GENERAL CONSIDERATIONS ON THE MOLLUSCA. 327 



acquisition of those characters which determine the typical Mollusc. 

 \Ve then touched chiefly upon features of the outer organisation, but 

 some points of internal organisation were also pointed out, such as 

 the supposed rise of the coelom from the gonads of the primitive form 

 and the primary excretory organ, the primitive kidney. A further 

 important characteristic of the Mollusca is the occurrence of the 

 (adult) nephridia and their connection with the coelom (pericardium). 



We believe the origin of the adult nephridia to be the same as 

 in the Annelida, i.e., we derive them from the protonephridium. 

 Although nothing is known ontogenetically on this subject, the adult 

 nephridia, like the primitive kidney, are derived from the middle 

 germ-layer, a fact which, indeed, indicates a common origin, if only 

 because the mesoderm originally distributed in the body-cavity 

 (mesenchyme) and the coelomic mesoderm (the former gonads) had 

 in any case the same origin, i.e.," were derived from the entoderm. 

 The connection now existing between the nephridia and the coelom 

 is secondary, for it is wanting in the primitive kidney. The nephridia 

 took over the transmission to the exterior of the genital products, 

 when special efferent ducts for them were not developed.* 



The circulatory system of the Mollusca, like the coelom and the 

 nephridia, shows great resemblance to that of the Annelida, a fact 

 which inclines us to ascribe it to the primitive form from which the 

 two stocks are derived. The simplest form of circulatory system was, 

 in any case, that of a contractile sac open at one end and lying 

 dorsally. The heart belonged to the primary body-cavity. Where 

 the coeloniic sacs were specially large it was found squeezed in 

 between them and the intestine, dorsally to the latter. The rise of 

 the heart between the entoderm and the splanchnic layer of the 

 mesoderm, still characteristic of many forms, and which, in the 

 Lamellibranchia, even leads to its development round the intestine 

 (Figs. 31-33, p. 75), led some authors to trace back the heart to one 

 of the blood-sinuses which encircles the intestine.f According to 

 this view, the blood-sinus would have to be located dorsally to the 

 intestine. The blood passed into the muscular sac which represented 

 the primitive heart and which carried on rhythmical contractions, by 

 means of which the blood was again driven out. There were no 

 vessels, but the blood ran through spaces and slits in the mesodermal 



* [See footnote p. 179 and GOODRICH, Quart. Jour. Micro. Sci. Vol. 

 XXXVII. ED.] 



t Tliis view which was adopted by GBOBBEN has been discussed in con- 

 nection witli the formation of the heart in the Lamellibranchia (p. 79). 



