298 CHORD ATA. 



end. It is flattened laterally, and the whole body is piano- 

 symmetric. It is of a milky white, semi-transparent 

 appearance, and a number of the organs may be seen 

 through the skin in the living animal. 



Amphioxus lives in moderate depths near the sandy 

 bottom. It may swim about actively or may lie on one 



_„. ,. side upon the sand, or on occasion it may 

 bury all but the anterior part of its body m 

 the sand and there remain in a resting condition. 



There are no definite external divisions of the body, but 

 the anterior part, to about the level of the mouth, is some- 

 times termed the head and the posterior quarter of the 

 body is often referred to as the tail. 



The anterior end forms a snout or rostrum, just below 



which is the mouth, surrounded by a ring of oral cirri or 



External tentacles. Along the mid-dorsal line is a 



Feat re "^^dian unpaired dorsal fin which is continuous 



behind with a caudal fin. The caudal fin is 



continued round the tip of the tail and forwards along 



the ventral surface for about a quarter of the length of the 



body as an anal fin. 



The tail of the animal runs symmetrically down the 

 centre of the caudal fin, hence Amphioxus is said to have 

 a protocercal tail. (See Pisces.) 



At the anterior termination of the anal fin there is a 

 median ventral aperture, the atriopore, and anterior to this, 

 as far forwards as the mouth, there is a pair of ventro-lateral 

 flaps of the body, called the metapleural folds. On the left 

 side of the body, at the base of the caudal fin, there opens 

 a minute aperture, the anus. 



The whole body is enveloped in a thin, transparent skin 

 _ formed of a single layer of ectodermal cells, 



which secrete on their outer surface a deli- 

 cate cuticle. Sensory cells are scattered throughout the skin. 



The mouth, surrounded by its oral cirri, leads into a 

 buccal cavity. The posterior wall of this cavity is formed 

 Alimentarv ^^ ^^ velum, a thin septum with a central • 



aperture leading into the pharynx. The apei:- 

 ture is surrounded by velar tentacles which protrude inwards. 

 The pharynx is a large, spacious chamber extending about 

 f of the length of the body. In general structure it 



