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 moves 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 
a body to the other. Hence at the anterior end 
eletal. . : : 
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. (4d nat.) 
Dorsal Aorta. 
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. arz. 
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 of fiz-rays. 
