THE DOGFISH 609 



is that the yolk is enclosed in a yolk-sac, with which the 

 embryo is finally connected only by a thin stalk — the 

 umbilical cord (see Fig. 343). 



The history of the yolk is briefly as follows : — It is accumulated 

 by the ovum from neighbouring cells, and from the vascular fluid ; it 

 is partly prepared for absorption by the merocytes or yolk-nuclei ; it is 

 at first absorbed by the blood vessels of the yolk-sac ; at a later stage, 

 absorption by blood vessels becomes less and less important, and the 

 yolk passes inside the embryo and into the gut, where it is digested. 

 Then the yolk-sac, empty of all but merocytes, degenerates, shrivels, 

 and disappears. 



Second type of Fishes. The Dogfish {Acanthtas vulgaris) 

 — A type of the Shark-shaped Elasmobranchs or 

 Selachians. 



The piked dogfish Acanthtas vulgaris is a common 

 British representative of the second main group of Elasmo- 

 branch fishes — the sharks. These agree with the skates 

 (Batoidei) in possessing cartilaginous skeletons, placoid 

 scales, a transverse ventral mouth, separate gill-openings, 

 a persistent ear aperture, in being without an air-bladder, 

 and in having large richly yolked eggs. They differ in 

 the form of the body, the use of the tail and hinder body as a 

 swimming organ, and in being of a more active and pelagic 

 habit. 



Acanthias is a small shark 2-3 feet long and abundant 

 in the North Atlantic. Great packs do much damage to 

 fishing nets and long lines as well as by harrying the shoals 

 of fish. The females are viviparous, giving birth in the 

 summer to as many as a dozen young 9 or 10 inches in 

 length. 



Externals. — The body is spindle shaped, roughly 

 circular in cross-section, with a blunt pointed snout and 

 tapering away rapidly behind to a markedly heterocercal 

 tail. It is covered with a very tough skin, rough to the 

 touch, in which small placoid scales are embedded 

 (" shagreen "). The colour is bluish-grey dorsally, shading 

 to white underneath, with a few not very conspicuous pale 

 spots. There are three unpaired fins — a large anterior 

 dorsal, a smaller posterior dorsal, and the caudal or tail 

 fin. In front of each dorsal fin lies a giant placoid scale, 



39 



