236 GILBERT 0. BOURNE. 



this there is a layer, for the most part thin, of connective 

 tissue channelled by blood-spaces; within this, again, and 

 lining the cavity of the branchial sac, is the endodermal 

 epithelium. The walls of the branchial sac are, in most 

 simple Ascidians, so extensively pierced by the branchial slits 

 or stigmata that the arrangement of their component layers 

 is not obvious, but in Oligotrema the branchial sac and the 

 number of the stigmata are so much reduced that the rela- 

 tions are very clearly seen in sections. The arrangements just 

 described are, of course, well understood, and have been 

 particularly clearly explained by Yves Delage, and Herouard 

 in the eighth volume of the ' Zoologie Concrete ; ' but these 

 authors, while showing a true appreciation of the morphology 

 of Tunicates in other respects, seem to me to make a grave 

 mistake in describing the complex of blood-spaces in the 

 connective-tissue layers of the " mantle " and branchial sac 

 as a " schizoccele." A schi/.ocoele, as defined by the authors 

 of the name, is a form of coelomic cavity distinguished by its 

 mode of development from an enterocoele. The blood-sinuses 

 in the " mantle " and branchial sac are not coelomic spaces, 

 but contain blood, and are appropriately classed under 

 Professor Laukester's term hsemocoele. Moreover we 

 know that, in some simple Ascidians at any rate, the ccelom 

 is developed as an enterocoele from outgrowths of the 

 primitive gut, but is obliterated early in larval life by the 

 conversion of its walls into mesenchyme, which fills up the 

 space not occupied by the nervous system and notochord, 

 between the ectoderm and endoderm. The term schizocccle, 

 therefore, is doubly unfortunate. 



Equally unfortunate, I think, is the term " mantle " applied 

 to the outer layers of the body-wall. It suggests, and Avas 

 originally intended to suggest, a genetic relationship with 

 the mantle of Mollusca, but this view has long since been 

 abandoned, and the name has now only the sanction of long 

 usage to recommend it. Furthermore it is misleading, for it 

 implies an external coat formed as an outgrowth from the 

 body-wall, whereas the so-called mantle is in fact the body- 



