172 AMPHIOXUS 



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- 

 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 

 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 contraction of these muscles 

 on opposite sides of the body, aided by the elasticity 

 of the notochord, that the lateral undulatory move- 

 ments of the animal when swimming are produced. 



2. The ventral muscles form a sheet covering the ventral 



surface of the body from the mouth to the atrial pore. 

 The muscle-fibres run transversely from side to side, 

 and by their contraction aid in expelling the water 

 from the atrial cavity. 



