60 BRITISH TUNICATA. 



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

 lateral lamina?, originating in the great ventral channel, and 

 extending to the endostyle ; and in Pyrusoma and Doliolum 

 we observe that these laminae are curtailed in their develop- 

 ment before they reach so far ; in the latter, in fact, they are 

 exceedingly limited. There is, therefore, no difficulty in sup- 

 posing 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 orifices, 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 Lamellibranchs. 

 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 blood-vessel leading directly to the 

 heart; while our supposed transformed organ has only one 

 such trunk vessel. It would therefore seem probable that the 

 branchial sac can represent but a single gill of the Lamelli- 

 branch, and that one fold 011 each side of the ventral lamina 

 (or great ventral channel) may be assumed to be the homo- 

 logue of the left gill of the higher mollusk. 



The branchial sac itself is not a perfectly symmetrical 

 organ; at least the oral lamina does not exactly divide it 

 into two equal lateral halves ; for it invariably passes to the 

 right of the oral aperture in all dextral species, and it never, 

 so far as my observations extend, abuts directly upon it. On 

 the other hand, the heart in the simple Ascidians usually 

 occupies a central position, being placed in the middle line of 

 the digestive organs; and the great vascular trunks, as they 

 leave its anterior or ventral extremity, exhibit a symmetrical 

 bilateral development, a trunk going to each side of the vis- 

 ceral mass, and there ramifying over these organs. That, 

 however, on the left side sends a large branch along by the 

 side of the intestine to the great ventral channel of the gill ; 

 while the corresponding branch of the right side dies out 

 before reaching the opposite margin of the visceral mass. 

 Here, then, ceases the bilateral symmetry of the vascular 

 organs; were it carried a little further, there would exist 

 two ventral branchial channels ; and thus a right pair of 

 gill-plates might be developed, one fold being 011 each side 

 of the channel ; and in this way the respiratory organ would 

 be exactly similar in all essential characters to that of a 

 Lainellibranch. And if the roots of the two lateral trunks 



