AND PHXSIOLOGT 01" THE TUNICATA. 331 



ture along the lamina. The anterior cord may perhaps also aid 

 in collecting sedimentary aliment, if it be cUiated, as its homologue 

 in Salpa 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 Clavelina and 

 PeropJiora the vascular network is not more complicated ; and in 

 the former, at least, the longitudinal bars have entirely disap- 

 peared. And in it there are numerous transverse laminae which 

 are adherent throughout to the walls of the transverse channels ; 

 they are united to the filaments of the oral lamina, and perhaps 

 are mainly instrumental in carrying the food in that direction. 

 The structure of the gill is equally simple in the compound Tuni- 

 cates ; and in them the longitudinal bars seem to be occasionally 

 present. 



In Ascidia mentula and A. sordida the branchial network is fun- 

 damentally the same as in A. venosa ; but in the two former, and in 

 some others, it is^minutely folded longitudinally, so that, on making 

 a transverse section of it, the edge presents a deeply undulated 

 line. The surface is not altogether unlike corduroy ; it is, in fact, 

 finely plaited (or crimped, as the laundress might say) ; but the 

 flutes or grooves between the ridges or plaits are interrupted 

 wherever the transverse vessels' cross them, the vessels at these 

 points filling up the hollows. Thus there are numerous septa 

 formed, turning the grooves into series of minute recesses or 

 pouches. 



The longitudinal bars are strong and raised considerably above 

 the inner surface in A. mentula ; and there are smaller interme- 

 diate papillae, as well as larger ones at the points where the bars 

 cross the transverse vessels. All the papillae bear ciliated disks ; 

 and a wide membrane stretches from the back of the larger pa- 

 pillae for a considerable way along the transverse vessels. In 

 A. sordida the bars are likewise strong ; but the papillae are rather 

 small, and there are no intermediate ones. Between the longitu- 

 dinal bars there are two oval ciliated disks, one on either side of 

 the middle line of the transverse vessels. 



The oral lamina in A. sordida is a wide plain membrane ; but in 

 A. mentula it is strongly ribbed transversely ; the ribs passing 

 beyond the margin as fine points give to it a pectinated appearance. 



* " Observations upon the Anatomy and Physiology of Salpa and Pyrosoma," 

 Phil. Trans. 1851, pt. 2. p. 567. 



