198 ZOOLOGY 



from the urinary and genital apertures ; and the brain has in 

 most cases no cerebral hemispheres, but an undivided prosence- 

 phalon with a non-nervous roof. 



1. Example of the Sub-class. — The Brown Trout 



{Salmo fario). 



The Brown Trout is common in the rivers and streams of Europe, 

 and has been acclimatised in other parts of the world, notably in 

 Australia and New Zealand. It varies greatly in size according to 

 the abundance of food and the extent of the water in which it lives : 

 it may attain sexual maturity, and therefore be looked upon as 

 adult, at a length of 18 — 20 cm. (seven or eight inches), but in 

 large lakes it may grow to nearly a metre in length. Other 

 species of Salmo, such as the Salmon {S. salar), the Lake Trout 

 (S.ferox), the American Brook Trout (S. fontinalis), are common 

 in the Northern Hemisphere and differ only in details from 

 S. fario. 



External Characters. — The body (Fig. 854) is elongated, com- 

 pressed, thickest in the middle, and tapering both to the head and 



Fig. So4. — Salmo fario. a. L adipose lobe of pelvic fin ; aa. anus ; c\ /. caudal fin ; d. f. 1, first 

 dorsal fin ; d. f. 2. second dorsal or adipose fin ; /. ^ lateral line ; op. operculum ; pet. f. pectorai 

 fin ; jjc. /; pelvic fin ; i-, /; ventral fin. (After Jardine.) 



tail. The mouth is terminal and very large : the upper jaw is 

 supported by two freely movable bones, the prcmaxilla (Fig. 855, 

 pmx.) in front and the maxilla (mx.) behind, both bearing sharp 

 curved teeth arranged in a single row. When the mouth is opened 

 a row oi palatine teeth is seen internal and parallel to those of the 

 maxilla, and in the middle line of the roof of the mouth is a double 

 row of vo7nerine teeth. The lower jaw {md.) is mainly supported by 

 a bone called the dentary and bears a row of teeth : on the throat 

 each ramus of the mandible is bounded mesially by a deep groove. 

 The floor of the mouth is produced into a prominent tongue (t.) 

 bearing a double row of teeth. In old males the apex of the lower 

 jaw becomes curved upwards like a hook. 



The large eyes have no eyelids, but the flat cornea is covered by 



