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 ccelome (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 pericardia! 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), 



