MR. J. D. MACDONALD ON THE ANATOMY OF EURYBIA GAUDICHAUDI. 247 
one on either side of the head, having the borders beset with large vibratile cilia. A 
frontal elevation rises up between them, from which, it would appear, the future tentacula 
spring. At this period of development the rudimentary external parts exhibit a loose 
cellular structure, the basis of the more complex tissues of the full-grown animal. 
The cephalic ganglia show themselves very distinctly, through the transparent enve- 
lopes, as disproportionately large and rounded masses; and the auditory sacs, containing 
a single primordial otolithe, lie quite in contact with their nervous centres. 
The intestine may be seen taking a simple horse-shoe flexure towards the neural 
surface, while the gastric dilatation fills up the hollow of the curve to a considerable 
extent. i 
The organ above described as corresponding with the spermatheca of the Pulmoniferous 
Gasteropods is just visible on the right side; and what appears to be the true vesicula 
seminalis on the left. 
The external surface of the mantle is pretty equally divided into three zones, by two 
ciliated rings disposed transversely, and consisting of a single series of prominent cells 
bearing moderately. large cilia. 
The development of the cartilage-cells of the mantle may be traced at any part, from 
the round transparent nucleus and its acquisition of a membranous investment, to their 
perfect condition as structural elements. 
The fibrous tissue may be also seen in course of evolution, from very thin-walled cells 
formed upon plastic but minutely granular nuclei. Having attained a certain size, the 
cell-wall shoots forth into delicate tubular processes, which either spring from two oppo- 
site ends; or irregularly from several parts of the circumference. As the nutrition of the 
cells goes forward, the simple tubular processes increase in length and in bulk, more 
particularly at the base, and break up into numerous and exquisitely delicate ramusculi, 
by which the ultimate fibrillation of the body of the cell seems to be effected. The 
elongated or fusiform nuclei permanently remain to strengthen the tissue, which now 
begins to assume a reticulated structure in the intercellular spaces. 
The remains of the ciliated rings of this embryo are distinguishable in Burybie of 
considerable size; and from what has been above noticed of the anatomy of the animal 
and the intimate structure of the mantle (which, though answering the purpose of a 
shell, is, strictly speaking, not homologous with such), it is evident that this genus is not 
conformable to the characters of the Thecosomata or first section of the Pteropoda pro- 
posed by De Blainville. 
The principal points of difference may be thus analysed :— 
EURYBIA. 
1. No external shell. 
2, Head and neck quite distinct. 
3. The foot is small, and only on this account 
rudimentary; the tentacula are large, and not 
combined with the fins. 
4. The mouth is not situated in a cavity formed 
by the locomotive organs. à 
5. This last particular may possibly apply. 
THECOSOMATA. 
1. Animal furnished with an external shell. 
2. Head indistinct. 
3. Foot and tentacula rudimentary, combined 
with the fins. 
4. Mouth situated in a cavity formed by the 
union of the locomotive organs. 
5. Respiratory organs contained within the man- 
.. tle-cavity. 
