6l2 PHYLUM CHORDATA : CLASS PISCES — FISHES 



The paired fins are chiefly for balancing. The whole method 

 of swimming is in marked contrast to that of the skate. 



Skeleton. — The pectoral girdle and fin skeleton is of 

 less marked development than that of the skate. There 

 is no vertebral plate. The propterygium is very small. 

 The propterygium and mesopterygium each bear but 

 one radial. The metapterygium bears several slender 

 radials (compare with Fig. 335). 



Brain and sense organs (see also Skate, Fig, 336). — 

 There is a well-developed lateral line system of sensory 

 canals. The " lateral line " itself is a groove covered 

 in for part of its length to form a canal with openings to 

 the exterior at intervals. On its floor are numbers of 



Vd Vll+V 



Fig. 347. — Side view of brain of Acanthias vulgaris. — After Purser. 



O.B., Olfactory bulb ; CBR., cerebral hemispheres ; OP.L., optic lobes ; CBL., 

 cerebellum; VEN., fourth ventricle enclosed laterally and ventrally by the 

 medulla oblongata ; INF., infundibulum, or stalk of pituitary body ; //., 

 opticnerves ; IV., V., VII., VIII., IX., rootsof these nerves ; A'., vagus nerve. 



sensory cells innervated by twigs from the lateral branch 

 of the tenth (vagus) nerve. The lateral line is continued 

 into the head region, forming a loop round the eye and 

 giving oflP several branches. Left and right lateral lines 

 are connected by a canal passing over the head immediately 

 behind the ear openings. In the head region the canals 

 are supplied chiefly by branches from the seventh (facial) 

 nerve. Clusters of sensory ampullae, opening by pores, 

 occur over the head region. 



Respiratory system. — The spiracles open laterally, not 

 dorsally, as in the skate. 



Alimentary system. — The short wide gullet iscontinued 

 into a hook-shaped stomach. The pyloric end of the 

 stomach opens into a bulky intestine with a spiral valve. 



