344 ME. A. Hancock; on the anatoky 



roborative facts in its favour : more information is still required 

 before tbis matter can be determined satisfactorily. Tbe inter- 

 pretation alluded to is, tbat the branchial sac is a new and dis- 

 tinct development, as tbe endostyle is, and as are tbe oral la- 

 mina, tbe branchial tubercle, and. tbe tentacular filaments of tbe in- 

 halant tube, — and that all these organs have equally their origin 

 in the lining membrane or inner tunic of Huxley, and have no 

 homologica] representatives in the Polyzoa. 



And, further, this interpretation of tbe facts leads to a belief 

 that the branchial sac is the rudiment of the Lamellibranchiate 

 gill, the structure of the two organs being essentially tbe same. 

 The principal blood-channels in tbe gill of the Lamellibranch are 

 simple transverse vessels ; and tbe most persistent and essential 

 parts in the structure of the branchial sac of the Tunicates are 

 the transverse or primary vessels. Thus, fundamentally, these 

 organs are similar. And when the branchial sac is furnished 

 with longitudinal folds, as generally is the case in the Cyntbiadae, 

 ■the primary vessels assume relatively tbe same position as their 

 sxipposed homologues do in the gill-plate. The folds, too, as the 

 nature of the structure implies, are formed of two laminse united 

 at their distal margins, and have the space between them divided 

 by septa into transverse pouches, which only want to be elon- 

 gated by the further development of the fold to make them cor- 

 respond in every respect to the interbranchial water-tubes of the 

 gill-plate of the Lamellibranch. And already the pouches sub- 

 serve tbe function of water-tubes. 



'Now we have seen that the branchial sac is composed of 

 two lateral laminae, originating in tbe great ventral channel, and 

 extending to tbe endostyle ; and in Pyrosoma and Doliolum we 

 observe that these laminae are curtailed in their development be- 

 fore they reach so far ; in the latter, in fact, they are exceed- 

 ingly limited. There is, therefore, no difficulty in supposing that 

 the branchial sac might be reduced to merely four such folds as 

 above alluded to, two being on each side of the mouth and oral 

 lamina. Were such the fact, there would be four rows of ori- 

 fices, corresponding to the pouches in the folds on the outside of 

 the gill, opening into the cloaca, exactly like the four rows of 

 openings of the interbranchial water-tubes communicating with 

 the anal chamber in the Lamellibranch s. Thus, in all external 

 characters, we should have here a very complete representation 

 of the four gill-plates of that group. Each pair of the gill-plates, 

 however, in the Lamellibranchiata has its own proper efferent 



