ANATOMY AND PHYSIOLOGY. 45 



the oral lamina, and perhaps are mainly instrumental in carry- 

 ing 1 the food in that direction. The structure of the gill is 

 equally simple in the compound Tunicates ; and in them the 

 longitudinal bars seem to be occasionally present. 



In Ascidia mentula and A. sordida the branchial network is 

 fundamentally the same as in A. venosa ; but in the two for- 

 mer, and in some others [J. scabm and A. affin!*, &c.], it is 

 minutely folded longitudinally, so that, on making' a trans- 

 verse 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 inter- 

 rupted 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 

 intermediate papillae, as well as larger ones at the points 

 where the bars cross the transverse vessels.. All the papilla? 

 bear ciliated disks : and a wide membrane stretches from the 

 back of the larger papillas for a considerable way along the 

 transverse vessels. In A. sordida the bars are likewise 

 strong ; but the papillte are rather small, and there are no 

 intermediate ones. Between the longitudinal 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. mentnla it is strongly ribbed transversely, and the 

 ribs, passing beyond the margin as fine points, give to it a 

 pectinated appearance. 



In Sfyela tuberosa and its immediate allies we have another 

 modification of the branchial network. In them it is provided 

 with eight simple longitudinal folds or lammas four on each 

 side of the oral lamina; these stretch from one end of the sac 

 to the other, and terminate below by the sides of the oral ori- 

 fice. The network is, in other respects, as simple as it is in 

 A. venosa, there being no minute plaiting such as is seen to exist 

 in A. mentnla. The folds, however, give to it a vei-y interest- 

 ing character, inasmuch as we observe in them a very ready 

 and efficient mode of increasing the aerating surface, as, indeed, 

 the same end is gained by the minute plaits in the vascular 

 network in A. meufubi and A. sordida. In Sti/da the folds 

 are formed in exactly the saine way as those minute plaits; 

 that is, they are each composed of a fold of the branchial sac, 



