48S Cresswell Shearer 



Organs occur only in tlie anterior \)iiYt of tlie larva, where no per- 

 iiunient (»rg'ans dcvelop, and where the coelom itself is frequently 

 wanting-. And also owing to the fact that the cells, thc nephroblasts, 

 which in the erabryo give rise to both sots of organs, consist of 

 cell rows, the provisionai organs arising from the anterior, the per- 

 manent, from tliC posterior portions of these rows; these rows in turn 

 bave their origin in a continuous, not a discontinuous embryological 

 basis. Moreover, reeentinvestigation has shown, that the adult nephridia 

 ean be closed internally l)y flame-cells , in a manner sirailar to the 

 provisionai organs; so far as simple structure is concerned, they are 

 esseutially the same. 



If we classify the Polychaeta according to the form of theip 

 nephridia, they are readily separable into two great groups. One 

 of these, which ineludes most of the Phyllodocids, Glycerids and 

 the Nephthyids (see 9), is furnished with nephridia which resemble the 

 provisionai organs of Oligochaets, the Polygordius larva, and the 

 protonephridia of Platyhelminths, being closed internally by flame- 

 cells. The other is provided with nephridia that open into the 

 coelom. The structure of this opening , in turn , separates this la8t 

 group into two subdivisious. In one of these the nephridium opens 

 by means of a large funnel, which arises from the peritoneal epi- 

 thelium of the coelom, ouly in the later stages of development 

 joining the nephridium, thereby increasing the effectiveness of this 

 organ as a collcetor of the products of the coelom. The nephridia 

 of the second subdivision are provided with true coelomic openings, 

 or nephrostomes, which are much smaller, and ne ver develop into 

 the great fiinnel-like oi)enings of the previous group. 



If we endeavour to determine which is the most primitive 

 of these three types of nephridia, we select the flame-cell form of 

 nephridium, from its resemblance to the excretory tubules of 

 Platyhelminths. This opinion is sui)ported by the fact, that this 

 type of nephridium is found in larvai forms, i\nd also by the fact 

 that in certain of these larvae, while the nephridia are at first of 

 tliis form, as development jiroceeds, they gradually lose their flame- 

 cells and acquire openings into the coelom. 



I have examined the head-kidney in a number of Trochophore 

 larvae ^ and I bave never found this structure opening into the body- 

 cavity (blastocoel). The whole organ is justly coraparable to an 



' Eupomatun micinatus, Hydroiiles pedinata and Pomatoceros trvßieter. 



