10 MR. E. RAY LANKESTER ON THE 
highly refringent, entirely devoid of structure, and very conspicuous. These -are sur 
rounded by a coarsely granular matter. What relation these nuclei bear to the original 
elements of the hypoblast is not known. Nucleoli commence to appear in these nuclei 
(Plate 4. fig. 41), and these enlarge very much, developing secondary and tertiary 
nucleoli, whilst more than one primary nucleolus makes its appearance in each of the 
original refringent bodies. The explanation of this sudden and striking cell-develop- 
ment requires more extended study, but it looks like a rapid process of endogenous 
proliferation. 
Plate 4. fig. 43 is chiefly interesting as showing the position of the mouth (0) and 
the form of the mantle (mn). The great gastric chamber obscures the full view of the 
root of the foot, which is seen lying symmetrically between the flaps of the mantle 
behind the mouth. ‘ 
Plate 4. fig. 38.¢ sufficiently explains itself. Fig. 43 gives a view of detached cel- 
lular masses lying within the faintly indicated wall of the gastric chamber at a stage 
corresponding to that of fig. 37. Their significance is obscure. 
Plate 4. figs. 44 & 45 have been already alluded to as giving highly enlarged views 
of processes from the epidermis budding inward, which appear to be the foundations of 
the organ of Bosanus of one side. ‘They are seen in their natural position close to the 
rectal peduncle in Plate 3. figs. 36 & 37, B. 
Plate 4. fig. 46 again brings us a large step forward in the developmental history 
of Pisidium ; and beyond the stage here presented I have not followed it. The shell- 
valves (v) have increased largely in size; the mantle-border (mz), hanging like a skirt all 
round the foot, covers it more and more. But the new feature which marks this stage 
of development is the appearance of the rudiments of the branchie (47). On each side 
they are seen as four blunt processes springing from a line which runs towards the mouth 
from the angle formed posteriorly by the junction of the foot with the mantle. They 
now appear confined to this region at the posterior root of the foot; but later, as 
appears from Lrypia’s researches on Cyclas cornea, extend in the direction of their basal 
line towards the mouth—that is to say, new buds make their appearance along this line 
on each side of the foot, progressing from the posterior to the anterior pedal region. 
The origin of the gill-lamelle of the Lamellibranchiata as short stump-like tentacles 
which become ciliated has long since been worked out by Lovin. Its significance 
has been, I am inclined to think, overlooked. In relation to this matter I will now 
merely draw attention to the close general agreement of the disposition of these tentacular 
branchiz, the foot, the mouth, and the anus in this embryo Pisidium (see Plate 4. 
fig. 52), with the disposition of tentacles, epistoma, mouth, and anus in a Hippocrepian 
Polyzoon, or more strikingly with the same parts in the exceedingly interesting form 
Rhabdopleura, as worked out by M. G. O. Sars*, where, if the so-called “buccal shield” 
be taken as the equivalent of the Lamellibranch’s foot, the homology of the gill-tentacles 
in the two cases cannot appear doubtful. 
* Quart. Journ. Mier. Sci., Jan. 1874. 
