576 The Animal Kingdom 



a single ventricle. Only venous blood is present in the heart. There 

 are from five to seven pairs of gills, each in a separate pocket. These 

 open to the outside by means of uncovered gill slits. The olfactory sac 

 is not connected with the mouth, and the intestine has a spiral valve 

 which increases the digestive surface. The sexes are separate, fertiliza- 

 tion is internal, and the eggs are provided with large amounts of yolk. 

 Most species are ovoviviparous, with the ^gg being retained in the 

 modified oviduct; others are oviparous. In all cases, development is 

 direct without intervening larval stages. 



The Classification of the Class Chondrichthyes. — The mem- 

 bers of this comparatively uniform group are divided into two sub- 

 classes and several orders as follows : 



Subclass 1. Elasmobranchii. Fish with numerous placoid scales, gills not 

 covered with an operculum, with paired olfactory sacs, spiracle and cloaca. 



Order 1. Selachii. With spindle-shaped bodies, five to seven lateral gill 

 slits. Examples: dogfish shark, Squalus; the hammerhead, Sphyrna; 

 whale shark, Rhincodon ; white shark, Carcharodon; and the basking 

 shark, Cetorhinus. 



Order 2. Batoidea. Body typically flattened, with large pectoral fins, 

 five pairs of ventral gill slits. Examples : common ray. Raja; the 

 electric ray, Torpedo; the sawfish, Pristis; manta or devil ray, Mania; 

 and the stingray, Dasyatis. 



Subclass 2. Holocephali. Fish with but one olfactory sac, gill slits covered 

 by operculum, no spiracle or cloaca present, with but few scales. 



Order 1. Chimaerae. With characters as above. Example : chimaeras, 

 Chimaera. 



The Anatomy of Squalus acanthias, the Dogfish Shark. — This 

 abundant, small shark is found along the coastal waters of the North 

 Atlantic. The adult attains a length of about 2^ feet when mature 

 and has a gray colored, spindle-shaped body. Covering the surface of 

 the body are many small placoid scales whose projecting spines give a 

 roughened texture. 



The head is pointed anteriorly and flattened above ; located on the 

 ventral surface is the transverse slitlike mouth which is well supplied 

 with teeth. On either side of the head is a large eye and anterior to 

 the mouth on the ventral surface are the paired olfactory organs. There 

 are paired pectoral and pelvic fins; the pectoral fins of the male possess 

 cartilaginous claspers which are used as intermittent organs during 



