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 alge 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 cesophageal 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’ 
