The alimentary tract has by this time become differentiated (fig. 117 C). It consists of a mouth (_m_) and ciliated oesophagus 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. [FIG. 119. THREE VIEWS OF AN EMBRYO OF PISIDIUM IMMEDIATELY AFTER THE CLOSURE OF THE BLASTOPORE. (After Lankester.) A. View from the surface. B. Optical section through the median plane. C. Optical section through a plane a little below the surface. _ep._ epiblast; _me._ mesoblast; _hy._ hypoblast; _p._ cells apparently budding from the hypoblast to form mesoblastic elements.]