428 SUB- PHYLUM UROCHORDA OR TUNIC AT A. 



with the cavity of the stomach is possibly a digestive 

 gland; 



The structure of the pharynx is exceedingly complex, as is 

 natural when we consider its double function respiratory 

 and nutritive and also that the breathing organs of sedentary 

 animals tend to be elaborate. The water which enters by 

 the branchial aperture is not only used in respiration, but 

 brings with it the minute food particles. Similarly, the out- 

 going current carries with it the water used in respiration, 

 the indigested residue of the food, and the spermatozoa 

 and ova. The water of respiration passes from the pharynx 

 through its numerous gill openings to the peribranchial 

 chamber, and so to the exterior. On its way it purifies the 

 blood which is contained in the vessels running in the 

 complex framework of the pharynx wall. The current is 

 produced and maintained by the action of the ciliated cells 

 lining the gill-slits, and its force necessitates special arrange- 

 ments to prevent the food particles being swept out before 

 they have entered the digestive region of the gut. To 

 accomplish this, there is a longitudinal glandular groove on 

 the ventral surface of the pharynx, and a ciliated fold on its 

 dorsal the regions being defined by the nerve ganglion. 

 According to Willey, the minute algae and so forth of the 

 food are entangled in the abundant mucus secreted by the 

 ventral groove or endostyle, and are swept forward in a cord 

 of slime, until at the anterior end of the endostyle they 

 reach a circular ciliated groove, whose two halves surround 

 the pharynx, and unite to form the dorsal lamina or fold. 

 The food particles pass round this groove, and are swept 

 backwards by the cilia of the dorsal lamina until they reach 

 the cesophageal opening. Other observers emphasise the 

 endostyle more particularly as the path along which the 

 food travels. 



In many Ascidians the dorsal lamina is replaced by a 

 series of processes --the dorsal languets, which may be 

 sensory, as well as food-wafting structures. 



Nervous system and sense organs. In the adult both of 

 these show marked degeneration. In the larva there is a 

 slightly-developed brain continued into a dorsal nerve-cord, 

 and having connected with it a median eye and an otocyst. 

 The two latter are completely absent in the adult, and the 



