SKELETAL AND MUSCULAR SYSTEMS. 191 



numerous as the segments of the body. A similar, 

 but double, structure extends along the ventral fin, 

 between the anus and the atrial pore. 



4. The branchial skeleton. A series of elastic rods are 



developed in the walls of the pharynx to support the 

 gill-arches. Their arrangement will be described 

 with the alimentary canal. 



5. The connective-tissue skeleton. The connective-tissue 



of Amphioxus is very dense, and forms a kind of 

 skeleton, of which the following are the chief parts : 



a. The sheath of the notochord is a thick sheath 



closely surrounding the notochord. 



b. The neural sheath surrounds the central nervous 



system, and is continuous below with the noto- 

 chordal sheath. 



c. The septa are sheets of connective tissue which 



run outwards from the notochordal and neural 

 sheaths to the integument. They separate the 

 successive muscle-segments, or myotomes, from 

 each other ; and their lines of insertion into the 

 integument form the > shaped markings along 

 the sides of the animal. The septa on the two 

 sides of the body are not paired, but alternate 

 with each other. 



C. The Muscular System. 



1. The lateral muscles are by far the largest part of the 

 muscular system. They form the side- walls of the 

 body along its whole length, and are divided, as 

 noticed above, into muscle-segments or myotomes 

 by the connective-tissue septa. The muscle-fibres 

 run longitudinally, i.e. parallel to the axis of the 

 body ; and it is by alternate contractions of these 

 muscles on opposite sides of the body, aided by 

 the elasticity of the notochord, that the lateral 

 undulatory swimming movements of the animal 

 are produced. 



