The foot has a byssus gland on its posterior surface.
In many Lamellibranchs a gland is found on the hinder surface of the foot in the mid line, which secretes a substance which sets into the form of threads - the so-called " byssus " - by means of which the animal can fix itself.
In Arca too and many others it carries a byssus-forming gland and a byssuscementing gland.
The glochidium quits the gillpouch of its parent and swims by alternate opening and shutting of the valves of its shell, as do adult Pecten and Lima, trailing at the same time a long byssus thread.
This byssus is not homologous with that of other Lamellibranchs, but originates from a single glandular epithelial cell embedded in the tissues on the dorsal anterior side of the adductor muscle.
In them the foot has a flat ventral surface used for creeping, as in Gastropods, the byssus gland is but slightly developed, the pleural ganglia are distinct, there is a relic of the pharyngeal cavity, in some forms with a pair of glandular sacs, the gonads retain their primitive connexion with the renal cavities, and the otocysts are open.
In the allied genus Cyclas, a byssus gland is formed in the foot and subsequently disappears, but no such gland occurs in Pisidium.
Anomia; byssus large and calcified; British.
Placuna; byssus atrophied in adult.
Pectunculus; foot without byssus; British.
Trigoniidae.-Shell thick; foot elongated, pointed in front and behind, ventral border sharp; byssus absent.
Lima; members of this genus form a nest by means of the byssus, or swim by clapping the valves of the shell together.
Tellinidae.-External gill-plate directed upwards; siphons separate and elongated; foot with byssus; palps very large; ligament external.
- Elongated burrowing forms; foot cylindrical, powerful, without byssus; shell long, truncated and gaping at each end.
The foot is a muscular mass without cuticle or skeleton, excepting certain cuticular structures such as the byssus of Lamellibranchs and the operculum of Gastropods, which do not aid in locomotion.
Chara, Fucus, Ulva and Conferva, and in ï¿½ part Tremella and Byssus - would to-day, in any sense in which the term is employed, be regarded as algae.
It is also remarkable for the small size of its foot and the large development of two glands in the foot - the byssus-forming and the byssus-cementing glands.
The byssus is a collection of horny threads by which the sea mussel (like many other Lamellibranch or bivalve molluscs) fixes itself to stones, rocks or submerged wood, but is not a permanent means of attachment, since it can be discarded by the animal, which, after a certain amount of locomotion, again fixes itself by new secretion of byssus from the foot.