NERVOUS SYSTEM AND SENSE ORGANS. 447 



over, the breathing organs of sedentary animals tend to be 

 elaborate. The water which enters by the branchial aper- 

 ture is not only used in respiration, but brings with it 

 the minute food particles. Similarly, the outgoing current 

 carries with it the water used in respiration, the undigested 

 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 in the 

 vessels running in the complex framework of the pharynx 

 wall. The water-current is produced and maintained by 

 the action of the ciliated cells lining the gill-slits, and its 

 force necessitates special arrangements to prevent the food 

 particles being swept out before they have entered the 

 digestive region of the gut. In this connection there is a 

 longitudinal glandular groove or endostyle along 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 the like 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 

 the ciliated peripharyngeal groove, whose two halves sur- 

 round the pharynx, and unite to form the dorsal lamina or 

 fold. The food particles passing round the peripharyngeal 

 groove are swept backwards by the cilia of the dorsal 

 lamina until they reach the O2sophageal opening. In many 

 Ascidians the dorsal lamina is replaced by a series of pro- 

 cesses 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 

 nervous system consists merely of a ganglionic mass lying 

 between the two apertures, giving off a few nerves forwards 

 and backwards. 



A structure of doubtful utility, but of considerable morphological 

 interest, is the small sub-neural gland which lies beneath the ganglion, 

 and communicates by a ciliated duct with the pharynx. The opening 



30 



