CCELOM OF MOLLUSCA. 121 



of the pericardium as opposed to the heart and vascular 

 spaces. 



Rabl (No. 43), Patten (No. 42 a), and Ziegler (No. 53) have 

 described mesoblastic bands in Planorbis, Patella, and 

 Cyclas respectively, arising in the typically Annelidan manner; 

 but Ziegler, so far as I am avrare, has alone succeeded in ascer- 

 taining what part these bands take in the formation of the peri- 

 cardium, generative organs and kidney. He has found that 

 the generative cells are derived from the mesoblastic bands; 

 that the pericardial cavity arises as two cavities — one in each 

 band — which subsequently unite; and finally, that the kidneys 

 are hollowed out in certain masses of cells of the bands. 

 These results, if generally applicable, appear to confirm abso- 

 lutely the anatomical proof of the coelomic nature of the 

 MoUuscan pericardium. It is interesting to notice that in 

 Ziegler's figure (fig. 27) the developing pericardia] vesicles have 

 exactly the same relation to the primary body cavity or vas- 

 cular space, i. e. they lie within it, as the nephridial vesicles of 

 Peripatus have to the vascular cavity of the leg. 



There are certain animals to which the above general con- 

 siderations as to the distinctness of the coelom and vascular 

 system do not apply. I refer more especially to the Nermer- 

 tinea and Hirudinea. In the Nermertinea, according to 

 Oudemans (No. 40), and in the Hirudinea, according to 

 Bourne (No. 21), structures which it is difficult to believe are 

 not nephridia open into the vascular system. I do not intend to 

 discuss these cases now, because, on the one hand, this paper is 

 already too long, and because, on the other, I do not think our 

 present knowledge is sufiicient to enable this to be done with 

 profit. But I venture to submit with regard to them that 

 it is not clear in either case that the vascular system into 

 which the nephridia open is homologous with that of other 

 types. The very fact that there is a communication with the 

 exterior is a strong point in favour of the space being coelomic ; 

 and it should be remembered that comparatively little is 

 known with regard to its development in either group. 



In conclusion, I may point out, that whereas in most 



