322 GENERAL CONSIDERATIONS ON THE MOLLUSCA. 



are either still connected with the gastral cavity or already open 

 externally through special efferent ducts (nephridia ?). A specially 

 important organ of this hypothetical form which also lies in the 

 primary body-cavity is the excretory organ, the origin of which is 

 one of the most difficult points to explain. Since we see the ex- 

 cretory organs rising from the mesoderm, and are inclined to derive 

 this latter from the entoderm, we hold it as most probable that the 

 primitive excretory organ was a diverticulum of the entoderm which 

 became secondarily connected with the ectoderm. At a later stage, 

 it loses its connection with the entoderm and thus becomes the 

 structure known as the primitive kidney (protonephridium). 



From a form possessing such an organisation, the Plathelmintb.es 

 also may be derived. Their excretory system remains on the level 

 of the protonephridium, merely branching and extending further 

 through the body. Their larval form corresponds more or less to 

 that described, but does not possess a primitive kidney. The Pilidium 

 of the Nemertini already shows a certain similarity to the Trocho- 

 phore, and it has been pointed out that transition forms between it 

 and the Muller's larva of the Turbellaria are to be found (Vol. i., 

 p. 168). The Pilidium is distinguished, like the Trochophore, by the 

 possession of an apical plate. 



Through the concentration of the ciliated apparatus and the 

 acquisition of an anus, the ancestral form ascended to the level of 

 the Trochophore, and then became the starting-point for the 

 Rotatoria, the Mollusca, and the Molluscoida. Its relation to the 

 Rotatoria and the Annelida, i.e., its ascent to the level of the latter, 

 has already been discussed (Vol. i., p. 342). The most important 

 features in this process are the appearance of segmentation and the 

 rise of the coelom, the hatter being perhaps explicable through the 

 enlargement of the gonads of the primitive form. As already 

 mentioned, the genital products originate from the epithelial wall 

 of the coelom, a fact which favours such an origin. The Mollusca 

 exhibit, on the whole, similar conditions, but show an important 

 difference in the absence of segmentation, remaining in this respect 

 more like the primitive form. 



We therefore assume that the Molluscan larva [Trochophore) still 

 closely resembles the ancestral form; it already shows, however, 

 some new characters which clearly indicate differentiation in a certain 

 direction, two of these, the shell and the foot, being specially dis- 

 tinctive of the whole organisation of the Mollusca. The first of these, 

 especially, can very early be recognised through the appearance of 



