468 '^' ^- NICKERSON, 



There is frequently present embeddcil in the material of which 

 these bodies are composed one or more small spherical particles; these 

 may lie either at the centre — and so appear to be a core around 

 which the layers have been laid down — or in among the concentric 

 layers in any other position (Fig. 17 c, d), as though caught by 

 chance during the deposition of the material and enclosed by it. 

 They appear to be of the same substance as the rest of the concretion 

 and when central in position are probably only the earliest stages in 

 the segregation of material to form the larger concretionary bodies. 

 Occasionally one of these concretions is found which has an irregular 

 form and seems to be built up around a somewhat irregular amorphous 

 mass, as in Fig. 17 e. Sometimes also there are seen bodies which 

 appear to have resulted from the fusion of two or more separate con- 

 cretions in an early stage of their formation and which by subsequent 

 increase have come to have a somewhat lobulated form (Fig. 17 d). 

 It is not unconnnon also to find them having one or, more rarely, 

 two projecting points upon the surface, as shown in Fig. 17 c. In 

 sections of one specimen there were also present in the vesicles 

 together with the larger concretions numerous very small spherical 

 bodies (Fig. 17 /"), which would appear to be very early stages in 

 the formation of the larger ones. That they are cells with membrane 

 and nucleus, such as Lejtenyi ('81) found in Gastrodiscus , is not 

 possilde. A complete series of stages from these small bodies to the 

 larger ones, I have not found. This may be due to the fact that 

 nearly all of my material was preserved in acid reagents, which 

 destroyed these concretions, and that consequently I was able to study 

 them under the more favorable conditions presented in sectioned 

 material in only two or three si)ecimens. In the living worms moreover 

 the smaller bodies were naturally less easily seen, being enclosed 

 among the larger and more prominent ones and being easily overlooked 

 under the less favorable conditions caused by the opacity of the over- 

 lying tissues. 



Little is known about the chemical composition of these struc- 

 tures. In killing one of the worms in an acid mixture ('i)icrosulphuric, 

 corrosive sublimate, acetic) while ol)serving it under a low power, a 

 single bubble of gas was observed to l)e formed. Considering the 

 small quantity of the material and the fact that the reagent was cold, 

 this seems to be sufficient evidence to warrant the assertion that 

 the material contains a carbonate. A somewhat crude test for 



