8 MR. E. RAY LANKESTER ON THE 
no anal opening. The arrangement of these parts is seen best in the diagram, Plate 4. 
fig. 51. 
The mantle (mn) originates simply as a continuation of the rim of the pharynx, carried 
along each side of the foot, as is well seen in fig. 35. 
Plate 3. fig. 835a@ gives in a diagrammatic way a view of the border of the pharynx 
surrounding the oral opening (0), and the similar border of the commencing mantle-flap 
surrounding the foot (f). 
In Plate 3. fig. 36 an embryo of this period is seen in approximately complete optical 
section. The shell-gland, which belongs of course really to the surface, is introduced 
(sh), showing the double appearance which it has when focused thus, being in reality 
saddle-shaped, and extending on each side of the embryo a little way. In both this 
section and in Plate 3. figs. 30 & 35, certain large cells are conspicuous (y), which lie 
above the pharynx in what ought to be the cephalic region. I merely draw attention 
to them, but cannot offer any explanation of their late differentiation. They present 
the appearance of the earlier embryonic cells, and soon after this stage disappear. 
It is perhaps well briefly to mention (what becomes obvious from the study of this 
development in full) that there is nothing which corresponds to the velum of the 
“veliger form” of Gasteropod development, though some marine Lamellibranchs, pro- 
bably most, do exhibit a veliger stage. And it is even still more curious to note that 
not even at the earliest stage, when such a differentiation of parts might make itself 
apparent for a brief period, is there any thing which indicates or corresponds in the 
remotest degree morphologically toa head. ‘There is a gap between the region marked 
y in fig. 35 and the pharynx, which might be filled by a head with paired eyes and 
tentacles. These have been as completely suppressed as though they had been cut 
away, and the sides of the wound so formed healed without leaving a trace. 
In the section fig. 36 the differentiation of the cell-elements in the foot is to be 
observed, and the attachment of some of these fusiform muscular corpuscles to the 
stomach-wall. The lumen in the rectal peduncle is obvious, but it is also certain that 
the peduncle is as yet imperforate at its termination. A mass of tissue projecting 
inwards from the epiblast by the side of the rectal peduncle marked B is the rudiment 
of one of the paired “segmental organs,” or organs of Bosanus, of the mollusk. In 
Plate 4. figs. 44 & 45 much more highly magnified views of similar in-buddings from the 
epidermal layer, which occupied similar positions in other embryos, are given. ‘The 
position occupied subsequently by what are clearly enough the rudimentary Bojanian 
organs, makes it highly probable that these buds are their first commencement. In 
fig. 37 the same bud-like process is marked B. 
Plate 3. fig. 837 is of value as a step in this developmental. isto, for it helps to 
connect the phase just described with that which perhaps may be best understood by 
looking at figs. 39 & 43. Up to this point the embryo usually and readily presents a 
more or less accurate profile view of itself, lying on the glass slip with the foot to the 
right and the pharynx to the left, or vice versd. But the result of the immediately 
