308 A. T. MASTERMAN. 



verse sections in PI. 19, fij^s. 16 and 17, and in longitudinal 

 sections in PI. 20, figs. 19—23 and 27, mes'. 



At the ventral posterior part of this collar area are seen in 

 the view of the whole larva (fig. 17, c. n., in section) a pair of 

 large bodies, apparently aggregations of amoeboid cells with 

 large nuclei. Their presence has been observed, figured, and 

 remarked upon by almost all the workers upon this larva, and 

 various interpretations have been put upon them. It has been 

 noticed that the masses break up at the metamorphosis, and 

 the constituent cells have in different cases been seen to float 

 freely in the cavity, pass into the tentacles, or into the vascular 

 trunks. 



In fig. 17 there is seen nothing but a mass of protoplasm 

 and large nuclei, but in sections further back we notice that 

 the cells are grouped round a few intercellular canals which 

 open into the collar cavity by broad funnels, join each other, 

 and pass outwards and downwards between the two layers of 

 the collar-trunk mesentery (mes' .) to open to the exterior near 

 the mid-ventral line immediately on either side of the "diver- 

 ticulum.^' 



This is the organ referred to as the " nephridium " by 

 Caldwell (2), as follows : — " The pair of nephridia lie on either 

 side of the body, their numerous excretory cells floating freely 

 in the body-cavity in front of the septum. The external open- 

 ings are placed one on either side of the opening of the foot." 

 He further describes in some detail the excretory cells, and 

 remarks, ''At no time during the free-swimming life of larva 

 does the excretory canal system open into the body cavity." 

 This statement does not apply to the species I have investi- 

 gated, and it is improbable that it does so to any other 

 Actinotrochse. The internal funnels, without doubt, open 

 into the collar cavities, and not into the trunk ccelom, as do 

 the adult nephridia. PL 21, fig. 40, shows a longitudinal section 

 of one of these collar nephridia as constructed from a series of 

 sections. Each is suspended and held in position by a long 

 cord of mesoblastic cells running from the front wall of the 

 collar. A transverse section of the duct is seen (fig. 41) to 



