638 PHYLUM CHORDATA : CLASS PISCES — FISHES 



The sturgeons are a group of old-fashioned fishes, 

 found chiefly in the Northern Hemisphere both in the sea 

 and in rivers and lakes. Acipenser is found in British 

 seas and rivers (R. Severn). Scaphirhynchus is represented 

 in Asia and the United States. Polyodon is the spoonbill 

 sturgeon of the Mississippi. An allied genus, Psephiaus, 

 grows to an enormous size in the Jangtszekiang in 

 China. Sturgeons are the largest fishes inhabiting fresh 

 water. A. sturio may attain a length of 18 ft. and a weight 

 of 600 lb., while A. huso of South-Eastern Russia may 

 measure 25 ft. and weigh 3000 lb. The roes or ovaries 

 form caviare ; the gelatinous internal layer of the air- 

 bladder is used as isinglass. 



The skeleton is cartilaginous, the notochord being un- 



FiG. 365. — Sturgeon {Acipenser sturio). 



Note the elongated snout, the barbules bounding the ventral mouth, 

 the operculum covering the gills, the rows of bony scutes, the 

 markedly heterocercal tail. 



segmented. There are large bony plates developed in the 

 skin, forming, in Acipenser, five longitudinal rows. Similar 

 plates form a shield to the cranium. The mouth is 

 ventral, the tail heterocercal. The gills are covered by 

 an operculum, but there is a spiracle, a spiral valve in the 

 intestine, and a contractile conus arteriosus with valves. 

 The air-bladder is physostomatous. It will be noticed 

 that the sturgeons retain many Elasmobranch characters. 

 They &pawn in fresh water in early summer, producing 

 enormous numbers of eggs 2-3 mm. in diameter. 



Order 3. HoLOSTEi — with bony skeleton 



Living examples : — Lepidosteus and Amia. 



Extinct examples : — Lepidotus, Pycnodus, Aspidorhynchus. 



