PISCES 



167 



Coelom. The skeletal muscular system is derived from the 

 upper division or myocoel of the coelomic mesoderm. This 

 part of the coelom, as in Amphioxus, is resolved into a series of 

 myotomes, the cavities of which open into the general body 

 cavity, or splanchnocoel, below them. As they are cut off 

 in succession from the splanchnocoel the myotomes spread 

 around the body wall outside the splanchnocoel, and their 

 inner layer yields the muscle fibres, while the outer layer is 

 converted into dermic mesoderm. The muscles related to the 

 skeleton of the body and limbs are thus originated but undergo 

 much modification in the body. It is only in the tail that the 

 myotomes retain the pri- 

 mitive meristic segmen- 

 tation. In the tail, more- 

 over, some of the fibres are 

 shortened and broadened 

 to form together an electric 

 organ which has the ap- 

 pearance of a gelatinous 

 mass embedded in the tail 



Coelom 



Myocoel 

 Notochord 

 Sclerotome 

 Aorta 

 Pronephric tubule 



FIG. 85. Raia. Diagrammatic transverse 

 section of embryo illustrating fate of 

 coelom. 



-Splanchnocoel 

 -Intestine 

 -Subintestinal vein 



muscles. In the rays it is 

 feebly developed, but in the 

 skate it is more prominent. 

 The torpedo ray possesses 

 a large and powerful pair of electric organs on each side of the 

 head from a similar modification of muscle fibres derived from 

 the muscles of the visceral arches. In other fishes, still other 

 muscles have been modified to produce electric organs as a 

 means of protection. Muscle fibres normally during contraction 

 give rise to an electric current, and these fish have evidently 

 found it possible to so modify the muscle as to produce an 

 intensified current without contraction. 



The body cavity, derived as has been said from the 

 splanchnocoel division of the coelom, is in Craniata not seg- 

 mented. At first it forms a continuous cavity, but it becomes 

 divided into a pericardial cavity and a peritoneal cavity which 

 communicate by a small canal. In the peritoneal cavity, 

 the mesentery is at first continuous, but it becomes absorbed 

 except in the gastric and the rectal region. The gonads are 



