174 AMPHIOXUS 



posterior ends, is a longitudinal series of connective- 

 tissue compartments filled with a gelatinous sub- 

 stance. The cordpartments are more than twice as 

 numerous as the segments of the body, and their 

 function is uncertain. A similar 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 tube surrounds the central nervous 



system, and is continuous below with the noto- 

 ehordal 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 

 one another ; 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 in pairs, but alternate 

 with one another. 



B. The Muscular System. 



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

 muscular system. They embrace the sides 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 



