316 MANUAL OF ZOOLOGY SECT. 



It has been already mentioned that the atrial cavity does not com- 

 pletely surround the pharynx on one side. This is owing to the fact that 

 on th side in question, which is ventral in position, the wall of the 

 pharynx is united with the mantle along the middle line. Along the 

 line of adhesion the inner surface of the pharynx presents a thickening 

 in the form of a pair of longitudinal folds separated by a groove. To 

 this structure, consisting of the two ventral longitudinal folds with the 

 groove between them, the term endostyle is applied (Fig. 185, end.}. 

 The cells covering the endostyle are large cells of two kinds ciliated 

 cells and gland cells the former beset at their free ends with cilia, the 

 action of which is to drive floating particles that come within their 

 influence outwards towards the oral aperture, the latter secreting and 

 discharging a viscid mucous matter. Anteriorly the endostyle is con- 

 tinuous with a ciliated ridge which runs circularjy round the anterior 

 end of the pharynx ; in front of this circular ridge, and running parallel 

 with it, separated from it only by a narrow groove, is another ridge of 

 similar character r these are termed the peripharyngeal ridges and the 

 groove between them is \h& peripharyngeal grcove. Dorsally, i.e. oppo- 

 site the endostyle, the posterior peripharyngeal ridge passes into a 

 median, much more prominent, longitudinal ridge, the dorsal lamina, 

 which runs along the middle of the dorsal surface of the pharynx 

 to the opening of the oesophagus. The mucus secreted by the gland 

 cells of the endostyle forms viscid threads which entangle food -particles- 

 (microscopic organisms of various kinds) ; the cilia of its ciliated cells 

 drive these forwards to the peribranchial groove, around which they 

 pass to the dorsal lamina, and the cilia of the cells of the latter 

 drive them backwards to the opening of the oesophagus. 



Some little distance in front of the anterior peripharyngeal ridge, at 

 the inner or posterior end of the oral siphon, is a circlet of delicate 

 tentacles (Fig. 186, tent.}. 



The cesophagus leads from the pharynx (near the posterior end of 

 the dorsal lamina) to the stomach, which, together with the intestine, 

 lies embedded in the mantle on the left-hand side. The stomach is a 

 large fusiform sac. The intestine is bent round into a double loop, and 

 runs forwards to terminate in an anal aperture situated in the atrial 

 cavity. There is no liver ; but the walls of the stomach are glandular, 

 and a system of delicate tubules which ramify over the wall of the 

 intestine is supposed to be of the nature of a digestive gland. 



The Ascidian has a well-developed blood system. The heart is a 



