MOLLUSC A. 



219 



B 



FIG. 120. DIAGRAMMATIC VIEW 

 OF ADVANCED LAKVA OF PlSIBIDM. 

 (Copied from Laukester.) 



m. mouth; a. anus; B. organ 

 ofBojanus; inn. mantle;/, foot. 



In a slightly later stage the shell gland rapidly increases in size and 

 flattens out, and on the two sides of it there appear the rudiments of the 

 two valves, which are at first quite distinct, and separated by a considerable 

 interval (fig. 120). Before the appearance of the valves of the shell, the 

 mantle folds have already grown out from the sides of the body. 



At a somewhat later stage the gills 

 appear as a linear series of small inde- 

 pendent buds within the folds of the mantle 

 behind the foot (fig. 120, br). The an- 

 terior adductor also becomes differentiated. 



The alimentary tract in the meantime 

 has undergone considerable changes. The 

 primitive lateral lobes dilate enormously 

 and become ciliated. At a still later stage 

 their walls undergo peculiar changes, the 

 nature of which is somewhat obscure, but 

 they appear to me to be of the same charac- 

 ter as those in many Pteropods and Gas- 

 teropods, where the cells of the hepatic 

 di verticu la, to which the lobes of Pisidium 

 apparently correspond, become filled with 

 an albuminous material. 



The later stages in Pisidium have not 

 been followed. 



It is remarkable that in Pisidium a 

 veliger stage does not occur. This is pro- 

 bably due to the development taking place within the brood-pouch. The 

 late development of the otocysts is also remarkable. A byssus-gland 

 was not formed up to the stage observed. In Cyclas calyculata (Schmidt), 

 a byssus-gland also appears to be absent. 



Cyclas. The development of Cyclas as described by Von Jhering 

 is very unlike that of Pisidium, and the differences would seem to be 

 too great to be accounted for except by errors of observation. 



The segmentation of Cyclas is similar to that of Anodon (vide p. 82), 

 and a mass of large cells enclosed by the smaller cells gives rise to the 

 hypoblast. In the interior of this mass there appears a lumen, and a pro- 

 cess from it grows towards and meets the epiblast, and gives rise to 

 the O3sophagus and mouth, a mode of development of these parts without 

 parallel amongst Mollusca. A very rudimentary velum would appear, 

 according to Leydig (No. 290), to be developed at the cephalic extremity. 

 A shell-gland is formed of the same character as in Gasteropoda. Accord- 

 ing to Leydig the shell appears as a single saddle-like structure on the 

 dorsal surface; the lateral parts of this become calcified, and give rise 

 to the two valves, but are united in the middle by the membranous median 

 portion. At the two sides of the body the mantle lobes are formed, as in 

 Pisidium. 



Very shortly after the formation of the shell the byssus-gland appears 

 as a pair of small follicles in the hinder part of the foot. It rapidly grows 

 larger and becomes a paired pyriform gland, in which are secreted the byssus 

 threads which serve to attach all the embryos at a common point to the 

 walls of the brood-pouch. 



