500 Cresswell Shearer 



the developing nepbridia of Phoronis to the anal or nephridial pit 

 is similar to the relationship existing betweeu the head-kidneys and 

 the pvoctodaeum in the Trochophove. As there are many resemblances 

 between the Actinotrocha larva and the Trochophore it is not im- 

 possible that the anal pit in the yonng Actinotrocha represents the 

 proctodaeum of the Trochophore. There is certainl}^ no proctodaeum 

 in the Actinotrocha, as the anns is a new formation appearing rather 

 late at a period when the larva has developed at least two pairs 

 of teutacles. This tends to Support this view. 



From the foregoing account of the origin and development of 

 the nephridial pit it will be seen that it has notbing to do with the 

 formation of mesoderm in the posterior part of the larva. It is 

 solely an ectodermal structure which gives rise to the nephridial 

 tubes, these in turn giving rise to the solenocytes. The distal ends 

 of the nepbridia hang free in the blastocoelic space in which they 

 develop, lying entirely outside the coelom, which at this stage is 

 quite small, the solenocytes having already formed on the ends of 

 the comparati vely long nephridial canals, Avhile the coelom is still 

 represented by a very small space on the dorsal side of the gut. 

 At no stage during tbeir development do the nepbridia open into 

 the blastocoelic space in which they lie. 



4. The development of the body cavity. 



The body cavity of the trunk region makes its appearance in 

 larvae of about the age represented in fig. 8 [tr.c). It appears first 

 as a small collection of cells on the dorsal side of the rectum, or 

 gut, dose under the ectoderm. At the time when they are first 

 distinguishable, they consist of a small mass of cells, some ten to 

 fifteen in number, closely applied to the wall of the gut. Whether 

 they are derived from this structure, or are only a locai accurau- 

 lation of the scattered cells of the blastocoel at this point, I bave 

 been unable to determine. Although I bave repeatedly looked for 

 them in larvae of this age, I bave never observed the actual steps 

 in tbeir origin. From their dose connection with the wall of the 

 gut at the time of their appearance, I believe they are really deri- 

 ved from this structure. They are at first closely packed together, 

 but a small space soon makes its appearance in their midst, and 

 this is tlie beginning of the trunk codoni. In pi. 32 fig. 36 this is 

 well seen as a small space surrounded by cells on the dorsal surface 



