A row of similar organs is present in the larva of Teredo in front of the foot.
Cardium. As an example of a marine Lamellibranchiate I may take Cardium pygmaeum, the development of which has been studied by Lovén (No. [291]). The ova, surrounded by a thickish capsule, are impregnated in the cloaca. The segmentation takes place much as in Nassa (vide p. [101]), and the small segments gradually envelop the large hypoblast spheres; so that there would seem to be a gastrula by epibole. After the hypoblast has become enveloped by the epiblast, one side of the embryo is somewhat flattened and marked by a deepish depression ([fig. 117] A). From Lovén’s description it appears to me probable that the depression on the flattened side occupies the position of the blastopore, and that the depression itself is the stomodæum. At this stage the embryo becomes covered with short cilia which cause it to rotate within the egg-capsule.
Close above the mouth there appear two small papillæ. These gradually separate and give rise to a circular ridge covered with long cilia, which encircles the embryo anteriorly to the ventrally-placed mouth. This structure is the velum. In its centre is a single long flagellum ([fig. 117] B). Shortly after this the shell appears as a saddle-shaped structure on the hinder part of the dorsal surface of the embryo. It is formed at first of two halves which meet behind without the trace of a hinge ([fig. 117] C). The two halves rapidly grow and partially cover over the velum, and below them the mantle folds soon sprout out as lateral flaps.
The alimentary tract has by this time become differentiated ([fig. 117] C). It consists of a mouth (m) and ciliated œsophagus probably derived from the stomodæum, a stomach and intestine derived from the true hypoblast, and an hepatic organ consisting of two separate lobes opening into the stomach. The anus (an) appears not far behind the mouth, and between the two is a very slightly developed rudiment of the foot (b). The anterior adductor muscle (cm) appears at this stage, though the posterior is not yet differentiated.
The larva is now ready to be hatched, but the further stages of its development were not followed.
Ostrea. The larvæ of Ostrea, figured by Salensky (No. [293]), shew a close resemblance to those of Cardium. The velum is however a simple ring of cilia without a central flagellum. The proctodæum would appear to be formed later than the stomodæum, and the earliest stage figured is too far advanced to throw light on the position of the blastopore.
Pisidium. The development of Pisidium has been investigated by Lankester (No. [239]). The ovum is invested by a vitelline membrane and undergoes development in a brood-pouch at the base of the inner gill lamella.
The segmentation commences by a division into four equal spheres, each of which, as in so many other Mollusca, then gives rise by budding to a small sphere. The later stages of segmentation have not been followed in detail, but the result of segmentation is a blastosphere. An invagination, presumably at the lower pole, now takes place, and gives rise to an archenteric sack.
