4i8 THE DOGFISH chap. 



a still harder tissue call-ed 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. 429), 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- 

 mg 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.' 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 coelome (Figs. 102 and 108), which, as in 

 other Vertebrates, is confined to the trunk, both head and 

 tail being, in the adult, acoelomate. The cavity is divisible 

 into two parts : a large abdominal cavity, containing most of 

 the viscera, and a small anterior and ventral compartment, 

 the pericardial cavity (Fig. 108, pcd. cav), containing the 

 heart and communicating with the abdominal cavity by a 

 canal which opens on the ventral surface of the gullet. 

 Both are lined by coelomic epithelium (Fig. 102, Cxi. 

 Epthm) underlain by 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- 

 cardial 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 (Fig. 16). 



