44 BRITISH TUNICATA. 



is really composed of two lateral membranes; and each 

 division is united to the lower member of the anterior collar, 

 much in the same manner as the latter is attached to the 

 dorsal folds connected with the endostyle. The upper 

 member of this collar is divided from the lower by a narrow 

 groove, and is uninterrupted by either the oral lamina or the 

 dorsal folds. The oral lamina is connected below by another 

 narrow cord to the posterior extremity of the dorsal folds : 

 this is the posterior cord. 



In this way are traced the boundaries of the two lateral 

 laminae composing the branchial sac. They are attached by 

 their upper borders to the walls of the pallial or respiratory 

 chamber a little below the anterior collar or cord ; the dorsal 

 margins are attached along the sides of the endostyle, and 

 the lower margins along the line of the posterior cord. In 

 all other parts the two lobes are free, except at the points 

 where the suspenders bind them to the walls of the chamber, 

 and where the extremity of the oesophagus penetrates the 

 branchial sac ; and here, of course, the latter is attached to 

 the alimentary tube. The supposed function of the endostyle 

 has been already indicated; the folds of the lining membrane 

 to which it is adherent are no part of the gill : neither can 

 the oral lamina be considered a portion of the breathing- 

 organ: it is certainly highly vascular; that is, minutely rami- 

 fying blood-channels can be traced in it : but similar vessels 

 (or channels) are seen in all the membranes of the organism, 

 and also occur in the dorsal folds in connexion with the endo- 

 style. The office of the oral lamina is to conduct the food to 

 the mouth. And it has already been stated that the sedi- 

 mentary matters are there accumulated and formed into a 

 cord, and so carried to the oral aperture along the lamina. 

 The anterior cord may perhaps also aid in collecting sedi- 

 mentary aliment, if it be ciliated, as its homologue in Sal pa 

 is stated by Professor Huxley* to be. 



The simple form of gill above described is not by any means 

 constant in Ascidia ; in fact, it seems but rarely to occur in 

 this genus. The same simplicity of structure, however, is 

 found in Pelonaia, with only some unimportant changes. In 

 Clarelina and PeropJiom the vascular network is not more 

 complicated ; and in the former, at least, the longitudinal bars 

 have entirely disappeared. And in it there are numerous 

 transverse laminae which are adherent throughout to the Avails 

 of the transverse channels; they are united to the filaments of 

 * " Observations \ipon the Anatomy and Physiology of Salpa and Pyro- 

 soma," ' Phil. Trans.,' 1851, pt. 2, p. 567. 



