290 CHORDATA. 
test is a single-layered ectoderm,* covering fairly well- 
developed J/ongitudinal and circular layers of muscles. The 
test may therefore be regarded as a modified and thickened 
form of cuticle produced from the ectoderm. 
On cutting open the body-wall the course of the alimen- 
tary canal can be made out. The mouth leads into a buccal 
sigs cavity, which is short, and expands into the enor- 
entary. 3 
mous pharynx. Between the two is a row of 
small zentacles. The pharynx extends nearly throughout the 
length of the body and forms a large sac, the lateral walls of 
which are perforated by rows of innumerable small slits, or 
stigmata. 
These are evidently clefts in the side-walls of the pharynx, but are 
not exactly the same as the pharyngeal clefts of the Chordata. They 
are produced from the less numerous true pharyngeal clefts of the larva 
by secondary division of the latter. 
The pharynx is surrounded on all sides except the mid- 
ventral line by the atrium, a large spacious cavity into which 
open the stigmata. It leads to the exterior by the atriopore. 
Along the mid-ventral line of the pharynx is a grooved 
ridge, the endostyle, formed of ciliated and glandular cells. 
At the oral end of the pharynx it is continuous with the 
peripharyngeal grooves, which pass up each side of the 
pharynx just behind the tentacles. The two peripharyngeal 
grooves meet in the mid-dorsal line, and are produced back- 
wards along the mid-dorsal line of the pharynx as the ef7- 
branchial groove, the edges of which hang down as the dorsal 
lamina. This groove terminates at the dorsal posterior corner 
of the pharynx, where a small wsophagus leads into a sac-like 
stomach. ‘This is continued by a bent dzdestine to the anus, 
opening into the atrium. The greater part of the alimentary 
canal is ciliated. 
The outstanding feature of this system is the pharynx, with 
its numerous clefts and its system of grooves. The endostyle 
secretes mucus, which is driven forward by the ciliated cells, 
up the peripharyngeal grooves and back along the epi- 
branchial groove. The mucus strands appear to form a 
complex meshwork of glutinous threads hanging across the 
cavity of the pharynx, the ultimate fate of which is to be 
carried into the stomach through the cesophagus. The cilia 
* Often termed the Mantle. 
