AND PHYSIOLOGY OP THE TUNICATA. 345 



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 Lamellibranch, and that one fold on each side 

 of the ventral lamina (or great ventral channel) may be assumed 

 to be the homologue 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 observa- 

 tions 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 extre- 

 tremrty, exhibit a symmetrical bilateral development, a trunk 

 going to each side of the visceral 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 on each side of the channel ; 

 and in this way the respiratory organ would be exactly similar 

 in all essential characters to that of a Lamellibranch. And if the 

 roots of the two lateral trunks that proceed from the heart were 

 dilated into auricles, the rudiments of the Lamellibranchiate 

 heart would also be established. This idea of an arrest of a bi- 

 lateral growth is somewhat strengthened by Krohn's description, 

 already quoted, of the development ot Ascidia mamillata, in which 

 the young at first has two distinct lateral atrial spaces and two 

 lateral exeurrent orifices ; the spaces ultimately coalesce, as do 

 also the orifices, the tendency to bilateral development termi- 

 nating at a very early period. 



If this view of the homologies of these organs be correct, then 

 the cloacal, or that which has been uniformly designated through- 

 out this communication the ventral surface, will correspond to 

 the dorsal region of the Lamellibranch ; and consequently the 

 opposite margin will be the ventral aspect, and the so-called 

 right and left sides will have to interchange appellations. Thus 

 the exeurrent tube will become dorsal, and the incurrent veu- 



