298 CHORDATA. 
end. It is flattened laterally, and the whole body is plano- 
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 in 
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 ead and the posterior quarter of the 
body is often referred to as the Zazv. 
The anterior end forms a snout or rostrum, just below 
which is the mouth, surrounded by a ring of oval cirri or 
External ‘emtacles. Along the mid-dorsal line is a 
Seuiancs median 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 a¢riopore, 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 azws. 
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 
by the velum, a thin septum with a central 
Beene snedinee leading into the pene The aper- 
ture is surrounded by velar tentacles which protrude inwards. 
The pharynx is a large, spacious chamber extending about 
2 of the length of the body. In general structure it 
Habits. 
Integumentary. 
