246 



VERTEBRATE ANIJIALS. 



The integumentary skeleton is in the form of placoid scales (fig. 

 11)4, c) — that is to say, of detached grains, tubercles, or plates. 

 There are two pairs of fins, corresponding to the fore and hind 

 limbs, and tlie ventral fins are placed far back, close to the anus. 

 The heart consists of an auricle and ventricle ; and the bulhus 

 (irU'i-iosics is rhythmically contractile, is provided with a distinct 

 coat of muscular fibres, and is furnished with several transverse 

 rows of valves. The gills are fixed, and form a number of pouches, 

 which open internally into the pharynx, and mostly communicate 

 with the outer world by a series of apertures placed on the side of 

 the neck (fig. 176, A). The intestine is very short, but to compensate 



for this, the mucous mem- 

 brane is thrown into a fold, 

 which winds round the intes- 

 tine in close turns from near 

 the pyloi-ic orifice of the stom- 

 ach to the anus, and which 

 thus greatly increases its ab- 

 sorbing surface. 



The most characteristic ex- 

 amples of the Elasmobranch 

 Fishes are the Sharks, Dog- 

 fishes, Skates, and Rays, usu- 

 ally called Plagiostomous 

 Fishes, because the mouth is 

 transverse, and is placed on 

 the under side of the head. 

 In all these types there are 

 five branchial pouches on each 

 side, which open by five dis- 

 tinct slits on the surface of 

 the Ijody. There are also two 

 apei'tures (" spiracles ") on the 

 top of the hen.l, just behind the eyes, which lead by tubes into the 

 pharynx, and which correspond with an anterior pair of branchial 

 punches. 



The Sharks ami Dog-fishes (Selachii) have pectoral fins of the 

 ordinary size and shai)e (fig. 176, A), and have the gill-slits placed 

 on the sides of the neck. They have shai'p conical teeth, in several 

 rows, and they are nearly all voracious and fierce animals. The 

 Sharks in particular attain a great size. 



The Port Jackson Sharks (Cestrarioa) of the Pacific Ocean, re- 

 semble the true Sharks in most points ; but they have blunt teeth 

 adapted for crushing shell-fish. 



Fig. 1V7. — B.-iticles. Rain mnrginatn, on", of 

 the Skates. Reduced one -sixth. (After 

 Gosse.) 



