XII 



PHYLUM CHORDATA 



395 



sion of an operculum or gill-cover, by the absence of a 

 cloaca, by having the primary skull and shoulder-girdle 

 complicated by the addition of membrane-bones, and by 

 possessing bony instead of horn-like fin-rays. 



A typical Teleostomian, such as a trout ( Fig. 235 ) ' or a 

 herring, has a long compressed body nearly half of which 

 is formed by the tail, pointed anterior and posterior ends, a 

 large vertical tail-fin, a head of moderate size, and a terminal 

 mouth. Such a form is eminently fitted for progression 

 through the water. But from this characteristic fish form 



Fig. 235. — Salmo fario. a. I, adipose lobe of pelvic fin; an, anus; c. /, caudal 

 fin; d.f. I, first dorsal; d /. 2, second dorsal or adipose fin; /. /, lateral line; 

 op, operculum ; pet. /, pectoral fin ; pv. f, pelvic fin ; v. f t ventral fin. (After 

 JardineJ 



there are many striking deviations. The body may be 

 greatly elongated and almost cylindrical, as in the eels ; or 

 of great length and strongly flattened from side to side, as 

 in the ribbon-fishes ; or the head may be of immense pro- 

 portional size and strongly depressed, as in certain shore- 

 fishes, such as the fishing-frog ; or, as in the beautiful reef- 

 fishes, the whole body may be as high as it is long. The 

 mouth sometimes has a ventral position, as in the Elasmo- 

 branchs, with the snout prolonged over it ; this is the case, 



1 Our common brook trout in the northeastern states is Salmo fontinalis. 



