DEVELOPMENTAL HISTOBY OE THE MOLLUSCA. 
11 
The mass of the central portion of the alimentary canal and its glands has in the 
present stage of development become dark granular, and its details very obscure. 
Anteriorly to the umbones of the shell-valves in the middle line appears a vesicle (?;) 
which lies below the surface, but is not imbedded in the tissue of the alimentary tract. 
One might take it for the commencing pericardium or cardiac ventricle, but that those 
structures certainly in later life lie posteriorly to the umbones. 
Plate 4. fig. 47 presents the same specimen as that of fig. 46, seen from the pedal 
aspect instead of the umbonal aspect. 
The drawing is not made so as to give a definite plane of optical section, but parts 
are allowed to show themselves in virtue of the partial translucency of the embryo. 
Plate 4. fig. 48 gives a more highly magnified view of the problematical vesicle, v, 
of figs. 47, 46. 
Plate 4. fig. 49. The same vesicle from another specimen, in which it is less strongly 
marked. 
Plate 4. fig. 50 represents an embryo a very little younger than that of Plate 4. 
figs. 46 & 47 (less developed by one gill-process), drawn with the camera lucida. The 
arrangement of the dark and clear masses in the central mass of tissue belonging to the 
alimentary tract is of interest as indicating an approaching differentiation into the coils 
of the intestine and the glandular adjacent liver. The rectum ( rp ) is here obvious, its 
walls having become thin and translucent as compared with their former condition, when 
we spoke of them as “ the rectal peduncle.” The anus is now perforate. At Ir the 
lumen of the rectum as it opens into the now much modified gastric chamber is seen. 
On either side the rectum two coiled tubes (B), the exact disposition of which it is 
impossible to make out on account of their delicacy and the not too great transparency 
of the body-wall, are to be observed. The position and character of these delicate 
structures renders it exceedingly probable that they are the future organs of Bojanus, 
and are developed from the rudiments marked B in earlier figures. 
In front of the shell-valves in this figure (50) a transverse striation lying below the 
surface Ad marks the commencing differentiation of the anterior adductor muscle. 
Plate 4. figs. 51 & 52 represent, somewhat schematically, an earlier and the present 
phase of the development of Pisidium. 
In figure 51 the arrows indicate the direction of ciliary currents, by which matters 
(chiefly or perhaps entirely liquid matter) are passed round the two lobes of the gastric 
chamber. 
At this stage my observations on Pisidium cease. There are some structures the 
rudiments of which I was continually in search of, which seem to deserve mention on 
account of their absence. For instance, the byssal gland figured by Leydjg in the foot 
of Cyclas cornea at an early period (quite within the period here gone over) was absent. 
No trace of any thickenings or invaginations to lay the foundation of the otocysts, 
nor of the cephalic, pedal, or branchial ganglia, was to be detected. 
c 2 
