BODY CAVITY. 468 
anterior part of the pharynx. 
The water-current which enters the mouth is, as in, 
Tunicates, connected both with respiration and nutrition. 
The food particles, entangled in mucus, are said to pass 
backwards along the hyperpharyngeal groove; the water 
passes down the pharynx, through its numerous gill-slits 
to the atrial chamber, and so to the exterior by the single 
atriopore. In the larva the gill-slits are few in number, and 
open directly to the exterior; in the adult they are con- 
cealed by the atrial chamber, and have greatly increased in 
number ; there may be more than 100 pairs. The water- 
currents are kept up by the cilia, probably assisted by the 
transverse muscles. 
The first sign of the development of the atrial chamber is the appear- 
ance of two lateral folds on the body wall, which form the metapleural 
folds of the adult. On their inner apposed, but not united, surfaces, 
two ridges appear. These grow towards one another and unite, leaving 
only the atriopore open. Thus the floor of the atrial chamber (Fig. 
250, II.) is produced. The chamber, as first formed, is a tube with a 
very small lumen, but, secondarily, it becomes enlarged, and extending 
upwards and inwards, constricts the ccelom, until it comes almost to 
surround the gut. The atrium eventually becomes a cavity, crescent- 
shaped in cross-section, surrounding the pharynx and extending back- 
wards as a blind pouch on the right side of the intestine. At the same 
time, the metapleural folds increase in size until they assume the adult 
appearance (Fig. 250, III.). During these processes the originally few 
gill-slits have been increasing in number, both by the addition of new 
slits and by the division of those first formed. The division is effected 
by the downward growth of a secondary bar or tongue-bar in the 
middle of each slit. The primary bars differ from these tongue-bars 
in being split at their lower ends, in enclosing a coelomic space, and 
in some other respects. 
The pharynx opens into the intestinal region of the gut, 
which is straight and simple. Near its commencement a 
pouch-like “liver” or cecum (Fig. 249, C.) arises, and 
extends forwards on the right side of the pharynx. The 
anus is some distance from the end of the body (cf. Fishes) ; 
in the larva it is close to the caudal fin. 
Body cavity.—This can only be understood when its development 
is studied (see Fig. 254). It is a fine example of what is called the 
enterocelic mode of origin. From the archenteron of the embryo a 
hollow ridge grows out on each side, and becomes almost at once 
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