538 I. TKEDA 



h. The Internal Structure of Actinotrocha. 



1. Body-Divhiom nml Bod y -Cavities. 



I have oiKloavoTod to show in tlio proeodinii; pn2;es, that the 

 l)0(lv-('avitio!^ of Actinotroclia do not arise from the enteric diver- 

 ticula, as was insisted upon l)y Caldwell, hut that they are 

 simply produced by mesohlastic cells applving' themselves to and 

 forming the lining of the ectohlastic, and the entoblastie, wall. 

 They may, therefore, he classed under the '* j^^^^^'^^o^^^l^ " or 

 " schizocœle " of Hertwig. ]\Ioreover, tlie body-cavities of Acti- 

 notrocha as a whole do not in their genetic relation correspond to 

 those of the adult, as I shall attempt to elucidate in the sequel. 

 Dui'ing the metamorphosis, the greater part of the former (the 

 preoral cavity) is almost wholly lost, while the other part (the 

 collar-cavity) is transformed into a vascular space, so that what is 

 known by the same name in the adult is of an entirely new origin. 

 Thus we see that the larval l)ody-cavity of Actinotrocha, ?'. e. the 

 ti'unk cavity, is the only jx'i'tion that persists among the body- 

 cavities of the adult, in wliich it is known as the foot or infraseptal 

 cavity. In correlation with this cii'cumstance are observable cer- 

 tain changes in the position of the ne])hri(lia and of the vascular 

 system. As described by Caldwell, the nephridia of Actinotrocha, 

 which are not pi'ovided with an internal opening, lie for the most 

 part in the collar cavity, while after the metamorphosis they are 

 found wholly in the infraseptal cavity of the worm. Moreover, 

 the paired corpuscle masses which are f )und only in the collar 

 cavity of the larva, are no longer seen in the same cavity of the 

 adult. These chanws to a cei-tain extent at least establish the 



