ELASMOBRANCH FISHES 



633 



ol.o. 



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 

 the pelvic fins of the 

 males. Except in Holo- 

 cephali there is no cover 

 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-fila- 

 ments are attached 

 throughout their length 

 to the septa. The 

 mouth extends trans- 

 versely 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 terminal cham- 

 ber (or cloaca) of the 



gut the genital and 



urinary ducts also open. 



The ventricle of the 



heart has a contractile 



conus arteriosus. Fer- 

 tilisation is internal. 



The ova are few and 



Fig. 361. — Young skate. — From Beard. 



The yolk-sac has been cut off, the yolk-stalk is 

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

 gills " ; a., cloaca ; c, claspers. 



large, i.e. with much 



yolk. Large egg-purses are common, but some Elasmo- 



branchs are viviparous. The embryos have gill-filaments 



