462 



SCANDINAVIAN FISHES. 



higli, liowevev, as those of tlic honiocercal fislies proper 

 — to support the middle rays of the caudal fin or even 

 the greater part of these rays. In these fishes the caudal 

 end of the spinal column is also bent upwards, at least 

 to sucli an extent that it lies, in the ordinary urostyloid 

 form, more or less clo.sely along the upper margin of the 

 hindmost (uppermost) hypural hone (fig. 117). The 

 diphycercal appearance of the Hn is also increased by 

 the marked development of tlie numerous suppoi-ting 

 rays, which are of similar fonu above and l)elo\v. The 

 character drawn fi'om the diphycercal caudal fin is thus 

 not absolutely valid, but is not entirely destitute of 

 significance, especially in comparison with the preceding 

 series, Avhere the hyj)ural bones are extraordinarily well- 

 developed, and the caudal end of the notochord is bent 

 sharply ujjwards for so long a. distance that the ori- 

 ginal caudal end of tlie body, with its end)i'y()nic (di- 



Fig. 117. Hind terininatioii of 1)ie spinal column in n Cod (Gadtts 

 callai'ia.i). 1','., times of llio natural size. 



pliycercal) caudal fin (fig. 1)8, e and fig. 99, ti, pp. 362 

 and 363, above), even in young Flatfishes 8 — 12 mm. 

 in length, still ]n'qjects above the ti'ue caudal fin, and 

 reminds us how this fin has devel()])ed from the strictly 

 hetei'ocercal caudal fin". 



This primitive, imperfectly specialized type of the 

 form of the caudal fin in the Onomorphous fishes is fre- 

 (juently accompanied by a no less imperfect specializa- 

 tion in the structtn-e of the other unpaired fins. Here, 

 as in several of the Blennomoi'phs and (jlobiomorphs, 

 we find these fins, especially in the deep-sea forms — 

 which also show several other embryonic characters'" — 

 continuous (united to the caudal fin) or separate. The 

 specialization is so far advanced, however, that in the 

 most typical Onomor[)hs the dorsal and anal fins are 

 more varial)le in form and nundier than is generally 

 the case in the other series. 



The jugular position of the ventral fins indicates 

 an advanced specialization, just as, in the preceding 

 part of this work, we ha\e repeatedlv seen the pro- 

 gressive development of the osseous fishes accompanied 

 by the removal in a forward direction of tliese fins. In 

 this respect too, the development sometimes advances so 

 far that it leads to degeneration, for the ventral fins dis- 

 appear in several forms, especially among deep-sea fishes. 



Another degeneration, which shows how the Ono- 

 morphous tv))e has succeeded in adapting itself to tlie 

 most unfavourable environments, is the reduction of the 

 organs of sight Avhich occurs in those fresh-water fishes 

 of the genus Lucifucja that inhabit the subterranean 

 caves and rivers of Cuba, and in Aplnioiiiis. which dwells 

 in tJie abyssal zone, in from 900 to 1,400 fathoms of 

 Avater, and which very closely resembles Liicifuf/a in 

 the form of tlie body as well as in this degeneration''. 

 Equally singular are the conditions of life to which the 

 genera Ficrasfer and EncheUo2)his have bound themselves, 

 a commensalism, in which they inhabit Mcdiisce, Holo- 

 fJnirifP, or shellfish. In these last forms the degenera- 

 tion mav involve not only the loss of the ventral fins, 

 but also of the pectoral, the vent assuming a jugular 

 position. 



During all these vai'\ing conditions of life, from 

 salt water to fresh, from the surface to the abyssal 

 depths of the ocean, from independent existence to a 

 sort of parasitism, the t^•lle of the series has undergone 

 so manifold modifications, and the degenerations speci- 

 fied above have exercised so many disturbing influences 

 on the regular course of development, that the char- 

 acters of the families are in many respects inconstant 

 and melt into each other. The validity of the follow- 

 ing division is, therefore, only general, though without 

 exception in the Scandinavian fauna. 



^.•raudal liu more or less distiuctly iude- 



pendcnt of und separated iVoni the dorsal 



and anal fins. Hind jiart of the body 



more or less terete. 



a: Jaws furnished with teeth Fam. (Jtuliilw. 



b: Jaws toothless.. Fam. A)innoJt/tid(v. 



U: Caudal (in not distinct, but, where pre- 

 sent, united cither to the dorsal iin. or 



to the anal fin, or to both. Hind part 



of the body Anguilliform or compressed, 



like a whip-hisli. 



" Cf. A. Ag,\ssiz, Proc. Amer. Acad. A., Sc, ii. ser., vol. V, p. 117 etc. (Boston, 1877). 



'' One of the remarkable instances of this we find in Apliyonus (G'rilli, Ecp. Deep-Sea Fisli., Ciiull. Kxp., part. LVIl, p. 

 XX\'I, lig. A), in which tlir eniliryonic notochord is persistent, sliowing merely external traces of vertebral segmentation. 

 '■ GLNTiiEii, liitriidiiction to the StuJij of Fis/ic.i, jip. 547 and 548; Hdiulh. /clitli., pp. 3'.10 and 'MH. 



1-20, pi. 



