270 



CHARACTERS OF VERTEBRATE ANIMALS 



means of the powerful lateral strokes of the large tail-fin, while 

 the remaining fins act as balancers and steerers. The fairly 

 large mouth, its jaws supported by bones bearing small teeth, 

 is placed at the end of the pointed head, and above it may be 

 seen two external nostrils, one on either side of the snout. 

 Farther back still come the large, round, expressionless eyes, 



DORSAL FINS 



OLFACTORY LOBE 

 OLFACTORY 

 NERVE 



LATERAL LINE 



GILLS 



VENTRAL AORTA 

 BULBUS ARTERIOSUS 

 ENTRICLE 



INUS VENOSUS } HEART 



AURICLE 



fYLORIC CAECA 



Fig. 163. Side-dissection of Perch (Percajluviatilis) 



devoid of eyelids, but there are no spiracles behind them. No 

 gill-slits are externally visible, but these may be seen by lifting 

 up the firm gill-cover or operculum placed at the side of the 

 head as in a Ganoid. Four such slits are present, instead of 

 five as in the Dog- Fish (see p. 258), with four narrow bar-like 

 gill-arches bearing the red, comb-like gills, the separate teeth 

 of the combs being the gill-filaments. The gill-openings here 

 are really slit-like, and not pouches as in Dog- Fish, and their 

 existence is practically demonstrated by every school-boy who 

 strings his small catches on a piece of grass, which he threads 

 through the mouth and gill -slits under the gill-cover to the 

 exterior. It is further of great interest to notice that on the 

 inner side of the gill-cover, close to the first gill-slit, there is 

 a small reddish projection. This false gill (pseudobranch) 

 is the vestige of the first gill present in a Dog- Fish, and must 

 not be confused with the false gill of that animal (see p. 263), 

 which is on the front wall of the spiracular cleft, here entirely 

 absent. 



The unpaired fins consist of two large dorsals, of which the 



