STRUCTURE OF ASCIDIANS. 359 



consists of cellulose, or some analogous carbohydrate. It is 

 interesting to find this product — characteristic of plants — in 

 the very passive cuticle of a very passive animal. But though 

 the tunic is at first truly a cuticle, and without cells, these 

 may migrate into it, and even blood-vessels may be formed. 



The Muscular System forms beneath the epidermis a netted 

 sheath of unstriped fibres, and special sphincters surround 

 the apertures. 



The Nervous System, comparable in the larva to a spinal 

 cord and slight brain, is represented in the adult by a ganglionic 

 mass lying between the two apertures, and giving off a few 

 nerves. 



Sensory Structures. — ^Beneath the ganglion lies a small 

 (sub-neural) gland, from which a ciliated duct opens into the 

 pharynx. It is possible that this corresponds to the pituitary 

 body (see page 380), possible that it may be excretory, pos- 

 sible also that it may have some sensory water-testing function. 



Some of the Ascidian larvae have an otocyst and an eye- 

 like pigment spot, both in close connection with the brain, 

 but these are not retained in the adults of this type. 



But some or all of the following structures, which will be 

 described below, are in all likelihood sensory — pigment 

 spots between the lobes of the apertures, tentacle-like pro- 

 cesses beneath the mouth, and other processes (languets) on 

 the dorsal wall of the pharynx. 



Alimentary System. — The mouth is the uppermost aper- 

 ture, slightly puckered, often with pigment spots between its 

 lobes. It leads into a large respiratory pharynx, near the 

 beginning of which is a circle of downward pointing tentacle- 

 like processes. Water, bearing minute algse and animals, 

 is drawn in by the action of cilia, which border numberless 

 small slits in the wall of the pharynx. The food-particles 

 are glued together by a mucous secretion, and swept back- 

 wards to the digestive and intestinal region of the gut which 

 begins at the base of the pharynx, while the water passes 

 through the slits of the pharynx walls into a surrounding 

 peribranchial chamber, and thence out by the exhalent orifice. 



On the internal ventral surface of the pharynx, i.e., on 

 the wall opposite that above which the ganglion lies, there 

 is a longitudinal groove or gutter called the endostyle. It 

 is ciliated and in part glandular, and from it some observers 



