366 MANUAL OF ZOOLOGY sect. 



ventral, and caudal ; these are all supported by fin-rays of 

 dermal (p. 372) origin. A hard external covering of scales 

 developed in the dermis is usually present. In the en- 

 doskeleton the notochord is usually replaced more or less 

 completely by cartilaginous or bony vertebrae ; there is a 

 well-developed skull and a system of well-formed visceral 

 arches, of which the first forms the upper and lower jaws, the 

 latter movably articulating with the skull, and both nearly 

 always bearing teeth. An air-bladder is frequently present, 

 and in certain exceptional cases acquires the function of a 

 lung or chamber for breathing air. 



Sub-class I. Elasmobranchii 



A dogfish may be selected as a convenient example of 

 the sub-class and of the class Pisces. Dogfishes occur at 

 slight depths off the coasts in all quarters of the globe. The 

 commonest European forms are the rough hound (Scrllium 

 canicula), the lesser spotted dogfish (S. ca lulus'), the 

 piked dogfish (Acanthias vulgaris), and the smooth hound 

 {Mustelus vulgaris). Allied species of the southern hemi- 

 sphere are Scyllium, Acanthias, and Mustelus anarcticus. 

 On the coast of Northeastern America the common dogfish 

 is Mustelus cam's. For the description which follows, any 

 of these species will be found to serve very well. 



A slight general account of the dogfish has already been 

 given in the introduction to the Craniata ; this has now to be 

 extended and supplemented. The general shape (Fig. 206) 

 may be described as fusiform ; at the anterior or head-end it 

 is broad and depressed ; posteriorly it tapers gradually and 

 is compressed from side to side. The head terminates 

 anteriorly in a short blunt snout. The tail is narrow and 

 bent upwards towards the extremity. The entire surface is 



