the Nephridla of the Actinotrocha. 75 



are just beginning to form and also in larvae with two, four, 

 and six tentacles. In one specimen with two tentacles, 

 however, I have found an arrangement of mesodermal cells 

 on the dorsal side of the intestine which seems to be the 

 beginning of a sac ; this, however, is not paired. Whether 

 or not this sac and its cavity give rise to the lining and 

 cavity of the trunk I cannot say, for I have found but a single 

 specimen in which this condition exists (fig. 4). 



One thing is certain — the fully developed trunk-cavity of 

 the Actinotrocha has a distinct mesodermal lining, consistii),2: 

 of a somatic and a splanchnic layer (fig. 5). As far as I 

 know, all Actinotrochce have a ventral mesentery ; this tends 

 to support tiie view that the lining of the cavity of the trunk 

 has its origin in a sac which grows around the rectum and 

 posterior part of tiie stomach. Whether or not the fact that 

 there is an indication of a dorsal mesentery in the posterior 

 region of some of the fully developed Actinotrochce (fig. 5) 

 has any bearing on the double origin of the cavity of the 

 trunk I cannot say, for I have never seen the very young 

 larvge of these forms. 



Besides the larval body-cavities there is a mesodermal sac 

 discovered by Ikeda, which arises between the mesodermal 

 lining of the collar-cavity and the ventral ectoderm at a rather 

 late stage in the development of the Actinotrocha (fig. 5). I 

 have nothing to add to Ikeda's description of this structure, the 

 cavity of which becomes the supraseptal cavity of the adult. 



My observations on the fate of the body-cavities and 

 mesenteries of the Actinotrocha agree with those of Ikeda. 

 The pieoral lobe and the lobe-collar mesentery are lost during 

 metamorphosis, the collar-cavity and its lining become tlie 

 ring-vessel of the adult, the cavity of the mesodermal sac 

 between the mesodermal lining of the collar-cavity and the 

 ventral ectoderm becomes the supraseptal cavity of the adult, 

 the trunk-cavity, including the cavity of the ventral pouch, 

 is transformed into the inf'raseptal body-cavity, and the 

 mesentery between the collar- and trunk-cavities becomes 

 the transverse septum of the adult. 



Nephridia. — Caldwell was the first to describe the nephridla 

 of the Actinotrocha, although earlier investigators saw the 

 bunches of excretory cells at the ends of the nephridial canals. 

 The account which Caldwell gives us is very brief, but he 

 makes the observation that the nephridial canals end blindly 

 within the collar-cavity. 



Longchamps^'s view agrees with that of Caldwell, while 

 Masterman and Menon find tliat the nephridial canals of the 

 Actinotrocha open into the collar-cavity through one or more 



