262 LAMELLIBRANCHIATA. 



Eyes are frequently present in the larva, though they dis- 

 appear in the adult. In Montacuta and other types a pair of 

 these organs is formed at the base of the velum on each side of 

 the oesophagus, not far from the auditory sacks. They are 

 provided with a lens. 



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 Loven 

 (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 flat- 

 tened 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 stomodaeum. 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 (ai) and ciliated oesophagus probably derived from 

 the stomodieum, a stomach and intestine derived from the true hypoblast, 

 and an hepatic organ consisting of two separate lobes opening into the 

 stomach. The anus (aii) appears not far behind the mouth, and between 

 the two is a very slightly developed rudiment of the foot (&). 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 proctodreum would appear to 

 be formed later than the stoinodaeum, 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 



