52 VERTEBRATE ZOOLOGY 



The Arteries (Continued). We shall now study the efferent 

 branchial arteries and the dorsal aorta and its branches. The af- 

 ferent arteries have already been seen; the efferent arteries lie 

 immediately posterior to them on the gill arches. Make a cross 

 section of a gill arch and note these two arteries ; the afferent is 

 the larger of the two. Remove the ventral wall of the pharynx 

 and the heart. Dissect away the mucous membrane which covers 

 the roof of the mouth and pharynx. Trace the efferent arteries 

 dorsally to the dorsal aorta. Follow the aorta and the arteries 

 which branch from it, as already explained (see page 49). 



Exercise 22. Draw a diagram showing the efferent branchial arteries 

 and the dorsal aorta and its branches, so far as observed. Care- 

 fully label all. 



Exercise 23. Draw a diagram of the entire vascular system. 



The Body Muscles. Skin the side of the body. Note the great 

 muscle which forms the entire side and extends from the head 

 to the caudal fin. This complex muscle is made up of a succes- 

 sion of muscle segments called myotomes or myomeres, which are 

 separated from one another by connective-tissue septa called myo- 

 commas. Each myotome is a plate, or lamella, between two myo- 

 commas, and consists of parallel fibers which run across from one 

 myocomma to the other. Thus the fibers are not bound together 

 by fasciae as in the muscles of the higher vertebrates. Note the 

 zigzag shape of the edge of the myotome, which appears on the 

 surface of the body ; note also that each myotome may be divided 

 into a dorsal and a ventral half. 



Exercise 24. Make an outline of the fish and in it draw a few of the 

 myotomes and myocommas. 



Cut the tail off an inch or two behind the anus and study the 

 cut surface. The myotomes appear here in groups of concentric 

 circles. This appearance is due partly to the fact that the inner 

 edge of each myotome, which is attached to the spinal column 

 and to its dorsal and ventral processes, is much anterior in position 

 to the outer edge which appears on the surface, and also partly to 

 the zigzag shape of the myotomes. Cut horizontal and dorsoven- 

 tral sections of the muscles and prove this. 



