290 CHORDA r A. 



test is a single-layered ectoderm ^^ covering fairly well- 

 developed lo?igitiidinal 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 

 All t catnty^ which is short, and expands into the enor- 

 mous pharynx. Between the two is a row of 

 small tentacles. 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 Choniata. 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 epi- 

 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 oesophagus leads into a sac-like 

 stomach. This is continued by a bent intestine 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 

 -^arried into the stomach through the oesophagus. The cilia 



* Often termed the Mantle. 



