhat different plan. From the connective-tissue membrane (Plate 7, Figs, 35, 87 ©) mentioned above membranous diverticula extend inward which join to form facets arranged radially. In the proximal spherical part of the organ these mem- branes are straight and each facet encloses a regular pyramidal space. Its apex lies near the centre of the sphere, and its base abuts on the connective-tissue membrane overlying the reflecting layer (Plate 7, Fig. 37). The radial membranes (facets) do not extend quite to the axis. A some- what irregular tubular cavity (Plate 7, Fig. 57 cc) oceupies that portion of the axial part of the organ which les between the centre of the proxi- mal sphere and the focus of the distal paraboloid. The margin of this tube is so well defined that it looks as if a special limiting membrane were present. Its cavity appears empty, neither blood corpuscles nor other bodies were found in it. In the pyramidal spaces enclosed by the facets, nuclei and residual protoplasmatic masses highly stainable with acid-fuchsin are met with. These adhere chiefly to the connective-tissue membrane at the base of the pyramid and to the adjoining parts of the radial membranes: the central parts of the facets (Plate 7, Fig. 57 ci) are pretty free from them. The facets in the distal paraboloid are not so regular as those in the proximal sphere. The inner (proximal) part of the paraboloid is occupied by pretty narrow, more or less curved facets (Plate 7, Fig. 37, em) extend- ing from the sides towards the distal end of the tubular cavity above described. These facets are pyramidal, distally broad and proximally con- tracted. The outer (distal) part of the paraboloid is occupied by regular 12 178 THE RADIATING ORGANS OF THE DEEP SEA FISHES. 94 straight, prismatic facets (Plate 7, Fig. 87 co) which are parallel to the axis of the organ; they are rounded off distally and drawn out proximally to points converging towards the distal end of the tubular cavity. The distal ends of these facets lie in a surface convex outwardly and i folo vertical to the axis of the whole organ. The triangular space outside these facets, between them and the surface, is occupied by gelatinous tissue (gl). The small curved facets and the proximal portions of the large straight ones which together occupy the inner part of the paraboloid and form the middle region of the organ (Plate 7, Fig. 87) are clothed with a single layer of cylindrical cells which form a kind of simple epithelium (Plate 7, Fig. 36). These cells take up the greater part of the space within the facet and only leave an exceedingly narrow central canal (Plate 7, Fig. 36 en) free. This canal extends longitudinally in the axis of the facet towards the tubular space in the interior of the organ. Probably it opens into it, as I have seen many indications of such a connection, but Iam not certain about it. The cylindrical cells forming the epithelial clothing of these facets and surround- ing the central canal are composed of two kinds of protoplasm. The proxi- mal three quarters of each cell (lying next the canal) (Plate 7, Fig. 36 ph) are transparent and only slightly stainable; the distal quarter (lying next the facet wall) (Plate 7, Fig. 36 pt) is granular and takes stains very readily, so that it appears dark and not transparent in the stained sections. In this highly stainable, basal portion of the cell the nucleus (nu) is situated. It is probable that the facets of the proximal spherical part of the organ described above contain the same kind of tissue, but the condition of the material does not allow of this being definitely determined. The tissue occupying the distal parts of the large straight cylindrical facets and forming the outer region of the organ (Plate 7, Fig. 37 s) is of an entirely different character. Here we find elongated, slender cells, which are rather indifferent to most stains, but become intensely yellow after treatment with picric acid. In the distal basal part of each of these facets these cells are very slender, spindle-shaped, and arranged longitudi- nally, parallel to the axis of the facet (Plate 7, Fig. 34 sp). Towards the proximal part they become stouter and shorter and here they often appear club-shaped. The cells appear to rise from the walls of the facet. Those joined to the terminal face of the facet arise more or less vertically ; those joined to the sides arise very obliquely and extend nearly parallel to the facet wall to which they are attached. The cells in the distal part of the THE RADIATING ORGANS OF THE DEEP SEA FISHES. 179 facets are nearly straight throughout the whole of their length. As we approach the proximal part of the facet we find these cells curving away more and more from the walls of the facet so that their free ends come to lie more or less obliquely (Plate 7, Fig. 54 sc). Finally they are situated nearly transversely, the cells arising from the different sides interlacing axially, In the sections one often finds a circular structure of considerable size (Plate 7, Figs. 34, 55 0) in the centre of the facet. Sometimes it appears as an empty space, and is perfectly colorless and structureless; sometimes it has —in sections stained with acid-fuchsin a bright red color and then one occasionally notices traces of a granular structure within it. The tissue surrounding the circle often shows a fine radial structure (Plate 7, Figs. 34, 35). Ihave not been able to ascertain the real nature of these circles and their coronas with any degree of certainty. The circles seem to be the expression of spherical spaces lying between the cells, and the radial structure surrounding them might be due to the fact that the slender inner terminations of the spindle cells abut vertically on this sphere. The organ is rich in blood vessels. These follow the facet walls and can often be traced for a considerable distance. The gelatinous tissue (Plate 7, Figs. 54, 57 gl) mentioned above is com- posed of a gelatinous ground substance traversed in all directions by exceed- ingly slender spindle cells. It passes into the connective-tissue membrane overlying the reflecting layer. A thick granular lamella or cushion (Plate 7, Fig. 87 er) underlies this gelatinous tissue and separates it proximally from the reflecting, and distally, where this is absent, from the pigment layer. As mentioned above, the anteorbital radiating organ differs in some respects from the others. It appears to be much larger and measures 1.5 mm, in diameter. Closer examination shows that it is a duplex organ composed of two, each similar to the other radiating organs of the fish. These two organs point different ways and are enclosed in a common pig- ment sheath which has the shape of a curved tube open at both ends. The two orifices of this tube lie in the surface ; one is oblique, directed upwards and forwards towards the eye, the other vertical, directed straight down- wards. The spherical parts (inner regions) of the two organs occupy its central, the paraboloidal parts (middle and outer regions) and the gelatinous and granular tissue the terminal portions of the tube. These two joined organs throw their radiation in different directions, one into the field of 180 THE RADIATING ORGANS OF THE DEEP SEA FISHES. vision and the other downward, neither inward nor backward, as Garman (99, p. 245), who also noticed that this organ differs in structure from the others, seems to think. Cyclothone acclinidens Garman. Plate 6, Figs. 28-30. This species has been described by Garman (’99, p. 247, Plate J, Fig. 4), whose statements concerning the radiating organs are, on the whole, in accordance with my observations; in his figure (4), however, 2 opercular and 5 posterior lateral organs are drawn which do not exist and which Garman himself does not mention in his description, The other organs are repre- sented in the figure as lateral whilst they are in reality situated on the ven- tral side of the body. The radiating organs are very small, only 700 in diameter, and are destitute of reflectors. On account of their insignificant size and want of lustre, they are far from conspicuous and not easy to make out. There are on each side 10 branchiostegal (Plate 6, Fig. 29 br), — Garman (99, p. 247) gives their number as 13 — 13 ventrothoracic (Figs. 28, 29 vt), 5 ventrome- dial (Figs. 28, 29 ve), and 16 ventroanal (Figs. 28, 29 a) radiating organs. All the organs seem to have the same structure. They are approxi- mately spherical and enclosed in a pigment sheath which forms over three quarters of a sphere and encloses them on all sides except where they abut on the surface of the fish. A straight line drawn through the centre of the sphere and the centre of the more or less circular superficial orifice of the pigment sheath is vertical to the surface of the body. The pigment sheath (Plate 6, Fig. 30 p) is composed of cells containing dark brown pigment granules. It is very stout. Its inner surface is pretty smooth, the outer covered with irregular protuberances. Within the pig- ment sheath a connective-tissue membrane (Plate 6, Fig. 30 c) is met with. Near the surface this splits up into two membranes, one covering the mass of radiating cells proper on the outer side (Plate 6, Fig. 50 er) the other forming a thick superficial cell layer (Plate 6, Fig. 30, 1 and ed). Three zones can be distinguished in the organ: an inner, occupying about three quarters of the whole sphere, an intermediate, and an outer. The inner zone is composed of a mass of large polyedrical cells and enclosed in a connective-tissue capsule. The part of this capsule which covers the radiating cell mass inwardly and laterally and separates it from THE RADIATING ORGANS OF THE DEEP SEA FISHES. 181 the pigment sheath (Plate 6, Fig. 30 c) is the connective-tissue membrane mentioned above. The part of the capsule covering the cell mass outwardly and separating it from the intermediate zone (Plate 6, Fig. 30 cr) is the innermost of the two layers into which the connective-tissue membrane splits up near the surface. From this outward part of the covering membrane lying near the surface diverticles extend inwards dividing the outer two thirds of the cell mass into somewhat irregularly pyramidal facets, the (distal) bases of which are closed by the outer capsule membrane whilst the (proxi- mal) apices are wide and open. ‘The innermost part of the cell mass is not thus divided into facets. The facets are occupied by large polyedrical cells (Plate 6, Fig. 30 r) arranged in radial rows. Each cell contains a large and conspicuous spherical nucleus; its protoplasm is readily stainable with acid- fuchsin. The same cells are met with on the inward side of the organ, but here they form layers parallel to the surface of the organ and are not arranged in radial rows. In_the interior, a little below the centre, a mass of much smaller cells (Plate 6, Fig. 30 m) is observed. These look like closely packed blood corpuscles. Possibly there exists here a blood sinus; I have not been able, however, to make out clearly a wall separating these smaller internal cells from the larger ones around them. The intermediate zone (Plate 6, Fig. 80 t) has in sections the appearance of a nearly empty space. It is in life perhaps occupied by a gelatinous sub- stance with few slender cells. It may, however, be a mere artifact pro- duced by shrinkage. The outer zone is composed of two layers, an inner layer of cylindrical cells arranged vertically to the surface (Plate 6, Fig. 30 1) and an outer layer of ordinary connective tissue composed of slender cells arranged para- tangentially (ed) which forms the supérficial covering of the organ in the orifice of the pigment sheath. The protoplasm of the cylinder cells of the inner layer is concentrated at their proximal ends which abut on the inter- mediate zone. Here also the nucleus is situated. The rest of the cell body is very transparent and the cell walls are difficult to make out. These cells do not take stains readily and thus essentially differ from the polyedric cells of the inner zone. In their microchemical behavior and their general ap- pearance they resemble the cells of the middle region of the radiating organs of Argyropelecus and Sternoptyx. A blood vessel, artery, (Plate 6, Fig. 30 b) of considerable size entering the organ at the innermost point was observed in several sections. 182 THE RADIATING ORGANS OF THE DEEP SEA FISHES. Myctophum aurolaternatum Garman. Plate 5, Figs. 21-23. This species has been described by Garman ('99, p. 254, Plate 55, Fig. 3). His statements concerning the distribution of the radiating organs agree with my observations, but his figure (99, Plate 55, Fig. 3) differs in several respects from what I find in the specimen examined. The radiating organs are for the most part nearly equal in size about 600 » in diameter. They are provided with reflectors, lustrous, and there- fore, although so small in size, quite conspicuous. There are on each side 3 mandibular (Plate 5, Figs. 21,22 ma), — these are mentioned by Garman (’99, p. 265), but not represented in his figure (’99, Plate 55, Fig. 3),— 4 branchiostegal, 2 opercular (op),— the lower one, which is smaller than the upper one, is not represented in Garman’s figure (99, Plate 55, Fig. 8),— 3 pectoral (pe) forming an are just behind the oill slit, 5 ventrothoracic (vt), over the last of these 1 anterior lateral (al), 4 ventromedials (ve), 3 mediolaterals (ml) forming an oblique row, 10 anterior ventroanal (va), 1 posterior lateral (pl), over the last one of the former 6 posterior ventroanal (vp) — according to Garman’s figure ('99, Plate 55, Fig. 8) 7, and 2 ventrocaudal radiating organs (vc) at the base of the tail. All the organs examined appear to have the same structure. Viewed from the surface each appears as a lustrous spot enclosed above and at the sides by a dark strip, which has the shape of a horseshoe open below. Each radiating organ of the body lies between two scales. The outer one which covers it (Plate 5, Fig. 25 ss) is convex and possesses near the centre an H-shaped thickening (t) produced by a local accumulation of calcareous laminae, which make the scale very brittle. On treatment with acid the thickening disappears. The horseshoe-shaped pigment stripe mentioned above is attached to the inner side of this scale. An axial section of the organ vertical to the medial plane of the fish passes through the apex of the horseshoe (p). The lower scale (si) is concave; on its inner side a continuous layer of pigment (pp) is observed. The lenticular space be- tween the convex outer and the concave inner scale is for the most part occupied by connective tissue (c). This is composed of a ground substance traversed by slender, elongated cells which are somewhat irregularly curved. THE RADIATING ORGANS OF THE DEEP SEA FISHES. 183 This tissue is rich in blood vessels. The radiating organ proper, that is, the mass of cells (pe) which probably emit the radiation, lies in the middle of this connective tissue. It is pear-shaped, the thinner end extending outward and downward. The cells composing it are flattened and extend parallel to the surface of the body. The protoplasm is quite transparent and each cell contains a large granular nucleus. The blood vessels form a fine-meshed capillary network (b) on the surface of this cell mass, but do not penetrate into its interior, Radiating organs of this kind seem to be rare in fishes. Somewhat simi- lar ones have been described by Emery (’84, p. 473) from the ventral series of Scopelus elongatus. Chauliodus barbatus Garman. Plate 8, Figs. 38-40; Plate 9, Figs. 43-46. This species has been described by Garman ('99, p. 271, Plate K, Figs. 2, 2a). The radiating organs are very numerous. Three different kinds of them can be distinguished. The differences between these are considerable and clearly discernible with the naked eye. Some of the organs are simple, some composed, as in the fishes described above, of an inner spherical and an outer paraboloidal part. The simple ones again are either uncovered or provided with a pigment sheath. I have designated these three kinds of organs accordingly as simple organs without pigment sheath, simple organs with pigment sheath, and compound organs with pigment sheath. The simple radiating organs without pigment sheath appear as very small whitish dots. They are arranged in groups of from 4 to 16, These groups are scattered over the medial part of the ventral surface (Plate 8, Fig. 39 u) and also occur in great numbers on other parts of the surface, chiefly on the back of the fish. The simple organs with pigment sheath appear as dark spots 300 4 in diameter. They are scattered in great numbers over the whole of the sur- face and seem to be most numerous on the sides of the head just behind the eyes (Plate 8, Fig. 38 x). In the barbel there are 8 of these organs (Plate 8, Figs. 58-40 Ba). The compound organs with pigment sheath are lustrous, elongated, about Imm. long and 600» broad. Of these organs there are on each side | anteorbital (Plate 8, Fig. 38 ao), 1 suborbital (Fig. 38 so), 18 branchio- 184 THE RADIATING ORGANS OF THE DEEP SEA FISHES. stegal, 8 guttural (Plate 8, Fig. 39 g), 19 ventrothoracic (Figs. 38, 59 vt), 22 11 ventroanal (Figs. 88, 39 a), 17 ante- rior lateral (Figs. 38, 39 al), and 22 medial lateral (Figs. 88, 39 mi). The simple radiating organs without pigment sheath are not in a partic- ventromedial (Figs. 38, 39 ve), d ularly good state of preservation. So far as they could be made out, their structure corresponds with the same organs of Chauliodus sloant described by Chiarini (799, pp. 16, 17). They are spherical or oval, enclosed in a capsule of connective tissue and composed of a layer of superficial, radial, cylin- drical elements surrounding a group of central polyedrical cells. The simple radiating organs with pigment sheath (Plate 9, Figs. 43, 44) have an oval (Fig. 43) or irregular (Fig. 44) shape. They seem, however, always to be elongated in the direction of their axis. The angle between the axis and the part of the surface of the fish where they are situated is very variable. It may be 90°, the organ then being vertical to the surface (Fig. 43), or it may be smaller, sometimes so small that the axis of the organ is nearly parallel to the surface of the fish (Fig. 44). It appears that this angle is correlated to the shape of the organs, the regularly oval ones being usually vertical to the surface, the irregular ones inclined to it ; and it seems that the organs are the more irregular the more obliquely they are situated. These organs are not, as is generally the case, imbedded entirely in the body of the fish, but protrude considerably over the surface, forming rather conspicuous rounded protuberances (Plate 9, Figs. 43, 44). They are covered distally by a layer of considerable thickness (Plate 9, Figs. 43, 44h) which appears structureless and transparent in the sections. Laterally and proximally they are enclosed in a pigment sheath (p). In the regu- larly oval organs of this kind the axis of which is vertical to the surface (Fig. 45) the pigment sheath extends only over the part of the organ imbedded in the body of the fish. In the irregular ones (Fig. 44) with oblique axis it covers also the proximal portion of the protruding’ part. The pigment sheath is thickest at the proximal apex of the organ and gradually thins out towards the margin. — Its distal orifice is of considerable extent, and occupies about a third of the whole surface. In this a fine connective-tissue membrane (Plate 9, Figs. 43, 44) is extended which here replaces the pigment sheath. In this thin layer which separates the radiat- ing cell mass proper from the structureless outer covering mentioned above, nuclei, appearing paratangentially elongated in axial sections of the organ, are observed. The interior of the organ is occupied by a mass of poly- THE RADIATING ORGANS OF THE DEFP SEA FISHES. 185 edrical cells in the centre, and cylindrical elements on the surface. ‘The latter (Plate 9, Figs. 43, 44 pe) resemble a cylinder epithelium clothing the pigment sheath and distal connective-tissue membrane internally. They are not vertical to this surface however, but so arranged as to converge to a point within the organ very near its proximal apex. In the distal part of the organ these cells are short, hardly longer than broad. Inwardly they increase very rapidly in length and those situated at the sides are nearly half as long as the organ itself. The nucleus is conspicuous, spheri- cal, or radially extended, oval, and always situated at the distal end of the cell. The protoplasm is granular, The limits between these cells are distinct enough distally, but become very indistinct centrally and finally vanish altogether; in the proximal part of the organ to which these cells converge, no trace of cellular division is discernible. This part of the organ is occupied by a granular protoplasmatic mass (Plate 9, Figs. 45, 44 m) in which neither cell walls nor nuclei are met with. The proximal apex itself is in the regular oval organs free from these cells, or cells of any kind, the granular mass (m) extending here right up to the pigment sheath (Plate 9, Fig. 45). In the axial sections of irregular organs, however, a net- work of cell limits with nuclei in the meshes is usually observed in the part appearing as the proximal apex (Plate 9, Fig. 44). It seems to me, however, that this point does not exactly correspond to the proximal apex of the regular organs and that the “cells” seen there are transverse sections of the same kind of superficial cylinder cells as appear cut longitudinally else- where. ‘These organs being so very irregular in shape, an irregularity in the arrangement of the superficial cylinder cells, giving rise to such an appearance of axial sections, would be not at all surprising. The inner mass of polyedrical cells (Plate 9, Figs. 43, 44 1) has the shape of a bee-hive or truneate cone resting with its broad base on the distal cylinder cells and extending inward a good way beyond the centre of the organ. ‘The distal cells of this mass are smaller and arranged irregularly, the proximal ones larger and situated in more or less clearly pronounced longitudinal rows. In their appearance these cells differ essentially from the cylindrical elements surrounding them. Their nuclei are pretty large, spherical, or flattened radially, and their protoplasm is transparent and free from granules. Picric acid stains them a bright yellow. The superficial cylindrical cells are comparable to the cells forming the inner region of such radiating organs as those described above of Argyropelecus and Ster- 186 THE RADIATING ORGANS OF THE DEEP SEA FISHES. noptyx; the inner polyedrical cells on the other hand resemble the cells of the middle region of the organs of these fishes. Three of the simple radiating organs with pigment sheath in the barbel are joined to form a composite mass. Two of these are irregular and lie symmetrically right and left opposite each other. The third is a regular one and lies between the other two. The axes of the two former lie in a plane vertical to the axis of the barbel; the axis of the latter is ori- entated longitudinally, parallel to the axis of the barbel. A close inspection of a continuous series of sections through this organ reveals the remarkable fact that the central one of these three organs is completely enclosed by the pigment layer on all sides, a thing I have not observed in any other radiat- ing organ, although Leydig (79, p. 369) mentions having occasionally seen simple organs with pigment sheath of C. sdoani thus entirely enclosed by pigment. Supposing, as one surely has a right to do, that the pigment sheath intercepts the radiation, we would have in that organ of the barbel, and in the similar ones observed by Leydig, radiating organs unable to radiate. A careful examination of these organs completely surrounded by pigment in Chauliodus may be warmly recommended to future investigators. According to Chiarini’s deseription ('00, p. 16, Plate 5, Fig. 6) the simple radiating organs with pigment sheath of Chauliodus sloani are very similar ; his figure (6) however differs somewhat from mine, (Plate 9, Figs. 43, 44). Chiarini states that in staining the superficial cylinder cells with haema- toxilin-eosin the nucleus absorbs the former, the protoplasm the latter. The compound radiating organs with pigment sheath (Plate 9, Figs. 45, 46) on the body are directed downward. ‘Their axis is vertical, and parallel to the median plane of the fish. Thus the angle between it and the adjacent part of the surface is much larger in the ventral rows than in the lateral. In the-latter it is very small, often appearing to be nearly wil, the axis then being approximately parallel to the adjacent surface. In their external shape these organs resemble the compound organs of Lychnopoles argenteolus described above. They are composed of a proximal spherical (Plate 9, Fig. 45 A) and a distal paraboloidal part (D). These parts are separated by a sharp and well-defined stricture (C) of considerable depth. The proximal part forms about three quarters of a very regular sphere. The distal part is short, broad, and cut off obliquely at the base, where it abuts on the sur- face. With the exception of the oblique terminal face of the distal para- boloidal part, the whole organ is enclosed in a stout pigment sheath (Plate THE RADIATING ORGANS OF THE DEEP SEA FISHES. 187 9, Figs. 45, 46 p). The cells composing the sheath contain brown pigment. They are very closely packed towards the inner well-defined and perfectly smooth surface. Towards the outer side they are farther apart and_ this surface is not so well defined, the pigmented tissue passing here somewhat gradually into the unpigmented tissue surrounding the pigment sheath. In the distal paraboloidal part of the organ a lustrous reflecting layer (Plate 9, Fig. 45 r) composed of slender fibres, is found within the pigment sheath. In the proximal spherical part no such reflecting layer could be made out. A connective-tissue membrane (Figs. 45, 46 ¢) clothes the whole organ. In the proximal spherical part it appears to rest immediately on the pigment sheath, in the distal paraboloidal part it rests on the reflecting layer. Distally this membrane merges into the transparent connective tissue (Fig. 45 d) which occupies the triangular space between the distal limit of the radiating cell mass proper and the outer surface of the fish. From the whole of the connective-tissue capsule fine radial septa extend inward. The radiating cell mass proper is composed of three regions, an inner (Fig. 45 i), a middle (m) and an outer (o). The inner region occupies the whole of the proximal spherical part of the organ with the exception of a distal conic sector. It is composed of large, elongate pyramidal cells (Figs. 45,46 pe). These are arranged radially in a very regular manner and nearly vertical to the connective-tissue capsule, on which their bases rest. They do not quite reach the centre to which they converge. This is occu- pied by a plug-shaped mass of small cells (Plate 9, Figs. 45, 46 a) which appears to be connected with the radial septa extending inward from the connective-tissue capsule enclosing the radiating cell mass. These septa can often be made out between the pyramidal cells. In sections large blood vessels (Plate 9, Fig. 46 b) are frequently seen extending radially inward from the surface to the central cell mass. Here they continue their course between the latter and the proximal ends of the pyramidal cells, and seem to give off exceedingly fine branches which extend radially outward between the pyramidal cells (Plate 9, Fig. 46) along the connective-tissue septa, separating the latter from one another. The pyramidal cells themselves (Plate 9, Figs. 45, 46 pe) are occupied at their distal end by protoplasm staining deeply with haematoxylin (Plate 9, Fig. 46 pt) whilst their central and proximal parts (ph), about five sixths of the whole, show no great affin- ity for haematoxylin and other stains. The protoplasm occupying this central part of the cell is, however, not hyaline but distinctly granular. In 188 THE RADIATING ORGANS OF THE DEEP SEA FISHES. the distal highly stainable terminal part of each cell one rather small nucleus (nu) is situated. The middle region (Plate 9, Fig. 45 m) occupies the distal sector of the proximal spherical part of the organ and the proximal portion of the distal paraboloidal part. The proximal limit is conic, the distal limit very regularly convex. The cells occupying the space between these limits (1) are well defined and polyedric in shape. The distal ones are small. From here they increase in size inwardly, the innermost ones being four to five times as large as the outermost ones. The protoplasm of these cells is homogeneous, not granular, and hardly stainable with haematoxylin. It shows a great affinity to picrie acid and the whole cell mass of this region appears bright yellow in sections stained with picric acid-haemotoxylin. The nucleus is nearly spherical and larger in the large cells than in the small. The distal limit of the outer region is nearly a plane vertical to the axis of the organ, and so, its inner limit being concave and some distance away, the outer region (Plate 9, Fig. 45 0) has the shape of a thick plane concave lens. From the mass of connective tissue which abuts distally on the outer region, septa, nearly vertical to the limiting surface and parallel to the axis of the organ, extend inward, dividing the outer region into prismatic facets rounded off distally. These facets are occupied by small elongated cells, the protoplasm of which behaves towards stains in a similar manner as the protoplasm of the cells of the middle region. The compound radiating organs of Chauliodus were first studied in C. sani by Leuckart (’64, pp. 158-155). According to his description these organs of C. sloani are similar to those of C. barbatus. Leuckart chiefly studied the branchiostegal radiating organs (1. ¢., p. 154). These are cylinders, 1 mm. long and 500 » broad. Their distal portion is occupied by a structure Leuckart considers as a lens. From the proximal face of this a conic protuberance arises, penetrating some distance into the inner cell mass; as mentioned above such a cone is also observed in C. barbatus. The reflecting layer consists of hexagonal pavement cells occupied by numerous crystals. In the outer part of the distal mass, that is, in the “lens,” Leuckart claims to have seen fibres, whilst the inner part, and the proximal cone are composed of closely packed rods 5 w thick radiating from the apex of the cone. The substance composing these rods is highly refractive. No fibres or rods of this description occur in the middle and outer regions, which cor- respond to Leuckart’s “lens,” in the compound organs of C. barbatus. The THE RADIATING ORGANS OF THE DEEP SEA FISHES. 189 proximal mass which Leuckart terms “ Glaskirper,” and which corresponds to the inner region, is composed of large crystal-clear radial cones similar to cells; Leuckart, however, does not consider them as such, not having been able to find a nucleus in them. Each organ is supplied by a slender nerve. Leuckart considers these organs as accessory eyes, “ Nebenaugen.” It is noteworthy that in the discussion following the reading of this paper Claus (Leuckart, 64, p. 155) drew attention to the similarity between these organs and the well-known structures of Euphausia, which were at that time also considered as accessory eyes. Ussow ('79, pp. 94-97, Plate 1, Figs. 1, 2, Plate 4, Figs. 25, 26) has also described the structure of the compound radiating organs of C. sloan’. He finds them composed of two semispherical parts, the outer one of which is occupied by a gelatinous mass, colorless and transparent in the living fish, but coagulating and becoming opaque on being treated with the reagents used for preserving the specimen. Below this gelatinous mass a mushroom- shaped “lens” (79, Plate 2, Fig. 6 Kl) is seen, the cylindrical stalk of which extends inwards, whilst the semispherical upper part, com- posed of conic elements, protrudes into the outer gelatinous mass. This “Jens”? appears to correspond to our middle region. The conic elements of its distal, semispherical part are according to Ussow attenuated proxi- mally to fine threads, which extend inward parallel to the axis of the organ and form the stalk of the mushroom, in which they are connected with granular multipolar cells. Concerning the inner region Ussow merely corroborates the previous statements of Leuckart. These, however, he does not seem correctly to have understood. Ussow also states that a slender nerve leads to each organ. His description and also his figures are very different from what subsequent authors have seen in these organs of C. sloani and show very little similarity to the structure of these organs in C. barbatus described above. Leydig (’79, pp. 365-382, Plate 15) examined the radiating organs of C. sloan’ simultaneously with and independently of Ussow. His material, a specimen preserved in spirits for several years, was not good, but nevertheless his results were more valuable than those of Leuckart and Ussow. He also chiefiy studied the compound organs; his notes on the others have been referred to above. The compound organs are surrounded by annular blood vessels and a slender nerve branch leads to each. The distal portion of the compound organ is composed of two parts, an outer cap and a lower mass. 190 THE RADIATING ORGANS OF THE DEEP SEA FISHES. The proximal cone described by Leuckart is part of this latter. In the distal cap club-shaped cells are met with, the distal end of which is the thicker one. It is laterally enclosed by stout cell walls, but destitute of a terminal membrane. The thin proximal end passes into a fine thread of considerable length. The nucleus is situated in the lower end of the thick part of the cell. The cells of the lower mass of the distal portion are of greater size, more finely granular, and have a larger nucleus. The proximal cone appears to be composed of processes of these cells which converge towards its apex. This cell mass is traversed by fibrous strands, extending inward from the surface and forming a kind of scaffolding which has the shape of a honey comb. The cells of this honey comb evidently correspond to the connective-tissue facets described above of C. barbatus. The cones occupy- ing the proximal part (inner region) of the organ are cells. Leydig found a nucleus in their granular outer end. The centre to which these conic cells converge is oecupied by a finely granular substance (‘f Punktsubstanz ”’). Leydig thinks that the nerve leading to the organ terminates in this “ Punkt- substanz” and that the cells both above (cone of the middle region) and below (inner region) are connected with it. A portion of the reflector form- ing half a ring outside the stricture has a distinctly golden lustre. Leydig calls these organs accessory eyes, but uses this term (“* Nebenaugen”) with acertain amount of mistrust. From this and from a quotation of Willemoes- Suhm, which he publishes, one can conclude that he was already at that time, although calling these organs eyes, quite open to the view that they had in fact a radiating function. The descriptions given by these authors are based on material insuffi- ciently preserved and studied without the application of modern methods. They can therefore claim hardly more than an historical interest. Now, however, we have to review the papers by Brandt (1899) and Chiarini (1900) m which the compound radiating organs of Chauliodus sloani are described in a far more exact manner. According to Brandt's description (99, pp. 447-450) these organs of C. sloan’ seem to be identical in structure with those of C. barbatus. THe considers the cells of the inner region as gland cells in which a secretion is formed and then oxidized whereby light is produced, whilst the middle and outer region have no other function than that of conveying the ra- diation produced to the outer world. Chiarini (00, pp. 14-17, Figs. 6-7) also describes the radiating organs of THE RADIATING ORGANS OF THE DEEP SEA FISHES. 191 C. sboani in detail. To his statements concerning the simple organs we have referred above. The compound organs of this fish (01, pp. 14, 15, Fig. 7) consist, according to him, as in C. barbatus, of a proximal, nearly exactly spherical part, to which a funnel-shaped neck, separated by a distinct incision, is attached. In the compound organs of the body the funnel is eut off obliquely where it abuts on the surface of the body. In the branchiostegal organs transversely. Each compound organ consists of an outer pigment layer, a reflector, a connective-tissue capsule, and a central cell mass, in which three regions, an outer, middle, and inner can be distin- guished. These regions correspond to the three regions in the organs of C. barbatus. Beyond the outer region gelatinous tissue is met with, occupy- ing the space between its outer face and the surface of the fish. The outer region is divided by connective-tissue membranes into very narrow facets di- verging distally. ‘The middle region is clearly divided into an outer portion, occupied by polyedrical cells, and an inner portion divided into radial facets. The inner region consists of radially arranged conic cells, the larger central parts of which are fine grained and stainable with eosin but not with haema- toxylin, whilst the much smaller peripheral portions forming about a quarter of the length of each cell are readily stainable with haematoxylin. Each of these cells contains one or two nuclei situated in the peripheral portion. It will be seen from this that the structure of these organs in C! s/oaii is very similar to that in C. darbalus. ‘The chief differences between them are the following: in C. slow the facets of the outer region are narrow, the middle region is distinctly divided into an outer and an inner portion, and the cone cells of the inner region contain one or two nuclei; in C. barbatus, on the other hand, the facets of the outer region are broad, the middle region is not divided into two clearly distinct parts, and the cone cells of the inner region always contain a single nucleus. Idiacanthus antrostomus Gilbert. Plate 8, Figs. 41, 42. This species was first described by Gilbert (90, p. 54), later more in detail by Garman (99, p. 280). It possesses two kinds of radiating organs, one without, the other with, a pigment sheath, The former are exceedingly small and arranged in two large elongate groups (Plate 8, Fig, 41 u) on the 192 THE RADIATING ORGANS OF THE DEEP SEA FISHES. sides of the head under the large suborbital organ with pigment sheath, and in six longitudinal rows, three on each side, of smaller groups on the body. The intervals between these small groups on the body are equal in extent to the intervals between the larger organs with pigment sheath, and the former alternate with the latter. The radiating organs with pigment sheath have a silvery lustre. Those of the body measure about 500 in diameter; the suborbitals are larger. There are on each side 1 suborbital (Plate 8, Fig. 41 so), 13 branchiostegal, 10 guttural (Fig. 42 ¢), 25 ventrothoracic (Figs. 41, 42 vt), 20 ventromedial (Figs. 41, 42 ve), 35 ventroanal (Figs. 41, 42 a), 25 anterior lateral (Fig. 41 al), 20 medial lateral (Fig. 41 ml), and 35 posterior lateral (Fig. 41 pl), radiating organs with pigment sheath, The small organs without pigment sheath are spherical, enclosed in a connective-tissue capsule, and composed of radially arranged cells, which usually enclose a central cavity. The organs with pigment sheath closely resemble the compound organs of Chauliodus barbatus described above. ‘The suborbital organs are so dis- posed as to throw their radiation into the field of vision; the organs of the body are directed downward. Stomias hexagonatus Garman. Plate 10, Figs. 47-51. This species has been described by Garman (99, pp. 276, 277, Plate 56, Fig. 5). It possesses five different kinds of radiating organs. An organ in the barbel, a pair of suborbital organs, and on the body simple organs without pigment sheath, simple organs with pigment sheath, and compound organs with pigment sheath. The radiating organ in the barbel (Plate 10, Figs. 47, 48, 50), lies in a thickening 1.5mm. long at the end of the barbel (Fig. 47,481). From its distal part three short terminal threads arise. The radiating organ itself (Plate 10, Fig. 50), is oval and placed transversely in the swelling of the barbel, the long axes of the two crossing nearly at right angles. It is enclosed in a stout connective-tissue sheath (Plate 10, Fig. 50 c), and com- posed of two parts, an upper, dorsal, smaller, dorsoventrally compressed (A), and a lower, ventral, spherical part (B). The smaller upper part is enclosed in a somewhat loose pigment sheath (p), lying within the upper part of the THE RADIATING ORGANS OF THE DEEP SEA FISHES. 193 connective-tissue sheath enclosing the whole organ. The upper portion of this pigment sheath forming the roof of the upper part is dome-shaped, the lower portion, forming its floor and separating it from the lower spherical part, is flat-or even a little concave. At the circular line where the upper part joins the lower a conspicuous ring-shaped thickening of the pigment layer (pt) is observed. From this a small annular ridge extends outwards and downwards, forming the rudiment of a cup enclosing the basal, upper portion of the lower, spherical part of the organ, The interior of the upper part of the organ is traversed by a horizontal, strongly pigmented membrane (pm), which divides it into a larger upper and a smaller lower chamber. In both these chambers large cavities (cv) are seen. In the upper one there seems to be only one such cavity. This is situated ventrally and surrounded by a special endothelial cell layer, separating it from the transparent connective tissue occupying the dorsal and lateral parts of the upper chamber. In the lower chamber more cavities than one are seen. These lie dorsally. Below them in the ventral part of this chamber large sinuous blood vessels (b) extend. The lower part of the organ (B) is covered dorsally by the pigmented floor of the upper part of the organ and the annular rudiment of a pigment cup referred to above. Laterally and ventrally it is surrounded by the transparent connective-tissue sheath only. From the middle of the roof of this lower spherical part of the organ a cylindrical thread composed of connective tissue (z) arises which extends vertically downward to its centre, This thread is composed of longitudinally arranged, spindle-shaped connee- tive-tissue cells with oval nuclei. Along this thread blood vessels (b) and probably also a nerve extend from above down to the centre of the sphere. The lower end of the thread is thickened to form a terminal knob in which a sinuous cavity, filled with blood corpuscles, is observed. Apart from this thread with its terminal knob the whole of the sphere is occupied by large radially arranged conic cells (pe) equal in length to its radius. — In arrange- ment and structure these cells are similar to the conic elements in the imner region of the compound organs of Chauliodus barbatus described above. They are, however, more slender. The inner seven eighths are occupied by finely granular, transparent protoplasm not readily stainable ; in the outer eighth of the length coarser grained protoplasm showing great affinity to hematoxylin and other stains is observed. In the peripheral part of each cell one small, spherical nucleus is situated. 13 194 THE RADIATING ORGANS OF THE DEEP SEA FISHES. The suborbital radiating organs (Plate 10, Fig. 47 so) are represented by Garman in his figure (’99, Plate 56, Fig. 3), but not mentioned in his description (’99, pp. 276, 277). The suborbital organ lies below and behind the eye; it appears nearly spherical, is 800 » in diameter, and enclosed in a pigment sheath and a connective-tissue capsule. The pigment sheath forms a sac, the opening of which abuts on the surface of the fish. It is composed of concentric layers of rather loosely scattered cells containing brown pigment. Within this pigment sheath lies the connective-tissue cap- sule. The structure of the inner cell mass could not clearly be made out, as these organs were somewhat injured in the specimens at my disposal. The simple radiating organs without pigment sheath appear as small white dots 100 in diameter. They are scattered in great numbers over the ventral side of the fish and also occur in the three terminal threads of the barbel mentioned above. They are more or less spherical, enclosed in connective-tissue capsules and composed of pretty large radially arranged cells with conspicuous nuclei, The simple radiating organs with pigment sheath are met with on the hexagonal scales of the body and also on the head of the fish. They appear as dark spots and measure 500 in diameter. On each ventral scale a group of 7 of these organs occurs, upwards they gradually become scarcer; and near the dorsal medial line of the fish only one such organ is found on each scale. In their structure these organs resemble the simple radiating organs with pigment sheath of Chauliodus barbatus described above. The hyaline mass which in the latter covers the organ outside, is, however, absent in these organs of Slomias hexagonalus. The compound radiating organs with pigment sheath (Plate 10, Figs. 47, 48, 49, 51) appear as conspicuous dots with a silvery lustre and are 300- 500 in diameter. On the whole they are largest in front and decrease in size backwards. There are on each side of the body 12 guttural (Plate 10, Vigs. 47, 48 ¢), 16 branchiostegal, 38 ventrothoracic (Figs. 47, 48 vt), above these in a parallel row 87 anterior lateral (Figs. 47, 48 al), 10 ventromedial (Figs. 47, 48 ve), 10 medial lateral (Figs. 47, 48 ml), and 19 ventroanal (Figs. 47, 48 a) compound radiating organs with pigment sheath. Two different kinds of these organs can be distinguished, The first somewhat larger kind is represented by the anterior compound organs of the body which form the ventrothoracic and anterior lateral rows. All the other compound organs belong to the second, smaller kind. THE RADIATING ORGANS OF THE DEEP SEA FISHES. 195 The compound organs of the first kind (Plate 10, Figs. 49, 51) are spherical and have a short process directed downwards towards the outer surface. They might be compared to spherical bottles with small necks placed upside down. The axis of the organ is vertical, parallel to the median plane of the fish. The ventral organs, that is, those belonging to the ventro- thoracic row, therefore abut nearly vertically on the surface, whilst the lateral ones, that is, those belonging to the anterior lateral row, are very oblique and enclose but small angles with the surface of the fish. In the former the “neck” appears cut off transversely ; in the latter, obliquely. The pigment sheath (Plate 10, Fig. 51 p) enclosing the organ is rather thin and composed of cells containing brown pigment. It surrounds all parts of the organ with the exception of the terminal face of the “ neck.” As in other compound radiating organs we also find in these below the pigment layer a reflecting layer. This is however not at all extensive, being confined to the distal part of the neck-shaped portion, where it forms a ring (Plate 10, Fig. 51 r) surrounding the “neck.” This reflecting layer is com- posed of highly refractive threads. Within the pigment sheath and, where this is developed, the reflecting layer, a connective-tissue membrane is met with which entirely surrounds the inner cell mass and forms a perfectly closed capsule, from which diver- ticula extend inwards. The greater part of the interior of the proximal, spherical part of the organ is occupied by large radially arranged conic cells which converge towards the centre of the sphere. These cells are very similar to the conic elements in the inner region of the compound organs of Chauliodus barbatus and like them consist of a long, proximal, finely granular part (ph) not readily stainable, and a short, distal part (pt) showing great affinity to haematoxylin. The nucleus is remarkably small and situated in the distal staining part of the cell. These cells are divided from each other by fine connective-tissue membranes extending from the outer connective-tissue capsule radially inward towards the centre. In these membranes small radial blood vessels (b) can be made out. The “neck” of the organ (Plate 10, Fig. 51 cy) above referred to is a stout cylinder, circular in transverse section, the axis of which coincides with the axis of the whole organ. This cylinder is surrounded by an extensive annular cavity (s) dividing it from the connective-tissue capsule forming the innermost layer of the outer covering of the organ. The outer, distal, ter- 196 THE RADIATING ORGANS OF THE DEEP SEA FISHES. minal face of the cylinder is rounded off, simple, convex; the inner, proxi- mal face is drawn out to form a regular cone about as high. as broad, which penetrates the mass of radial conic cells occupying the proximal spherical part of the organ. The apex of this cone lies a little way above the centre of this sphere. The whole appears as a cylindroconic plug (Plate 10, Fig. 51). From that part of the connective-tissue capsule of the organ which covers the terminal face of this plug, diverticula extend inwards parallel to the axis. ‘These form parallel facets. The central (axial) facet (ec) is the longest; it extends to the apex of the cone, whilst towards the sides of the cylinder the facets become shorter. Transverse (Plate 10, Fig. 49) and longitudinal axial (Plate 10, Fig. 51) sections of the plug-shaped outer part of the organ show that the contents of the central facet (Plate 10, Figs. 49, 51 ec) are different from those of the other facets (pce). In staining with Van Gieson’s haematoxylin-picric acid-fuchsin one finds that the latter take up haematoxylin and acid-fuchsin very readily, whilst the substance in the central facet absorbs only the picric acid. It also stains with eosin. In the distal parts of the outer facets nuclei and cell limits can be distinctly seen ; here pretty large elongate cells are situated. In transverse sections of the distal part of the plug one sees two or three cell sections lying side by side in each facet (Plate 10, Fi g. 49). In the proximal parts of the outer facets and throughout the whole length of the central facet cell limits cannot be made out and nuclei are hardly to be seen. Ussow (79, p. 91) has described these organs in Sfomius barbatus. He says that they are similar to those of Chauliodus, but that the middle region, which he represents as a mushroom-shaped body (see above) in Chauliodus sloani, is absent in Stomias barbatus. Organs similar to these have been described by Chiarini (00, p. 12, Fig. 3) from Jaurolicus powertae, He says that each of the cells of the inner spheri- cal part and also of the cylindroconic plug, that is, the “neck” of the organ, often contain to nuclei. These cells are always mononuclear in Sfomias hexagonalus, In longitudinal sections the limits between the cells are often so indistinct that one can indeed be easily misled on this point and imagine that the two or three nuclei seen imbedded in an apparently continous pro- toplasmatic mass belong to one and the same cell. Transverse sections, how- ever, show, at least in the distal part, the cell limits well enough and make it clear that here each nucleus belongs to a separate cell. r . . . . . The compound radiating organs of the second kind which are met with THE RADIATING ORGANS OF THE DEEP SEA FISHES. 197 on the hinder part of the body have the same structure as the compound organs of Chauliodus barbatus described above. It is therefore not necessary to give a detailed account of them. (2) THE FISHES WITH RADIATING DISCS. Bassozetus nasus Garman. Plate 4, Figs. 14-17, This species has been described by Garman (’99, pp. 159, 361, Plates 77, 78). Its head shows a number of deep depressions (Plate 4, Fig. 14). On remoy- ing the semitransparent skin from it a number of subdermal, clearly defined, white, somewhat elongated, rhombical patches 1-3 mm. long are exposed to view. NS . ‘a °. 6 x av d «6 gar De aS SASSY POR RASNS t a PLATE 8. PLATE 8. Figs. 388-40. Chauliodus barbatus Garman. Fig. 38. The fish seen from the side. Drawing; natural size. a, ventroanal compound radiating organs; al, anterior lateral compound radiating organs; ao, anteor- bital compound radiating organs; Ba, simple radiating organs with pigment sheath in the barbel; ml, medial lateral compound radiating organs; so, subor- bital compound radiating organ; ve, ventromedial compound radiating organs; yt, ventrothoracic compound radiating organs; x, simple radiating organs with pigment sheath on the side of the head. Fig. 89. The fish seen from below. Drawing; natural size. a, ventroanal compound radiating organs; al, anterior lateral compound radiating organs; Ba, sim- ple radiating organs with pigment sheath in the barbel; g, guttural compound radiating organs; ml, medial lateral compound radiating organs; u, groups of simple radiating organs without pigment sheath; ve, ventromedial compound radiating organs; vt, ventrothoracic compound radiating organs. Fig. 40. The lower jaw seen from below. Drawing; magnified 1:2; Ba, simple radiating organs with pigment sheath in the barbel. Figs. 41, 42. Idiacanthus antrostomus Gilbert. Fig. 41. The fish seen from the side. Drawing; natural size. a, ventroanal radiating organs with pigment sheath; al, anterior lateral radiating organs with pigment sheath; ml, medial lateral radiating organs with pigment sheath; pl, posterior lateral radiating organs with pigment sheath; so, suborbital radiating organ with pigment sheath; u, suborbital group of small radiating organs without pigment sheath; ye, ventromedial radiating organs with pigment sheath; vt, ventrothoracie radiating organs with pigment sheath. Fig. 42. The fish seen from below. Drawing; natural size. a, ventroanal radiating organs with pigment sheath; @, guttural radiating organs with pigment sheath; ve, ventromedial radiating organs with pigment sheath; vt, ventrothoracie radiating organs with pigment sheath. Qt) SNWOLSOULNVY SNHLNVOVIG] 2E Lp BED SNIVEUVE SNAOTINVHD OF EC BE vorsog Wai) Jastayy g IHL E SUIT Ee s ne ——— ; ‘ vg 8 Sivig I68I XB.SSOULvaTY, PLATE 9. PLATE 9. Figs. 43-46. Chauliodus barbatus Garman, Fig. 43. Axial section vertical to the median plane of the fish through a regular simple radiating organ with pigment sheath. Stained: haematoxylin-picrie acid-fuchsin. Drawing; magnified 1: 280. ¢, connective-tissue membrane forming the covyer- ing of the mass of radiating cells on the outer side; h, outer apparently struct- ureless covering of organ; 1, central mass of polyedrical cells; m, granular protoplasmic mass at the base of the organ; p, pigment sheath; pe, peripheral cylindrical radiating cells. rig. 44. Axial section vertical to the median plane of the fish through an irregular simple radiating organ with pigment sheath. Stained: haematoxylin-picrie acid-fuchsin. Drawing ; magnified 1: 280. ¢, connective-tissue membrane forming the cover- ing of the mass of radiating cells on the outer side; h, outer, apparently structureless covering of organ; 1, central mass of polyedrical cells; m, gran- ular protoplasmatie mass at the base of the organ; p, pigment sheath; pe, peripheral cylindrical radiating cells. Fig. 45. Axial section vertical to the median plane of the fish through a medial lateral compound radiating organ. Stained: haematoxylin-picric acid-fuchsin. Draw- ing; magnified 1: 140. A, proximal spherical part of the organ; a, mass of small cells in the central portion of the proximal spherical part of the organ; C, constriction between the proximal and distal parts of the organ; c, connective- tissue capsule within the pigment sheath; D, distal paraboloidal part of the organ; d, gelatinous connective tissue occupying the distal part of the organ ; f, cells in the facets of the outer region; i, inner region; 1, cells composing the middle region; m, middle region; 0, outer region; p, pigment sheath; pe, large conic cells of the inner region; r, reflecting layer. Fig. 46. Transverse section through the centre of the proximal spherical part of a medial lateral compound radiating organ. Stained: haematoxylin-picric acid-fucbsin. Drawing; magnified 1: 340. a, mass of small cells in the central portion of the proximal spherical part of the organ; b, blood vessels extending radially to the centre of the sphere; ¢, connective-tissue capsule; nu, nucleus of the conic cells; p, pigment sheath; ph, proximal only slightly stainable part of the conic cells; pt, peripheral highly stainable part of the conic cells. Puate 10. Fig. 47. Fig. 4 Fig. 49. Fig. 50. CO Puate 10. Figs. 47-51. Stomias hexagonatus Garman. The fish seen from the side. Drawing; natural size. a, ventrocaudal compound radiating organs; al, anterior lateral compound radiating organs; g, guttural compound radiating organs; L, radiating ovean of the barbel; ml, medial lateral compound radiating organs; so, suborbital radiating organ; ve, ventro- medial compound radiating organs; vt, ventrothoracic compound radiating organs. The fish seen from below. Drawing; natural size. a, ventrocaudal compound radiating organs; al, anterior lateral compound radiating organs; g, guttural compound radiating organs ; ml, medial lateral compound radiating organs; ve, ventromedial compound radiating organs; vt, ventrothoracie compound radiating organs. Transverse section through the outer distal part of the cylindroconic plug-shaped part of a yentrothoracic compound radiating organ. Stained: haematoxylin- picrie acid-fuchsin. Drawing; magnified 1: 280. ce, central facet of cylindro- conic plug; pe, transverse sections of cells in the outer facets of the eylindroconic plug. Axial section transverse to the barbel through the radiating organ of the barbel. Stained: haematoxylin-picrie acid-fuchsin. Drawing; magnified 1: 120. A, upper (dorsal) part of the organ; B, lower (ventral) part of the organ; b, blood vessel following the axial thread, and sinuous blood vessels in the lower chamber of the dorsal part of the organ; ec, connective-tissue sheath inclosing the organ; cv, cavities in the upper part of the organ; p, dorsal pigment sheath; pe, large radial conic cells of the lower spherical part of the organ; pm, pigmented membrane traversing the upper part of the organ; pt, annular thickening of the pigment layer; z, axial thread in the lower spherical part of the organ. Fig. 51. Axial section vertical to the medial plane of the fish through a ventrothoracic com- pound radiating organ. Stained: haematoxylin-picric acid-fuchsin. Drawing ; magnified 1: 160. ad, blood vessels in the septa between the conic cells of the proximal spherical part of the organ; ¢c, connective tissue covering the distal part of the organ on the outer side; cc, central facet of cylindroconic plug forming the distal part of the organ; co, conic proximal end of the outer plug- shaped part of the organ; cy, eylindroconic plug-shaped outer part of the organ ; p, pigment sheath; pe, longitudinally elongated cells in the outer facets of the plug-shaped distal part of the organ; ph, inner only slightly stainable part of the radiating conic cells in the proximal spherical part of the organ; r, annular reflecting membrane surrounding the plug-shaped distal part of the organ; s, annular cavity surrounding the plug-shaped distal part of the organ. By % \ SLE Yel obals}elches sane e2s 1S Oe RE ers aced AN ole, oat | ee, NES og ml PLATE 11. PLATE 11, Figs. 52-57. Halosaurus radiatus Garman. Fig. 52. The fish seen from the side. Drawing; natural size. x, 53. Radiating cephalic dise in transmitted light. Photograph; magnified 1: 17. b’, large blood vessel leading to the disc; n, main dise nerve; n/, smaller cessory dise nerves. Fig. 54. Radiating cephalic disc in transmitted light. Photograph; magnified 1 : 41. b’, large blood vessels leading to the disc; n, main dise nerve; n’, smaller cessory disc nerves. Fig. 55. Radiating cephalic dise in transmitted light. Photograph; magnified 1: 17. b’, large blood vessel leading to the disc; n, main dise nerve; n’, smaller ac- cessory dise nerves. ac- ac- Fig. 56. Transyerse section through a cephalic radiating dise along its longer axis. Stained : haematoxylin-picric acid-fuchsin. Drawing; magnified 1:42. b, close capillary network of blood vessels underlying the cylinder cell layer; b’, large blood vessel leading to the dise; ¢, thickened connective tissue of the inner wall of the slime canal on which the cylinder cell layer rests; cv, cavity of slime canal; d, cyl- indrical cells of the radiating disc; m, marginal zone of the radiating disc; n, main dise nerve; s, outer wall of slime canal. . Part of a transverse section through the cylinder cell layer of a cephalic radiating disc. Stained: haemotoxylin-picric acid-fuchsin. Drawing ; magnified 1: 200. b, close capillary network of blood vessels underlying the cylinder cell layer: e, thickened connective tissue of the inner wall of the slime canal on which the cylinder cell layer rests; d, cylindrical cells of the radiating discs. 7 ‘ Mo Fig. 5 ‘ALBATR ETrojan del Ss pete} uliee 56 189] “4 wy aa ia) ip) S-RADIATING OR 52 52-57 Halosaurus radiatus Garman. 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