404 UROCHORDA OR TUNIC ATA. 



Alimentary System. 



The mouth is surrounded by eight rounded lobes, between 

 which are pigment spots. It leads into a large pharynx, or 

 branchial sac, the walls of which are perforated by numerous 

 gill slits. At the beginning of this pharynx there is a circle 

 of tentacles, which point downwards from the posterior 

 edge of the sphincter muscle. At its base in the dorsal 

 middle line the pharynx opens into a short, curved, ciliated 

 oesophagus, which leads to the large stomach, placed, like 

 the rest of the gut, on the left side of the body. The 

 intestine describes an S-shaped curve, and terminates in a 

 cloacal chamber below the exhalent opening. Its wall is 

 folded inwards as in the earthworm. With the stomach a 

 mass of ramifying tubules is associated, which opens by 

 means of a duct into the cavity of the stomach, and may be 

 a digestive gland. 



In feeding, water is drawn into the mouth by the action 

 of the numerous small cilia which border the pharyngeal 

 slits. This water contains small algae and other organisms. 

 These are glued together by a mucus secretion, and swept 

 backwards to the oesophagus at the base of the pharynx, 

 while the water passes through the slits into the peribranchial 

 chamber. This lies between the body wall and the outer 

 wall of the pharynx, and communicates with the exterior by 

 the exhalent aperture. 



On the internal ventral surface of the pharynx (the side 

 on which the ganglion lies being morphologically dorsal) there 

 is a longitudinal groove the endostyle. This is furnished 

 with very long cilia, and is in part a glandular region. It 

 may secrete part of the mucus mentioned above. On the 

 dorsal surface of the pharynx there is a projecting ciliated 

 fold, called the dorsal lamina. In many Ascidians this is 

 broken up into a series of processes, the languets. 

 Herdman considers that, as few of the endostyle cells are 

 glandular, much of the mucus must be secreted elsewhere, 

 and the food particles probably pass backwards, both on 

 the ventral and dorsal surfaces. 



The ventral endostyle is morphologically comparable to the ventral 

 portion of the respiratory pharynx in Balanoglosstis, and to the "hypo- 

 branchial groove " of Amphioxus. It may even be compared to the 

 thyroid gland of Vertebrates, for that organ in the Ammocoete larva of 



