of the Fishery Hoard for Scotland. 
297 
some distance along the surface from the blastopore, and as Horst * has 
pointed out in the case of the European oyster, as development advances, 
it is seen at the dorsal pole of the animal (PI. VII. fig. 30). This figure 
shows that the primitive shell of the embryo is in the same position as 
that occupied by the depression in figs. 27 and 28, and I believe that this 
depression in Pecten operadaris, as figured in 27, corresponds to the 
depression noticed by Lacaze-Duthiers f and by Davaine X in the oyster, 
and so fully described by Horst in the same species. On comparing 
this figure with that of the same stage (fig. 32) of Ostrea virginiana, 
observed by Brooks,§ it appears to me likely that the dorsal depression 
which lie regards as the blastopore, or opening of the primitive intestinal 
canal, is, as Horst rightly suggests, the depression of the preconchylian 
gland. Lankester§ was the first to discover the organ in Pisidium, and 
in some gasteropods, and subsequently Hatschek || described the same 
organ in Teredo. 
The relative position of the blastopore and the preconchylian gland is 
shown in figs. 28, 29, and 30. In fig. 29, which is a drawing of this stage, the 
embryo exhibits the velum, v, as foot-like structure projecting between the 
blastopore and preconchylian gland. Here it is seen to occupy only a very 
small portion of the oosphere, but in fig. 30 the velum is of much greater 
size, and the blastopore is further apart from the developing shell. In one 
specimen a distinct area, covered by the growing and extending valves of 
the shell, was manifest on one side, and a few clear vesicles were seen within 
the body of the embryo. The form is very irregular, and differs from the 
spherical form by a lobate process which projects (figured at the bottom 
of the drawing), and may possibly be the representative of the velum, 
though the cilia with which it is beset do not differ in length from the cilia 
covering other parts of the body. It is a highly irregular form, and but for 
the indication of the shell valve its right to be regarded as a stage at this 
point of development might be denied. 
In my observations a hiatus occurred here, and the next stage seen had 
a pair of very well developed shell valves. Fig. 32 is a view of the dorsal 
surface of the embryo, which shows the slightly divaricated valves of the 
shell above the large velum, which is extended beyond the margins of the 
valves. An indication of the liver is apparent through the valves by the 
dark patches shown on either side of the middle or hinge line. Fig. 33 
gives a lateral view of the embryo, with the velum partially extended. At 
the middle of the crown of the velum is a darkened patch, e.d., indicating 
the cephalic disk present also in other Lamellibranchs, but there is no 
flagellum present on the velum. The long row of cilia are seen on the 
velum, and posterior to it is the opening of the mouth, m. This leads by a 
ciliated canal or oesophagus, ce, the cilia being in constant motion upwards 
till it is hidden by the dark lobe of the liver, Iv, which is also seen to 
obscure the ventral half of the stomach, si. The intestine, in, is visible 
at one or two points, a loop being covered by the stomach and liver ; the 
rectum, r, is seen to ruu downwards and backwards till it opens (near the 
mouth) at the anus, a. Two velar muscles for retraction are seen, one being 
posterior, pr, and the other anterior, ar, both indicating a bifurcation at 
* hoc. ext., pp. 287, et seq. 
i " Memoire sur le developpement des Acephales lamellibranches," Comp. Rend, 
de VAc. de Sc. de Paris, vol. xxxix. p. 103; and " Nouvelles observations sur le de- 
velopement des huitres," Idem., p. 1197. 
$ " Recherches sur le generation des huitres," Comp. Mend, de la 8oc.de Biol., vol. 
iv., p. 297. 
§ "Contributions to the Developmental History of the Mollusca," Phil. Trans. , vol. 
165. 
|| " Ueber die Entwicklungsgeschichte van Teredo,''' Arb. Zool. Inst. Vienna, vol. iii. 
