MORSE: EARLY STAGE OF ACMAEA. 
315 
Fig. 4. 
with a stage 1 have discovered in Acmaca testudinaUs. Jn the young 
of this species, less than half a millimeter in length, though freely 
crawling about (Figs.' 2 and 3) I found a nucleus, as may be seen bv 
the figures, in which there was no suggestion of a coiled nautiloid shell. 
It was simply a caecal-like shell with slight dorsal flexure, and 
minutely granulated without lines of growth. In this stage the ten- 
tacles were short and thick and the eyes distinctly marked. In the 
anterior portion of this embryonic shell, on each side of its aperture 
as it joins the permanent shell, is 
a slightly elongated area rounded 
anteriorly — the outline of this 
area being parallel to the edge of 
the aperture. These markings 
may be called for convenience 
lateral folds (Fig. 4). The lines 
of growth of the permanent shell 
seem to be depressed at the middle part of these folds. Precisely what 
these folds may be in the embryo I do not know. Patten's figure of 
the advanced embryo in P. vulgaia gives no clue as to their meaning. 
No structure shows which might be instru- 
mental in making these folds. In the cicatrix 
left on the shell of Acmaea alveus (Fig. 5) 
after the disappearance of the embryo shell 
are right and left circular marks between the 
divided scar which may possibly be associ- 
ated with these folds. The lateral folds are 
exceedingly faint and can only be detected 
by direct light. This embryo shell is early 
separated by decollation, and apparently 
never passes beyond the condition of the 
early veliger stage. It may be observed that 
a portion of the viscera occupies the embryo 
shell, there being free communication be- 
tween the embryo shell and the new patelloid 
shell forming. A partition evidently forms 
before the embryo shell breaks away. It will be observed also that the 
embryo shell turns posteriorly and the increments of growth of the 
permanent patelloid shell are at the outset mainly on its anterior 
margin. In a short time the increments increase posteriorly, so that 
Fig. 5. 
