S 66 



PISCES FISHES. 



the view that the connection serves to make the fish aware of the 

 varying tensions of gas in the bladder, due to the varying hydrostatic 

 pressure. 



CLASSIFICATION OF FISHES 



Sub-Class I. ELASMOBRANCHII. Cartilaginous Fishes, 

 e.g. Sharks and Skates . 



Voracious carnivorous fishes, with cartilaginous skeleton, 

 placoid scales, usually heterocercal tails, "claspers" on 



ol.o. 



the pelvic fins of the 

 males. Except in Holo- 

 cephali there is no cover 

 gtgi over the (5-7) gill-aper- 

 tures ; anterior to these 

 there is often a spiracle 

 the first gill-cleft 

 with a rudimentary gill. 

 The gill -clefts are 

 separated by complete 

 septa, and the gill- 

 filaments are attached 

 throughout their length 

 to the septa. The mouth 

 extends transversely on 

 the under side of the 

 head. The nostrils are 

 also ventral. There is 

 no air-bladder. A spiral 

 fold extends along the 

 internal wall of the large 

 intestine. Into the ter- 

 minal chamber (or clo- 

 aca) of the gut the genital 

 and urinary ducts also 

 open. The ventricle of 

 the heart has a con- 

 tractile conus arteriosus. 



FIG. 301. Young skate. From Beard. Fertilisation is internal. 



The yolk-sac has been cut off, the yolk-stalk is The OVa are few and 



left, m., Mouth ; ol.o.. nostril, e.g: "external i . . , 



gills " ; a., cloaca ; c., claspers. large, I.e. With much yolk. 



