224 GASTROPODA. 



groove-like vas deferens leads down to an introvertible cephalic penis. The 

 next change which occurs is the closure of the groove-like vas deferens and its 

 separation from the ectoderm as a tube. This brings us to the condition seen, 

 in Actaeon and then to the Basommatophorous Pulmonata, where the male 

 aperture is distinct from the female. The only break in the series is that 

 between the Basommatophorous and Stylommatophorous Pulmonata, due to 

 the development in the latter of a secondary oviduct which extends from the 

 primitive genital aperture (now closed) down to the cephalic penis, and opens 

 with that structure through an atrium. With regard to the origin of this 

 secondary oviduct, two possible interpretations present themselves, one being 

 that the secondary oviduct has arisen as a groove, like the vas deferens, and, 

 like that structure, has secondarily closed together with the primary genital 

 aperture, so that both products are discharged by a single anterior aperture, 

 or it may be that the primitive genital orifice has shifted by growth down the 

 side of the body towards the penis and, finally, an atrial involution has caught 

 up both these apertures, so that they now communicate with the exterior by 

 a common aperture. The fact that these stages are not recapitulated in the 

 ontogeny is not, we think, of much importance, for we know that ontogeny 

 by no means always recapitulates phylogeny, and that this is especially the 

 case in forms which, like these Mollusca, have a considerable amount of yolk. 

 ED.] 



LITERATURE. 



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 1848. 



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