8o 



OUTLINES OF NATURAL HISTORY. 



head, and is armed with small teeth, which have a uniform 

 size, and are bent backwards. The throat is pierced with 

 a series of slits, which, as we shall see, allow the water to 

 reach the gills or breathing-organs. Behind the throat 

 the digestive canal consists of a gullet, stomach, and 

 intestine ; and the latter terminates in a distinct vent, 

 placed on the lower surface of the body. There is a large 

 and well-developed liver, and behind the stomach are 

 placed three closed tubes, which are believed to represent 

 another of the digestive glands (the sweetbread or pan- 

 creas). 



At the sides of the throat, on the under surface of the 

 head, are placed the breathing-organs and heart. The 

 former constitute the "gills," and are adapted solely for 

 breathing air dissolved in water, and not for breathing air 

 directly. Hence the fish dies if removed from its natural 

 element. The gills (fig. 36, A, h h) have the form of a 

 series of fringes, supported upon four boiiy arches (a), 



Fig. 36.— A, Gills and heart of the I'ertli exposed by the removal of the gill- 

 cover on the left side, a First of the four bony arches which carry the 

 gills (h b) ; V The lower edges of the gills on the right side ; h Heart (after 

 Van der Hoeven). B, Scale of the Perch, magnified. 



and having a bright red colour from the number of blood- 

 vessels distributed to them. A similar set of gills, sup- 

 ported upon four similar arches, is placed upon the oppo- 



