156 ESSENTIALS OF ZOOLOGY 



situated far forward on the dorsal surface. A small caudal 

 fin occupies the upper surface of the end of the tail. A ventral 

 fin is absent, but it is present in the embryo. The anterior 

 angle is supported by a rostral extension of the skull, and it is 

 more or less prominent and pointed or rounded, according to 

 the species. In the skates it has the form of a sharp-pointed 

 process, in the rays it is rounded. 



The eyes, originally lateral in position as in fishes generally, 

 are in the adult situated dorsally, one on each side of the 

 flattened head ; and behind them are the spiracles, a pair of 

 large openings which have likewise been moved from a lateral 

 position. The spiracles open into the pharynx. Each is 

 furnished with a valve, and these openings are used to intro- 

 duce water to the pharynx, and so to the gills for respiration. 

 According to circumstances, the water so introduced may be 

 passed through the gills to the exterior, or it may be returned 

 to be ejected through the spiracles. 



On the under side the transverse mouth lies far from the 

 anterior end of the body. The jaws are furnished with many 

 rows of teeth. The- nasal openings are placed in front of the 

 mouth, with which they are connected by grooves bounded by 

 lips. The rectangular space bounded by the nasal grooves 

 and the mouth is the fronto-nasal process. The external 

 openings of the gills are a paired series of five slits, the two 

 series converging posteriorly towards the median line. A 

 rudimentary sixth slit is sometimes quite plain as a depression 

 of the skin which covers the ventral part of the pectoral 

 girdle. Except this rudimentary gill slit, all the gills open 

 into the pharynx. 



The cloaca is a longitudinal opening in the mid-line between 

 the pelvic fins, and a pair of abdominal pores may be seen near 

 its hind margin. 



Sensory tubes or mucous canals ,and their apertures are 

 plainly visible on the under surface. The canals, which radiate 

 over the pectoral fin from a region near the anterior pair of 

 gills, are usually prominent, and others may be traced between 

 the mouth and the rostrum. 



The male is distinguished externally by bearing distinct 

 processes of the internal borders of the pelvic fins. These are 



