THE DOGFISH 25 



called myotomes or myomeres, which are separated from one an- 

 other by connective-tissue septa called myocommas. Each myo- 

 tome forms an irregular plate and is composed of parallel muscle 

 fibers which will be seen running across from one myocomma to 

 the other. The fibers are thus not bound together by fasciae into 

 distinct muscles, as is the case in the higher vertebrates. Note the 

 zigzag shape of the outer edge of the myotome between the dorsal 

 and ventral sides of the body. 



Exercise 28. Make an outline drawing of the side of the body, show- 

 ing a few of the myotomes, with their fibers, and the myocommas. 



Cut the tail off a short distance behind the anus and examine 

 the cut surface. The myotomes appear here in groups of concen- 

 tric circles. This appearance is partly due to the zigzag shape of 

 the myotomes. Cut horizontal and dorsoventral sections of the 

 muscles and determine the exact shape and arrangement of the 

 myotomes. 



Observe the other structures in the cross section. In the center 

 of it is the spinal column. The neural arch, within which is the 

 spinal cord, forms its dorsal portion, and the haemal arch, contain- 

 ing the caudal artery and the caudal vein, forms its ventral por- 

 tion ; the vein is beneath the artery. 

 Exercise 29, Draw the cross section, and label all these features. 



The Skeletal System. The skeleton of the dogfish is made up of 

 two distinct parts, the exoskeleton and the endoskeleton. The 

 former is of integumental origin and consists of the placoid scales, 

 the teeth, and the delicate horny rods in the fins. Each placoid 

 scale consists of a basal plate of bone, which is sunk in the dermis 

 of the skin, and a flattened toothlike spine, which projects back- 

 ward through the epidermis. The basal plates are set close to- 

 gether so that the entire body of the animal, including the fins, 

 is covered with a coat of mail of bone. The scales pass over the 

 lips into the mouth and are continuous with the teeth, which have 

 essentially the same structure as the spines of placoid scales. 



Placoid scales have furnished the beginnings, throughout the 

 group of vertebrates, not only of teeth but also of certain of the 

 bones of the skull. Dogfish and their allies have no bony skull ; but 



