462 
SCANDINAVIAN FISHES. 
high, however, as those of the homocercal fishes 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 such an extent that it lies, in the ordinary urostyloid 
form, more or less closely along the upper margin of the 
hindmost (uppermost) hypural bone (fig. 117). The 
diphycercal appearance of the fin is also increased by 
the marked development of the numerous supporting 
rays, which are of similar form above and below. The 
character drawn from the diphycercal caudal fin is thus 
not absolutely valid, but is not entirely destitute of 
significance, especially in comparison with the preceding 
series, where the hypural bones are extraordinarily well- 
developed, and the caudal end of the notochord is bent 
sharply upwards for so long a distance that the ori- 
ginal caudal end of the bodv, with its embryonic (di- 
Fig. 117. Hind termination of the spinal column in a Cod ( Gaclus 
callarias). 1(4, times of the natural size. 
phycercal) caudal fin (fig. 98, e and fig. 99, a, pp. 362 
and 363, above), even in young Flatfishes 8 — 12 mm. 
in length, still projects above the true caudal fin, and 
reminds us how this fin has developed from the strictly 
heterocercal caudal fin". 
This primitive, imperfectly specialized type of the 
form of the caudal fin in the Onomorphous fishes is fre- 
quently accompanied by a no less imperfect specializa- 
tion in the structure of the other unpaired fins. Here, 
as in several of the Blennomorphs and Gobiomorphs, 
we find these fins, especially in the deep-sea forms — 
which also show several other embryonic characters 5 — 
continuous (united to the caudal fin) or separate. The 
specialization is so far advanced, however, that in the 
most typical Onomorphs the dorsal and anal fins are 
more variable in form and number 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 have repeatedly seen the pro- 
gressive development of the osseous fishes accompanied 
by the removal in a forward direction of these 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 type has succeeded in adapting itself to the 
most unfavourable environments, is the reduction of the 
organs of sight which occurs in those fresh-water fishes 
of the genus Lucifuga that inhabit the subterranean 
caves and rivers of Cuba, and in Aphyonus, which dwells 
in the abyssal zone, in from 900 to 1,400 fathoms of 
water, and which very closely resembles Lucifuga in 
the form of the body as well as in this degeneration c . 
Equally singular are the conditions of life to which the 
genera Fierasfer and EnclieliopMi s' have bound themselves, 
a commensalism, in which the} - inhabit Medusae , HoJo- 
thurice, or shellfish. In these last forms the degenera- 
tion may involve not only the loss of the ventral fins, 
but also of the pectoral, the vent assuming a jugular 
position. 
During all these varying conditions of life, from 
I salt water to fresh, from the surface to the abyssal 
depths of the ocean, from independent existence to a 
sort of parasitism, the type 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. 
A : Caudal fin more or less distinctly inde- 
pendent of and separated from the dorsal 
and anal fins. Hind part of the body 
more or less terete. 
a: Jaws furnished with teeth Fam. Gad Ida;. 
b: Jaws toothless Fam. Ainmodytidoi. 
B : Caudal fin not distinct, brit, where pre- 
sent, united either to the dorsal fin, or 
to the anal fin, or to both. Hind part 
of the body Anguilliform or compressed, 
like a whip-lash. 
a Cf. A. Agassiz, Proc. Amer. Acad. A., Sc., n. ser., vol. V, p. 117 etc. (Boston, 1877). 
1 One of the remarkable instances of this we find in Aplvjonus (Grim, Rep. Deep-Sea Fish., Chall. Exp., part. LVI1, p. 120, pi. 
XXVI, fig. A), in which the embryonic notochord is persistent, showing merely external traces of vertebral segmentation. 
c Gunther, Introduction to the Study of Fishes , pp. 547 and 548; Handh. Ichth., pp. 300 and 391. 
