110 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
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 archenteron ; in 
the latest stages I have examined it still persists. This is the region 
where the anus is ultimately found, and Iam therefore inclinod 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 tho ecto- 
derm on the ventral surface of the embryo. Figure 55 (Plate VIII.) 
shows an embryo of a stage where the posterior opening leads into tho 
archenteron, and the development of the head vesicle has obliterated all 
traces of the volar 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). 
'l'he conclusions to which I have arrived as to the fato of tho 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- 
XVIIL.) 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 Bütschli (77), Lankester (76), and Blochmann (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 tho 
anterior part. It seems probable that in this case the mouth must be 
regarded as an entirely new structure. 
CAMBRIDGE, Mass., May 1, 1894. 
