At a somewhat later stage the gills appear as a linear series of small independent buds within the folds of the mantle behind the foot ([fig. 120], br). The anterior 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 character as those in many Pteropods and Gasteropods, where the cells of the hepatic diverticula, 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 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 œsophagus 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[108].