234 Introduction to A7iimal Alorphology. 



lose lining in Cynthia. On the wall of the stomach is 

 often a layer of yellow or green cells, some of which 

 project inwards (Appendicularia). A somewhat similar 

 coating of gland cells exists in Sidnyum ; these may 

 represent a liver. One or two gastric cseca in Salpa 

 are also supposed to be hepatic. The short caecal 

 tubes of Chelyosoma, the long follicles of Cynthia, the 

 separate lobular bodies of Phallusia intestinalis or 

 Boltenia, are more specialized forms of hepatic organ. 

 Amauroucium has a streak of glandular caeca on the 

 stomach w^all. The stomach is usually directed to- 

 wards the haemal surface, and ends in the intestine, 

 which passes neurad ; thus the digestive canal has 

 first a haemal and secondly a neural flexure, the latter 

 being the most conspicuous and constant, while the 

 former is due to a secondary spiral folding. No part 

 of the intestine is protrusible. The tube has usually 

 a thin muscular wall (absent in Salpse), and is often 

 suspended by thread-like mesenteries. It may be 

 spirally folded throughout (Sidnyum), or only at its 

 terminal part (Aplidium, Polyclinium). In Clavellina 

 it is divisible into three regions, a clear, thin-walled 

 part, a thicker glandular tract, and a thin rectal 

 portion. Glandular caeca may open into the intestine, 

 or it may have a coating of hepatic cells. In Boltenia 

 reniformis, small sub-conical masses of nucleated 

 connective tissue and protoplasm are attached to the 

 upper surface of the rectum. The intestine contains 

 usually diatom frustules, often Radiolarians, &c The 

 hinder part of the canal may be surrounded by a 

 narrow meshed plexus of vessels with whitish con- 

 tents, forming a peculiar vascular appendage. The 

 anus is on the surface beneath the tail in Appendicu- 



