142 ORGANIZATION OF THE HEAD v. 4- 



as in a carpet, giving a tissue of great strength and flexibility, able to 

 maintain the shape of the body. Scattered over the skin are the charac- 

 teristic denticles or placoid scales (Fig. 95). Each of these consists of a 

 pulp cavity, around the edge of which lies a layer of odontoblasts 

 secreting the calcareous matter of the scale, known as dentine. This 

 has a characteristic structure resulting from the fact that the odonto- 

 blasts send fine processes throughout its substance. The outside of the 

 dentine is covered by a layer of enamel, secreted by the overlying 

 ectoderm. Usually the denticles pierce through the ectoderm, after 

 which no further enamel can be added to their surface. Obviously the 

 scales are similar to teeth, which are indeed to be considered as 

 specialized denticles developed on the skin of the jaws. It has often 

 been supposed that the denticle is the primitive type of fish scale, 

 from which others have been derived, but it now seems more likely 

 that the earliest covering was a continuous layer, later broken into 

 large scales, from which the denticle was ultimately derived (p. 269). 

 The skin also gives protection to the fish by its colour, produced 

 by a layer of chromatophores beneath the epidermis. Many sharks 

 have a spotted or wavy pattern, which breaks up their visible outline 

 as they move in the water, especially near the surface. They are able 

 to change their colour, though only slowly, becoming darker on a dark 

 background (see p. 164). 



5. The skull and branchial arches 



In general organization a dogfish follows closely the fish plan, 

 which we have already considered. Most of its special new features 

 are in the head, and we may now turn to a consideration of the 

 organization of the head and jaws of a gnathostome vertebrate. The 

 jawless vertebrates of the Silurian and Devonian included fresh- 

 water animals of various sorts, but the vertebrate type began to 

 flourish and increase more abundantly with the appearance of creatures 

 with jaws in the late Silurian. From this stage onwards we have to 

 follow the parallel history of numerous orders and families, as the 

 vertebrate plan of structure became adapted for various habitats. It 

 seems likely that the development of a biting mouth greatly increased 

 the range of possibilities of vertebrate life. The most obvious use of 

 a mouth is for attacking other animals, but it may also have been used 

 to collect plant food from all sorts of situations where it would not 

 be available to the microphagous or shovelling Agnatha. Probably the 

 mouth was also early used for defence, and in this way influenced the 

 whole bodily organization, making unnecessary the heavy armature 



