AND PHYSIOLOGY OF THE TUNICATA. 311 



however, quite so obvious in the species which have these two 

 tunics comparatively free, as they are universally in Ascidia and 

 Molgula. But we have just seen that, in such instances, the 

 inner surface of the test, and the outer surface of the mantle, lie 

 in close contact with each other. Now, as under all ordinary 

 circumstances, the pressure of the water inside the mantle must 

 be as great as that of the water resting against the outer surface 

 of the test, and as no water can possibly enter between these two 

 tunics, it is clear enough that they will be held together with no 

 inconsiderable force. Thus, when the muscles of the mantle con- 

 tract, diminishing the bulk of that organ, the test will be drawn 

 in after it ; and so soon as the muscles of the former relax, the 

 latter, through the elasticity of its walls, will expand, and the 

 mantle will be constrained to do so likewise. 



The most interesting matter that I have to communicate re- 

 specting the digestive system relates to the biliary apparatus. 

 A remark or two, however, may be made, in the first place, upon 

 the alimentary canal, which, in all the species that have come 

 under my inspection, makes its first bend towards the dorsal 

 region, assuming that to be the dorsal aspect where the endostyle 

 is placed. The intestine then usually ascends and crosses over (in 

 a more or less undulatory course, sometimes forming one or two 

 loops) to the opposite or ventral side, where it again ascends to reach 

 the cloaca, into which, in the Ascidians, it invariably opens. The 

 walls, from one end of the organ to the other, are particularly firm, 

 and do not collapse even in preserved specimens. The lower portion 

 of the intestine is the most delicate ; but even here the wall rarely 

 shrinks. The stomach is well marked, though it is never very 

 bulky, and is usually lined with a stout mucous membrane, which 

 is frequently plaited or wrinkled, sometimes in a symmetrical 

 manner, the plaits extending into the oesophagus on the one 

 hand, and into the intestine on the other. In the latter organ 

 this membrane is thrown up so as to form a very conspicuous 

 groove, which extends from the stomach to that portion of the 

 intestine which may be termed the rectum. In Stycla tuherosa, 

 and some other species, however, this groove extends the whole 

 length of the intestine. 



The food of the Tunicaries is extracted from sedimentary 

 matters ; there is no power of selection in the first instance ; 

 those particles which can be, are digested ; the others, chiefly 

 composed of sand and mud, are rejected in the usual manner. 

 This sedimentary aliment is sifted from the water in the respi- 



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