AND PHYSIOLOGY OF THE TTJNICATA. 337 



" branchial band," and the characters of the latter, suflBLciently 

 prove this. 



The so-called branchial band has the appearance of a cylindrical 

 tube ; but it is easily seen that it is formed of two laminae, the 

 lower or ventral margins of which are a little separated, while 

 the upper or dorsal are brought together, forming a ridge along 

 this margin. A large blood-channel runs along in connexion 

 with, and immediately below, the ventral margins ; this is the ho- 

 mologue of the ventral branchial channel. The two laminse rise, 

 as it were, from the sides of this channel, and inclining towards 

 each other, are united along the dorsal ridge as just stated ; but 

 towards the anterior extremity of the organ they separate, and 

 become united to the ends of the ciliated cord or band in exactly 

 the same way as the lateral divisions of the oral lamina join the 

 anterior cord or coUar. In fact, in both Salpa and Ascidia the 

 one organ seems to be a continuation of the other, as they are, 

 no doubt in both, productions of the lining membrane, the blood- 

 channel itself beiug developed in connexion with -the same mem- 

 brane. The "branchial band," we thus see, corresponds to the 

 oral lamina in being composed of two laminse, in its relation to 

 the anterior ciliated band or collar, and in its connexion with the 

 great ventral blood-channel. And, moreover, like the oral lamina 

 in several of the Ascidice, it is transversely ribbed. In Salpa the 

 ribs are stout and strongly ciliated ; and there can be no doubt 

 they are also ciliated in Ascidia. 



In Salpa, then, the so-called branchial band cannot be looked 

 upon as a true gUl ; and homologically it does not represent the 

 branchial sac at all, but only that appendage of it the oral lamina. 

 And as the function of the latter seems to be chiefly, if not ex- 

 clusively, to carry the food to the mouth, the same ofiice is pro- 

 bably performed by the so-called gill in Salpa. And, indeed, 

 without some such help it is difficult to see how such an animal 

 would be able to take its food. There can be little doubt that the 

 walls of the respiratory cavity, as has been suggested by Professor 

 Huxley *, chiefly efl'ect the decarbonization of the blood ; while 

 the so-called gill will aid in this operation in proportion to its 

 vascularity ; as must, indeed, all the tissues bathed by the respira- 

 tory currents. 



The ganglion lies on the ventral side of the respiratory cavity, 

 between the lining membrane and inner tunic, a little in advance 

 of the ciliated band, and directly in front of the anterior extremity 

 * Op. ctt. p. o70. 



