434 THE DOGFISH CHAP. 



fied tissue harder than bone known as dentine, capped with 

 a still harder tissue called enamel which projects through 

 the epiderm and gives a rough, sandpaper-like character to 

 the skin. These placoid scales or dermal teeth together con- 

 stitute the exoskeleton of the dogfish : it is a discontinuous, 

 mainly dermal, exoskeleton (p. 445), and not a continuous 

 cuticular one like that of the crayfish. 



Beneath the derm is the muscular layer, which, as in 

 Amphioxus and in the tail of the tadpole, is metamerically 

 segmented. The muscles are divided into myomeres, follow- 

 ing one another from before backwards, and having a zigzag 

 disposition. The fibres composing them are longitudinal, 

 and are inserted at either end into fibrous partitions or 

 myocommas which separate the myomeres from one 

 another. 1 The muscular layer is of great thickness, espe- 

 cially its dorsal portion. The fibres of all the body-muscles 

 are, as in the frog and Vertebrates generally, of the striped 

 kind. 



There is a large c&lome (Figs, in and 117), which, as in 

 other Vertebrates, is confined to the trunk. The cavity is 

 divisible into two parts : a large abdominal cavity, containing 

 most of the viscera, and a small anterior and ventral com- 

 partment, the pericardia I cavity (Fig. \\>], pcd. cav\ contain- 

 ing the heart and communicating with the abdominal cavity 

 by a canal which opens on the ventral surface of the gullet. 

 Both are lined by ccelomic epithelium (Fig, in, Cffl. 

 Epthm\ underlain bv a layer of connective-tissue, a strong 

 lining membrane being thus produced, called, as in the 

 frog, peritoneum in the abdominal, pericardium in the peri- 

 card ial cavity. 



Another very characteristic Vertebrate feature is that the 



1 In the adult frog a segmentation can still be seen in the rectus 

 muscle of the abdomen (Figs. 2 and 16). 



