332 ME. A. HANCOCK ON THE ANATOMY 



In Siycla tuberosa and its immediate allies we have another 

 modification of the branchial network. In them it is provided 

 with eight simple longitudinal folds or laminae — 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 orifice. 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. mentula. 

 The folds, however, give to it a very interesting character, inas- 

 much as we observe in them a very ready and efficient mode of in- 

 creasing the aerating surface, as, indeed, the same end is gained 

 by the minute plaits in the vascular network in A. mentula and 

 A. sordida. In Stycla the folds are formed in exactly the same 

 way as those minute plaits ; that is, they are each composed of a 

 fold of the branchial sac, and the space within is divided into 

 pouches by septa situated at nearly equidistant points. In this 

 genus the transverse vessels vary considerably in size, there being 

 usually one or two smaller between larger ones ; and the septa are 

 placed wherever the latter cross the structure. Thus a series of 

 pouches of nearly equal size occupy the interior of the folds, and 

 open at the outer surface of the branchial sac into every part of 

 the atrium. In fact, we see here an arrangement very similar to 

 that observed in the interbranchial water-channels in the Lamelli- 

 brauchs ; and in this case, as in them, the purpose is to allow 

 the water, after permeating the walls of the fold or lamina, to es- 

 cape extei-nally. 



The longitudinal bars in Stycla assume the form of delicate 

 ribbon-like membranes attached by one edge to the principal 

 transverse vessels ; they are numerous, and are found on the folds 

 as well as on every other part of the organ. The oral lamina is a 

 wide simple membrane. 



The branchial sac in Molgula conchilega and its allies is charac- 

 terized by longitudinal folds or laminae, formed much in the same 

 Avay as those in Stycla. In the former there are six such folds 

 on each side of the sac. The vascular network, however, is very 

 different, having the secondary vessels, or those which are usually 

 arranged at right angles to the transverse channels, disposed in 

 imperfect spiral coils or convolutions, the vessels themselves fre- 

 quently intercommunicating ; consequently the stigmata or open 

 spaces separating them are broken into various lengths. There 

 are also a few delicate radiating vessels which pass from the centre 

 of the coils to the circumference, but mostly in the direction of 

 the transverse channels that convey the blood to and from the 



