372 THE DOGFISH less, xxix 



meet, for the first time in our present subject, with distinct 

 metameric segmentation. The muscles are divided into 

 segments or myomeres (Fig. 98, mym) 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 {myc), 

 which separate the myomeres from one another. The mus- 

 cular layer is of great thickness, especially its dorsal portion 

 (Fig. 99, c). The fibres of all the body muscles are of the 

 striped kind. 



There is a large ccelome (Fig. 99, Ccel), remarkable for being 

 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 {Fed), 

 containing the heart. Both are lined by ccelomic epithelium 

 {Coel. Epthm), underlain by a layer of connective tissue, 

 a strong lining membrane being thus produced, called /m- 

 toneum in the ahdormndX, pericardiutn in the pericardial cavity. 



Another very characteristic feature is that the dorsal body- 

 wall is tunnelled, from end to end, by a median longitudinal 

 neural cavity, in which the central nervous system is con- 

 tained. The greater part of the cavity is narrow and cylin- 

 drical, and contains the spinal cord : its anterior or cerebral 

 portion is dilated, and contains the brain. 



Imbedded in the body-wall and extending into the fins 

 are the various parts of the endoskeleton. This characteristic 

 supporting framework is formed of a tough elastic tissue 

 called cartilage or gristle, more or less impregnated with 

 lime salts, so as to have, in part, the appearance of bone. 

 As, however, the hard parts of the dogfish's skeleton have a 

 different microscopic structure from the bones of the higher 

 vertebrates, they are best described as calcified cartilage. 



