GENERAL CONSIDERATIONS ON THE MOLLUSCA. 319 



the Arthropoda, in which also relations were proved to exist between 

 the mouth and the aims on the one hand and the blastopore on the 



other. 



The intestine, which like the oesophagus, is usually formed by an 

 ectodermal invagination in the Invertebrata, appears to be derived 

 entirely from the entoderm in the Mollusca ; thus the enteron 

 usually fuses direct with the ectoderm, without any depression of 

 the latter. This has been regarded as an important peculiarity in 

 the organisation of the Mollusca, and much stress has been laid 

 upon it, but if, as has been stated, an ectodermal intestine actually 

 occurs in a few Molluscs {Chiton, Teredo, etc., pp. 15, 66, 208), the 

 usual absence of the proctodaeum cannot be considered as a dis- 

 tinctive feature. 



At the opposite end of the larval body, i.e., at the cephalic pole, 

 and in the midst of the velar area, an ectodermal thickening known 

 as the apical, plate is found, occupying the same position as in many 

 other pelagic larvae. In the Annelida, the supra-oesophageal ganglion 

 is thought to originate from the apical plate, and in the Mollusca 

 also the cerebral ganglion is said to arise from it (Chiton, Lamelli- 

 branchia) or else to bear some relation to it {Dentalium). In the 

 Gastropoda, indeed, the cerebral ganglia originate as two ectodermal 

 thickenings of the pre-oral part of the body, but these also do not 

 differ much in position from the apical plate, so that even here there 

 may be some relation to the latter. 



An organ of special importance in comparing the Molluscan Trocho- 

 phore with other larval forms is the prim it in' kidney. This arises 

 from cells derived from the mesoderm-bands,* in the same way as 

 in the Annelida, as a tubular paired structure. Its relations to the 

 primary body-cavity also seem to be the same as those met with in 

 the Annelida and in the c;tse of the adult excretory organ of the 

 Plathelminthes. 



It is evident from the above that the resemblance of the Molluscan 

 larva with the Trochophore of other animal phyla, and especially 

 with that of the Annelida is exceedingly striking not only in external 

 form but in internal structure. Attempts have been made to account 

 for this resemblance by the supposition that the larvae of these 

 phyla gradually assumed the same organisation as a consequence 

 of similarity in their manner of life. We do not share this view, 

 and can only explain the remarkable external and internal similarity 



* [See footnote, p. 179. Ed.] 



