566 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 
Fic. 301.—Young skate.—From Beard. 
The yolk-sac has been cut off, the yolk-stalk is 
left. 1., Mouth; o/.0., nostril, e.g. ‘external 
gills”; a., cloaca; c¢., claspers. 
the pelvic fins of the 
males. Except in Holo- 
cephali there 1s 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- 
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. 
Fertilisation is internal. 
The ova are few and 
large, z.e. with much yolk. 
