52 BRITISH TCJNICATA. 



suppose a vascular network extended from either side of the 

 endostyle to the lateral margins of the "branchial band/' 

 and imagine it to be attached in front and behind along the 

 ciliated bands, we shall see how readily a Salpa may be made 

 to assume the most striking feature of an ordinary Ascidiaii. 



Now the ciliated bands are the homologues of the pos- 

 terior cord and the lower member of the anterior cord or 

 collar of the branchial sac of the simple Ascidians ; and the 

 "branchial band" is the equivalent of the oral or ventral 

 lamina of the same group. The relation of the ciliated bands, 

 particularly the anterior, to the "branchial band," and the 

 characters of the latter, sufficiently prove this. 



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

 drical tube ; but it is easily seen that it is formed of two 

 laminas, 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 homologue of the ventral branchial 

 channel. The two lamina? 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 ex- 

 tremity 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 collar. 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 being developed in connexion with the same membrane. 

 The "branchial band," we thus see, corresponds to the oral 

 lamina in being composed of two latninas, 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 Ascidise, it is transversely ribbed. In 

 Sfil/ifi the ribs are stout and strongly ciliated ; and there can 

 be no doubt that they are also ciliated in Avidia. 



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

 looked upon as a true gill; 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 exclusively, to carry the food to the mouth, 

 the same office is probably 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 



