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 Ijetween them, from vi^hich, it wovdd appear, the future tentacula 

 spring. At this period of development tlie rudimentary external parts exhibit a loose 

 ecUular structure, the basis of the more complex tissues of the full-grown animal. 



The ceplialic 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 cui-ve to a consideral)le 

 extent. 



The organ aboA-^ described as corresponding with the spermatheca of the Pulmoniferous 

 Gasteropods is just visiljle on the right side ; and what appears to be the true vesicula 

 seminalis on the left. 



The external sm-face of the mantle is pretty equally divided into three zones, by two 

 ciliated rings disposed transversely, and consisting of a single series of prominent ceDs 

 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 dehcate 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 tul)ular 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, winch now 

 begins to assiime a reticulated structure in the intercellular spaces. 



The remains of the ciliated rings of this embryo are distinguishable in Eurybim 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 : — 



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. 



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. 



5. This last particular may possibly apply. 



