THE PERCH 199 



consist of a black central kernel — the body of the cell — sur- 

 rounded by a halo of fine dots, which constitute its outlying pro- 

 jections. Many fishes have the power of changing their color in 

 a remarkable degree, although it is probably a reflex action in 

 them and not under the control of the will. It is accomplished 

 by the often very rapid variation in the extent of these pigment 

 cells, which in such cases are amoeboid. 



The body may be divided into three regions, the head, trunk, 

 and tail, the boundary between the latter two regions being the 

 anus. There is no neck. 



Vertebrates which live in the water differ much from those 

 living on the land in the arrangement of the body regions. Water 

 animals must force their way through a dense medium, and 

 hence the forward portion of the body is rigid and usually more 

 or less wedge-shaped. A neck region is thus absent, since a neck 

 region is essentially flexible. Even in those mammals which have 

 adapted themselves to a wholly aquatic life, as the cetaceans, 

 the neck region is so much reduced that the head and trunk are 

 in direct contact with each other. In every animal which moves 

 rapidly, however, at least one flexible body region must be pres- 

 ent, where the body can turn when the direction of movement 

 is to be changed. In the fish this is accomplished in the caudal 

 region ; in most mammals it is in the lumbar region. 



The Head. The head of fishes differs from that of land verte- 

 brates in that it contains the organs of respiration and the heart. 

 The head of the perch is flattened ventrally and dorsally, with 

 the large mouth at its anterior and the gills at its posterior end. 

 The opening of the mouth is bounded ventrally by the paired 

 mandibles, and dorsally by the paired premaxillae, above which 

 on each side is the flattened maxilla. The large eyes are without 

 Hds. A transparent membrane called the conjunctiva passes over 

 the front of the eye and is continuous with the epidermal layer 

 of the skin ; a deep fold of the skin is also present around the eye, 

 joining it with the skin of the head, and yet permitting it con- 

 siderable freedom of motion in its socket. 



In front of the eyes are two pairs of nostrils ; there is, however, 

 but a single pair of nasal capsules, each capsule having two ex- 



