The pectoral fins of the Sharks (fig.496, 497) and Sturgeons (fig. 498) are formed after the same plan as that of the Skate, only upon a considerably smaller scale, representing as it were only a moiety of the posterior division of the fin of the preceding genus.
The posterior extremities or ventral fins are attached to a zone similar to that which supports the pectorals (fig. 500,14). The pelvic zone is, however, very incomplete, the superior or iliac portion being quite deficient, so that it has no connection with the spine, but is simply im bedded among the muscles at the posterior part of the abdomen.
Externally the pelvis supports the first ray of the ventral fin rfig. 500, 15), which is very large, and likewise a long stem (16) composed of nume rous articulations, to the commence ment of which the succeeding fin rays are appended. Inferiorly the former is prolonged in the male Rays into a very curious club shaped apparatus called the "clasp er," of the nature of which we shall have occasion to speak further on.
Skeleton of Dermapterygii.—In the cyclostomatous Fishes, such as the Lamprey, the skeleton is of still more simple structure than in the plagiostomatous genera.
The cranium exhibits through life a soft carti laginous texture; nevertheless it is not difficult to identify the different pieces of which it con sists, and to point out their analogies with those of the osseous Fishes.
The spine consists of a soft cartilaginous stemovhich passes along the entire length of the body. It is enclosed in a strong membra nous investment, from which prolongations are given off that perform the office of spinal apo physes ; but the only indications of distinct vertebrx exist in the presence of slight and almost imperceptible rings of osseous substance distinguishable upon the surface of the cartila ginous stem above mentioned.
The Cyclostomata have neither pectoral nor ventral fins, so that in this respect they are the most imperfect of all Fishes : even the vertical fins situated above and beneath the tail are only supported by a few soft and scarcely sen sible fibres representing the fin-rays.
The most remarkable part of the skeleton of the Lampreys ( Petromyzon) is the cartilagi nous framework situated on the sides of the neck, enclosing the branchial apparatus, and allowing respiration to be accomplished by a peculiar mechanism which will be described in its proper place. This singular structure con
sists of seven pairs of cartilaginous arches de rived from a kind of sternum situated in the rnesial line beneath the throat. These arches, which have nothing in common with the bran chial arches of Fishes, mount upwards inter ruptedly towards the spine, giving off anterior and posterior processes to form a kind of frame to the branchial orifices. Anteriorly this re markable apparatus is attached to the cranium, while posteriorly it terminates in a thin cartila ginous capsule, which encloses the heart; it will, however, be better to describe it further on.
In Anunocetes the skeleton is even more im perfect than in the Lamprey, all its parts remain ing permanently in a membranous condition, so that they would seem to resemble worms more than vertebmted animals, and in fact were ab solutely classed as worms even by the great Linnmus.
Skeleton of Branchiostoma.— In this re markable Fish the entire spine is made up of a succession of verrdelicate membranous rings without any apophyses whatever ; neither in young specimens is the slightest trace of a cranial dilatation of the vertebral column appa rent, probably owing to its being at tliis period quite gelatinous in its texture, and conse quently translucent; and even in adults such is its softness that it is impossible to distin guish it satisfactorily; but along the back from sixty to seventy vertebr& are easily counted, the divisions between them being indicated by slight bulgings and lines passing obliquely from above downwards on the sides of the column. In this way a separation between the rachidian rings is rather indicated than proved to exist.; for, although there is, so to express it, a tendency to divide at the points indicated, the division is rather artificial than natural.
According to Mr. Goodsir, the chorda dor salis is formed externally of a fibrous sheath, and internally of an