MOLLUSC A. 217 



tenerl and marked by a deepish depression (fig. 117 A). From Loven'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 stompdasum. 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 papillae. 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 valves 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 (TO) and ciliated oesophagus probably derived from 

 the stomodseum, 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 larvae 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 proctodseum would appear to 

 be formed later than the stomodseum, 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 invagina- 

 tion, presumably at the lower pole, now takes place, and gives rise to an 

 archenteric sack. 



The embryo now rapidly grows in size. The blastopore becomes 

 closed and the archenteric sack forms a small mass attached at one point 

 to the walls of the embryonic vesicle (fig. 119, hy). In the space be- 

 tween the walls of the archenteron and those of the embryonic vesicle 

 stellate mesoblast cells make their appearance, derived in the main from 

 the epiblast, though probably in part also from the hypoblastic vesicle 

 (vide fig. 119 C, p). The cavity between the hypoblast and epiblast, 

 which contains these cells, is the body cavity. Fig. 119 represents three 

 views of the embryo at this stage. A is a surface view shewing the 

 epiblast ; B is an optical section through the median plane shewing 

 the hypoblast and some of the mesoblast cells ; and C is an optical section 

 shewing the mesoblast cells. A prominence on one side of the embryo 



