290 CLASSIFICATION OF ANIMALS 



fins are all dermal expansions supported on cartilaginous rays 

 and bony spines. Fishes are all aquatic and respiration is by 

 means of gills. There are often accessory respiratory organs — 

 the swim bladder and true lungs. The swim bladder is an 

 unpaired sack filled with air which serves to give the body of 

 the fish the same specific gravity as the water. The heart 

 consists of three chambers, a thin-walled auricle opening ante- 

 riorly into a very strong thick-walled ventricle which in turn is 

 continued anteriorly by the conus arteriosus. In the bony 

 fishes the latter is wanting and in its place the truncus arteriosus 

 is enlarged into a bulbus arteriosus. The vessels have prac- 

 tically the same arrangement as in the lancelet. The organs of 

 special sense are tw^o nasal chambers which do not communicate 

 with the mouth but have each two openings on the surface, one 

 incurrent, one excurrent orifice; two eyes; two statocysts with 

 utriculus and sacculus and three semicircular canals; and the 

 lateral line system. The lateral line organs line depressions or 

 tubes which communicate with the surface. These organs are 

 arranged in a line along each side of the body and other shorter 

 lines along the side and over the dorsal surface of the head. 

 The sensory cells are clustered and their sensory bristles 

 project into the canal of the lateral line. The function is to 

 sense the currents in the water. The cerebral hemispheres are 

 small, the cerebellum comparatively well developed. 



629. Order i. — The Selachii are the sharks and rays; fishes with a 

 cartilaginous skeleton, placoid scales, 5-7 gill clefts with separate openings, 

 a spiral valve, and upper jaw not united with the skull. The body is 

 spindle shaped in the sharks and flattened dorso-ventrally in the rays. 

 The spiral valve is a much enlarged posterior portion of the intestine with 

 a spiral shelf-like fold projecting inward from the wall. These fishes are 

 chiefly marine. 



630. Order 2. — The Holocephali are not numerous in point of genera. 

 They^have a cartilaginous skeleton with the upper jaw articulated with 

 the skull. The notochord persists and the vertebrae are represented only 



