34 VERTEBRATE ZOOLOGY 



black central kernel,— the body of the cell, — surrounded by a halo 

 of fine dots which constitute its outlying projections. Many fishes 

 have the power of changing their color in a remarkable degree, 

 although it is probably a reflex action 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, it being essentially 

 a flexible one. Even in those mammals which have adapted them- 

 selves 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 present 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 teleost fishes differs from that of land 

 vertebrates in that it contains the organs of respiration and the 

 heart. The head 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 lids. 

 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 considerable 

 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 external 

 openings. Note the difference in shape of these two openings on 



