DEVELOPMENTAL HISTORY OF THE MOLLUSCA, 23 
glass slip) are a little different, and the plane of focus differs a little in each case, being, 
however, nearly median, but more superficial in fig. 9 than in figs. 7 & 11. Plate 7. 
figs. 7 & 9 are so disposed by focus and attitude as to catch strongly the shell-groove 
and the thickened mass of tissue at the aboral pole. In fig. 11 the indentation or groove 
itself is out of focus, but the thick epiblast (ep) is well shown, and the mass of adherent 
cells ( pme), which, as stated above, undoubtedly take a chief part in forming the intes- 
tinal portion of the alimentary canal. 
Plate 7. figs. 12,13, 14,15 show a great advance in the development of the foot and 
of the pharynx. They are, however, chiefly of interest in relation to the groove of the 
shell-patch, which they show with remarkable clearness. Figs. 12 & 15 present the 
same embryo, with the least possible change of focus, fig. 12 being a very minute bit 
higher than fig. 15. The result of this little change of plane on the appearance of 
both pharynx and of shell-groove enables one to make out the direction and character 
of these structures. 
The shell-groove is perhaps the most important structure to which I have to draw 
attention in this paper, and I may therefore now say a few more words about it. Is 
this groove connected with the secretion of the shell? or is it perhaps an invagination 
to give rise to a block of tissue connected subsequently with the rectum and anus? 
That question occurred to me; and if the former supposition could not be supported, 
the apparent analogy with the shell-groove of Pisidiwm would be a false one; also 
the possibility that this primitive groove in Mollusca generally may represent the 
closed epidermal sac, in which the pen of Loligo is developed, would have no basis. 
I am able definitely and conclusively to show that the “ shell-groove” in Aplysia does 
really belong to the shell, and in fact sometimes contains a plug of chitinous secretion, 
an imbedded shell in fact, the possible homogen of the internal pen of Cephalopoda. 
The specimens which gave this interesting result, and which also throw light on the 
connexion of the rectal portion of the alimentary canal with the early aboral thick- 
ening of the epiblast and the cell marked pme, are artificially produced deformities. 
I was in the habit of keeping egg-coils of Aplysia minor in a basin, through which 
there ran a constant current of sea-water. From the same egg-coil I cut from day to 
day a small piece of the coil, in order to examine the embryos contained in its capsules. 
I noticed that in some cases which had been left for several days untouched, the bit of 
the coil near the cut edge had assumed an opaque and curiously pink appearance. 
The capsules at this part on examination proved to contain most strange and irregular- 
looking embryos, which were, however, in a high state of activity, moving about by 
means of their cilia, as though their distorted conformation made little difference to 
their vitality. Before proceeding further, I may, however, say that I did not succeed 
in bringing such embryos on to an advanced stage of development. ‘Two of these 
embryos are represented in Plate 6. figs. A, B, C. Most were similar in condition to 
that represented in figs. B, C; but some were as abortive and shapeless as that of 
fig. A. Of that embryo I have nothing special to say beyond drawing attention to its 
