334 ARTHUR WILLEY. 



Ascidians and Amphioxus are far from being unintelligible. 

 The epithelial lining of the prseoral cavity of Amphioxus is 

 represented by scattered cells in the Ascidians. The anterior 

 body-cavity of BalanoglossuSj as described by Bateson (2 and 

 3), seems to oflfer an intermediate condition in that an epithe- 

 lium and a loose mesenchymatous tissue co-exist there. Bate- 

 son found that the rounded extremities of the cells lining the 

 anterior body-cavity of the embryo of Balanoglossus are con- 

 tinually budding off cells into the cavity itself, and this pro- 

 liferation of cells continues until the proboscis cavity is to a 

 great extent occupied by loose masses of cells. 



The similar position, relations, and behaviour of the endo- 

 style in the larvae of Ascidians and Amphioxus are the founda- 

 tion on which the present system of homologies is being built. 

 The actual way in which the endostyle arrives at its definitive 

 antero-posterior position is somewhat different in the two cases, 

 but the main features are essentially the same (author, 33). 



From what has been said above, therefore, I draw the con- 

 clusion that the anterior body-cavity of the Ascidians, that is 

 the cavity of the prseoral lobe or adhering stolon, which 

 appears in a comparatively unmodified form in Ciona, in con- 

 trast to the condition in Clavelina and the compound Ascidians 

 generally, with its loose mesodermic cells, is homologous with 

 the anterior coelomic cavity and prseoral pit of Amphioxus 

 with their epithelial lining, the two latter structures being 

 taken together as representing a primitively symmetrical and 

 bilateral prseoral body-cavity, the two halves of which have 

 become secondarily differentiated from one another. 



4. Gill-slits of Ascidians and Amphioxus, with 



Remarks on Ammoccetes. 

 If we consider the succession of the various organs from 

 before backwards, and allow for the secondary change of posi- 

 tion of the mouth in Amphioxus (author, 33), we find that the 

 following is the order in which they occur in both Ciona 

 and Amphioxus: — 1. Praeoral lobe or anterior body- 

 cavity. 2. Endostyle. 3. Mouth. 4. Neuroporus (vide infra). 



