2i6 



VKKTElSliATR ANHIALS. 



Tl\e integumentary skeleton is in the furni of placoid scales (fig. 

 I(i4, r) — that is to say, of detached gtains, tnbercle.s, or plates. 

 There are two ]>a.irs of fins, corresponding to the fore and hind 

 limbs, anil tlie ventral fins are placed far back, close to tlie aims. 

 The heart consi.sts of an ani'icle and ventricle ; and the hulljiis 

 arteriosus is rhvtlimically contractile, is proviiled with a distinct 

 coat of nmscnlar fil.ires, and is furnished with sevei'al transverse 

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

 which open internally into the pharynx, and mostly conmuinicate 

 with the o\iter world by a series of apei-tnres j)laced on the side of 

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



for this, the nnicous mem- 

 brane is thrown into a fold, 

 which winds round the intes- 

 tine in close turns from near 

 the |iyloiic orifice of the stom- 

 ach to the anus, and wdiich 

 thus greatly increases its ab- 

 sorliing surface. 



The most characteristic ex- 

 ani|iles of the Elasmobranch 

 Fishes are the Sharks, Dog- 

 fishes, Skate.s, and Kays, usu- 

 ally called Plagiostomous 

 Fishes, because the moiith 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 body. There are also two 

 apertures ("spiracles") on the 

 to]) of the bead, just behind the eyes, which lead by tnbes into the 

 j>hai\ iix, and which correspond with an anterior pair of branchial 

 jiouclies. 



The Sharks ami Dng-fislies {S</hii-h>i) have jiectoral fins of the 

 ordinarv size and sli.-ipe (fig, 176, A), and have the gill-.slits jilaced 

 on the sides of the neck. They have sh.-ii'p conic.-il teeth, in .several 

 rows, and they are nearly all vor.-icidUs and fierce animals. The 

 Shai'ks in jia.rticular att.ain ;i. great size. 



The Port Jackson Sharks (Ci'sl rariiui) of the Pacific Ocean, re- 

 send ili' (he true Sharks in most jioints ; but they have blunt teeth 

 adapted foe crushing shell-fish. 



r'jinaUi, <nv rif 

 -sixth. (Altri- 



