AMPHIOXUS. 



301 



arranged in V-shaped bars. These characteristic Vs can be 

 seen through the skin in the living animal. Contraction of 

 the myomeric muscles moyes the tail from side to side, driving 

 the animal forwards. The ventral longitudinal muscles ex- 

 tend from the region of the mouth to the atrium. 



The most important skeletal organ is the notochord. It 



extends as a long cylindrical elastic rod from one end of the 



Sk 1 tai t)ody to the other. Hence at the anterior end 



it passes forwards to the tip of the rostrum, in 



front of the brain. It consists of chordoid tissue and is 



enveloped in a mesoblastic sheath. Amphioxus burrows 



Fig. 215. — Transverse Section of Amphioxus behind 

 THE Atrium. {Ad nat.) 



Dorsal Aorta. 



■ Perivisceral 



Coelom. 



Subintestinal 

 Vein. 

 Anal Fin 

 (Skeleton). 



The dark shading is the connective tissue and the light outside is the simple ecto- 

 derm. The section is taken across E F in Fig. 211. 



with its rostrum and the notochord apparently gives it 

 the necessary solidity. It also assists the motor muscles 

 by its elasticity. 



Around the notochord and nervous system and between 

 the myomeric muscles is a continuous mass of mesoblastic 

 connective-tissue, which at the bases of the dorsal and anal 

 fins forms a row oi fin-rays. 



