ON DE\'ELOrME:S'ï ETC. OF i'HORONIS. 533 



the excretory cells of Actinotroclia, is always found attached to 

 the 1)liiid ends of the nephridial canals. Caldwell say>s that he 

 saw the excretory cells aggregated around the apex of each canal 

 and that they had numerous plasmic processes, giving them a 

 strong resemblance to the perforated cells known in J^chiiirua. It 

 seems, however, highly prol)al)le to me that this strange appear- 

 ance of the excretory cells is an artefact, since, as I shall point 

 out later, the same cells in Actinotroclia are certainly not provided 

 with any such processes. 



I have very frequently detected some gigantic meso1)last cells 

 Iloating freely in the ]:>ostoral 1)ody-cavity of larvie with (jne or 

 two pairs of tentacles (tig. 44, corp.). They are round and 

 nucleated and contain numerous large yolk-spheres. After repeated 

 examination I have come to regard them as mother-cells of blood 

 corpuscles which are found as corpuscle-inassess in the collar cavity 

 of Actinotroclia. This point will again Ije treated of in detail in 

 the proper [)lace in the following section. 



II. The Structure of Actin otrocha. 



a. External Appearance. 



It can scarcely be dou])ted that each species of the PJloi'o- 

 n'idœ has a characteristic lorm of Actinotroclia peculiar to it. Bonie 

 of the previous observers {c. g., Wilson and Masterman) have 

 mentioned two distinct types of larvie as occurring in the same 

 locality. Among the larva^ which I ol »served at Misaki, I was 

 able to distinguish four ditlerent types, each of which had a 

 characteristic form and a more or less delinitc topographical 



