110 bulletin: museum of comparative zoology. 



end of the embryo. This shifting is similar to that described by 

 Heymons ('93) for the velar field of Umbrella, and is perhaps homolo- 

 gous with the shifting of the troch in the trochophore larva. 



A prominent funnel-shaped opening leads into the arehenteron ; in 

 the latest stages I have examined it still persists. This is the region 

 where the anus is ultimately found, and I am therefore inclined to the 

 view that this remnant of the posterior part of the blastopore becomes 

 the anus of the adult ; it certainly lies in the anal region. 



The mouth is formed at a later period by an invagination of the ecto- 

 derm on the ventral surface of the embryo. Figure 55 (Plate VIII.) 

 shows an embryo of a stage whei'e the posterior opening leads into the 

 arehenteron, and the development of the head vesicle has obliterated all 

 traces of the velar prominences. It will be seen that the ventral lip of 

 the opening is prolonged posteriorly. This is the pedal elongation, which 

 later becomes the podocyst. The shell gland arises on the dorsal sur- 

 face as an invagination, posterior to the level of the velar region. The 

 enlarged cells premonitory of the invagination are seen in Figure 57 

 (Plate VIII.) . 



The conclusions to which I have arrived as to the fate of the blasto- 

 pore in Limax agrestis are directly contradictory to those of Fol upon 

 L. maximus. According to him the blastopore becomes the permanent 

 mouth. He seems, however, to have entirely overlooked the early stages 

 in the history of the blastopore. His Figures 1 and 2 (Plate XVII.- 

 XVIII.) represent stages in which the overgrowth of the blastopore has 

 already taken place. The polar globules in his Figure 1 have also, it 

 seems, been shifted somewhat, for they lie at the anterior end of the 

 embryo. Furthermore, it is impossible to reconcile his Figures 3 and 4 

 with his later stages, e. g. Figure 7. It seems much more probable 

 that what he calls the mouth in Figures 3 and 4 is the anus, and that 

 his shell gland is in reality the oral invagination. 



In concluding that the anus is formed from this persisting posterior 

 part of the blastopore (actually or potentially), my conclusions agree 

 with those of Biitschli ('77), Lankester ('76), and Dlochmann ('83) upon 

 Paludina. Although the anus can thus be referred to the posterior 

 region of the blastopore, it is not so certain, owing to the peculiar man- 

 ner in which the anus is formed, that the mouth corresponds to the 

 anterior part. It seems probable that in this case the mouth must be 

 regarded as an entirely new structure. 



Cambridge, Mass., May 1, 1894. 



