Sometimes this gland is found in the young and not in the adult (Anodonta, Unio, Cyclas).
The otocysts of Cyclas are peculiarly favourable for study on account of the transparency of the small foot in which they lie, and may be taken as typical of those of Lamellibranchs generally.
The next stage of modification is seen in Ostraea, Cyclas and some Lucinidae, in which the generative and renal ducts FIG.
In the allied genus Cyclas, a byssus gland is formed in the foot and subsequently disappears, but no such gland occurs in Pisidium.
Cycladidae.-One siphon or two free siphons with simple orifices; pallial line simple; hermaphrodite, embryos incubated in external gill-plate; freshwater, Cyclas; British.