MOLLUSCA. 265 



It is remarkable that in Pisidium a veliger stage does not occur. This is 

 probably 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 process from it 

 grows towards and meets the epiblast, and gives rise to the oesophagus 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 Gasteropods. According 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. 



The foot is large, and ciliated anteriorly. Otolithic sacks and peda 

 ganglia are developed in it very early. 



Unio. The ovum of Anodonta and Unio is enveloped in a vitelline 

 membrane, the surface of which is raised into a projecting trumpet-like tube 

 perforated at its extremity (fig. 12). This structure is the micropyle. The 

 micropyle disappears in Anodonta piscinalis when the egg is ripe, but in 

 Unio persists during the whole development. The ova are transported, in a 

 manner not certainly made out, into the space between the two limbs of the 

 outer gills of the mother, and there undergo their early development. The 

 animal or upper pole of the egg is placed at the pole opposite to the 

 micropyle. 



The segmentation is unequal (vide p. 100) and results in the formation of 

 a blastosphere with a large segmentation cavity. The greater part of the 

 circumference of the egg is formed of small uniform spheres, but the lower 

 (with reference to the segmentation) pole is taken up by a single large cell. 

 The small spheres become the epiblast, and the large cell gives rise to 

 hypoblast and mesoblast 1 . 



1 The account of the remainder of the development till the larva becomes hatched 

 is taken from Rabl, No. 292. 



