372 ZOOLOGY 



392. Classification of Fishes. 



Subclass I. Elasmobranchii (Sharks, Dog-fishes, Rays, Skates). Marine 

 fishes with essentially cartilaginous skeleton; no operculum or gill-cover; mouth on 

 the ventral surface of the head; heterocercal tail; external skeleton of placoid 

 scales; spiral valve in the intestine; no air bladder. The elasmobranchs are 

 regarded by some as being the nearest present relatives of the primitive fishes. 

 They occur most abundantly and are larger individually in warm seas. They are 

 powerful swimmers as befits carnivorous, preying animals. They feed on Crustacea, 

 Mollusca, and fish. 



Subclass II. Ganoidei (Ganoid Fishes: Sturgeon, Gar-pike). Fishes^with 

 bony cartilaginous skeleton; gills covered by an operculum; exoskeleton of ganoid 

 scales or enameled plates; air-bladder present; spiral intestinal valve; tail either 

 homo- or heterocercal. 



The group was very important in the early history of the earth, and the present 

 species are a mere remnant of the former glory of the ganoids. They occur now 



FIG. 187. 



FIG. 187. Long-eared Sunfish (Lpomis auritus). Adult. Photo from life by Dr. R. W. Shufeldt. 



chiefly in the rivers and lakes, though the sturgeon is also found in the sea. North 

 America is as well represented as any other region in the living species of this 

 remarkable group. 



Subclass III. Teleostei (Bony Fishes). Fishes with well-ossified skeletons; 

 body covered with cycloid or ctenoid scales; exoskeleton of bony plates in the head 

 region which become associated with bones of the internal skeleton to form the 

 skull; mouth terminal rather than ventral; gills covered; spiral valve lacking; air- 

 bladder usually present; homocercal tail. 



This subclass embraces the great majority of the forms ordinarily known as 

 fishes. There are estimated to be 6,000 or more species of teleosts, more than 2,000 

 of which inhabit fresh water. The group is variously divided by different authors 

 and the student must be referred to more advanced texts for fuller classification. 

 The principal orders are outlined below. 



Pneumatic duct (from air-bladder to intestine) open Order Physostomi. 



(Carp, cat-fish, sucker, salmon, trout, shad, herring, eel, etc.) 

 Pneumatic duct closed. 



Dorsal, anal, and pelvic fins spiny in front. 



Bones of the pharynx (branchial arches) distinct ... Order Acanthopteri. 

 (Perch, sunfish, mackerel, stickleback, silverside, etc.) 



