BLASTOPORE AND ANUS IN PALUDINA VIVIPARA. 383 



the dorsal surface is ^jresented a large portion of the velar 

 area — that area enclosed by the ciliated ridge of the velum — 

 i;5 visible, since the velar area forms a plane having a dorsal 

 rather than a polar aspect, as may be best seen in the lateral 

 view (fig. 12). In the centre of the pallial region is a 

 small strongly marked pit, from which the cells of the epi- 

 dermis are seen to radiate, each cell having an elongated 

 form ; the area occupied by these elongated radiating cells is 

 sharply marked off and is of a circular form (fig. 10.) This 

 area, with its central groove or cup, is the structure which I 

 discoveredinthe Lamellibranch Pisidium and in the Gastropod 

 Aplysiain 1871, and subsequently in the ' Annals and Maga- 

 zine of Natural History' for February, 1873, described as 

 the *' shell-gland," and further in this Journal, in 1874, 

 showed to be an organ common to the various classes of 

 Mollusca either in their adult or embryonic conditions.^ Its 

 occurrence has since been noted by other observers. In 

 Paludina, when the shell has already assumed a dome-like 

 form, the pit of the shell-gland is still in existence and is 

 filled by a small chitinous knob of the shell. 



In fig. 11 the same embryo as that seen dorsally in fig. 10 

 is represented as seen from the pedal or ventral aspect. The 

 anterior margin as this view is formed by the anterior border 

 of the velum. It is just below this at the spot marked sm 

 that the mouth subsequently makes its appearance. At this 

 stage there is here no trace of mouth or stomodseum — no 

 thickening of the ectoderm or special connection between 

 it and the close well-rounded upper end of the archenteron 

 or endodermal sac. In specimens of a very little later growth 

 than this a broad shallow depression can be detected between 

 the margin of the velum and the protuberance of the foot 

 at the spot marked sm in figs. 11 and 12, and it is thus as shown 

 by the appearance of still further developed embryos such 

 as fig. 14 represents — that the mouth and stomodeeum make 

 their appearance. The woodcut (figure 2) in my paper in 

 this journal, April, 1875, represents an intermediate stage. 



The blastopore is seen to lie on the ventral surface, in the 

 stage represented in figs. 10, 11, and 12, a position to which 

 it tends from the earliest assumption of the Trochosphere 

 phase of development (see figs. 7, 8, and 9) . 



Up to this period the invaginated archenteron (primitive 

 or gastrula stomach) has retained a simple oval form nar- 

 rowing to a short neck which opens by the blastopore : it 



' In the Proc. Rojfal Society, March, 1874, 1 mentioned that I had found 

 the embryonic shell-gland in the following genera, Pisidium, Aphysia, Neri- 

 tiua, Limneeus. 



