434 THE DOGFISH CHAP. 



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 constitute the 

 exoskeleton of the dogfish : it is a discontinuous, mainly 

 dermal, exoskeleton (compare p. 445), and not a 

 continuous cuticular one like that of the crayfish 



(P- 369)- 



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, 

 following 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, especially 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 ccdome (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 compartment, the pericardial cavity 

 (Fig. 117, pcd. cav), containing the heart and communi- 

 cating with the abdominal cavity by a canal which opens 

 on the ventral surface of the gullet. Both are lined by 

 coelomic epithelium (Fig. in, Ccel. Epthm] underlain by 

 a layer of connective-tissue, a strong lining membrane 

 being thus produced, called, as in the frog, perito- 

 neum in the abdominal, pericardium in the pericardial 

 cavity. 



Another very characteristic Vertebrate feature is that 



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

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



