the Nephridia of the Actiuotroclia. 71 



transverse mesentery inserted along the line of the bases of 

 the tentacles, a ventral mesentery in the posterior body-cavity, 

 and an incomplete transverse mesentery separating the hood 

 from the rest of the body. This latter, however, he believes 

 is a secondary structure, and so considers the Actinotrocha to 

 have two body-cavities — one in front of the mesentery along 

 tlie line of the tentacles, and one back of this. 



The lining of the former he finds has its origin from meso- 

 derm-cells arising in a diffuse manner from the walls of the 

 archenteron ; the lining of the latter, he seems to think, may 

 arise from cells of the nephridial pits. 



Menon^s paper deals only with the full-grown Actinotrocha^ 

 and his study leads him to believe that this creature has three 

 body-cavities, separated from one another by two complete 

 transverse mesenteries corresponding to the incomplete 

 mesentery and the mesentery along the line of the tentacles 

 of which Longchamps speaks. He also recognizes the exist- 

 ence of a ventral mesentery and indications of a dorsal 

 mesentery in the posterior body-cavity. 



Masterman, from a study of the early stages in the deve- 

 lopment of P. Buskii, considers that five body-cavities are 

 represented in the Actinotrocha — one median anterior and 

 two pairs of body-cavities back of this one. According to 

 his account of the anatomy of the Actinotrocha, however, 

 there are but three body-cavities, which he calls the preoral, 

 collar-, and trunk-cavities. He describes the same mesen- 

 teries that Menon speaks of, and beside these a dorsal mesentery 

 in the collar-cavity. 



Ikeda's description of the body-cavities of the Japanese 

 Actinotrochce is of special interest because he is the first to 

 describe a mesodermal sac arising immediately in front of the 

 mesentery along the line of the tentacles. This sac makes its 

 appearance rather late in the life of the larva and, after meta- 

 morphosis, gives rise to the lining of the supraseptal cavity of 

 the adult. Ikeda finds the same mesenteries in the Actino- 

 trocha that Longchamps does, and also describes the mesentery 

 back of the preoral lobe as incomplete. He has scarcely 

 anything to say concerning the origin of the mesenteries, but 

 he finds that the mesoderm arises, in part at least, from two 

 bilaterally placed archenteric diverticula. 



Goodrich has recently published a paper on the body- 

 cavities of the Actinotrocha^ and his observations confirm 

 those of Ikeda. 



From the above review of the recent literature on the 

 body-cavities and mesenteries of the Actinotrocha it is seen 

 that all but Roule, and possibly Caldwell, recognize the 



