THE ANIMAL KINGDOM 447 



(Fig. 267). The tip of the tongue is also provided with similar 

 teeth. These animals attach themselves to fishes; and feed by 

 sucking the blood and shredded tissues from the rasped sur- 

 faces. There are seven pairs of gill pouches, opening separately 

 on the outside and communicating internally with a median 

 respiratory canal, located ventral to the esophagus and opening 

 from the mouth in front. When the animal is attached to prey 

 or to a stone, both inspiration and expiration of water take 

 place through the external gill openings. 



Example: Petromyzon marinus, the sea lamprey, reaches a 

 length of 3 ft. and is found in the North Atlantic and also in 

 fresh water. 



CLASS II. ELASMOBRANCHII. Sharks and rays. These 

 have a cartilaginous skeleton, in which the notochord is par- 

 tially replaced by the centra of the vertebrae; a skull with 

 jaws; a ventral subterminal mouth and ventral paired nostrils; 

 paired and median fins; a spiral valve in the intestine; and a 

 heterocercal tail (one in which the axis of the tail curves dorsally). 

 The skin is invested with dermal denticles or scales of the 

 placoid type. On the margins of the jaws the scales are 

 enlarged into teeth. The placoid scale is thought to be the 

 forerunner of the vertebrate tooth. The first gill cleft or 

 spiracle has only rudimentary gill filaments and with the mouth 

 serves as an incurrent respiratory passage. An operculum 

 covering the gill slits is usually lacking. There is no swim 

 bladder. All are marine. 



Examples : Mustelus canis, a dogfish shark, 2 to 3 ft. in length ; 

 Carcharias taurus, the sandshark, 10 to 12 ft. long; Pristis 

 pectinatis, the sawfish of Florida and the Gulf of Mexico, 16 to 

 18 ft. long; Raji erinacea, a common ray or skate, 1 to 2 ft. 

 long. Rays and skates have gill slits on the ventral surface 

 of the body. The spiracle or first gill cleft through which water 

 enters is located on the dorsal surface just back of the eye. 

 Torpedo marmorata, an electric ray. 



CLASS III. PISCES. True fishes. These have a more or 

 less ossified skeleton; a skull with membrane bones, and usually 

 distinct maxillary and premaxillary bones; paired nostrils; 

 median fins; and usually two sets of paired fins. 



SUBCLASS 1. TELEOSTOMI. Cartilaginous or bony 

 skeleton. Breathe principally by gills, which are usually 



