Foot with anterior transverse groove; a posterior pallial tentacle; generally burrowing.
Arcidae.-Borders of the mantle bear compound pallial eyes.
In the Arcidae the pallial eyes are compound or faceted somewhat like those of Arthropods.
Foot without parapodia; no pallial cavity, but always a single ctenidium situated on the right side between mantle and foot.
Cl, Cloacal or pallial chamber of Neomeniae and Chaetoderma.
There are two pairs of longitudinal cords, a pedal pair situated ventrally and united beneath the intestine by numerous commissures, and a pallial pair situated laterally and continuous with one another above the rectum (fig.
A simple gonaduct on each side arises from the gonad near its posterior end and passes first forwards, then backwards, and lastly outwards to the external opening in the pallial groove, anterior to the renal aperture.
Posteriorly the mantle forms a large pallial lobe FIG.
Mantle with two posterior appendages; ctenidium large and capable of protrusion from pallial cavity.
The eggs may be laid separately invested by a chitinous envelope, or as in Ischnochiton magdalenensis they may form strings containing nearly 200,000 eggs, or the ova may be retained in the pallial groove and undergo development there, as in Chiton polii and Hemiarthrum setulosum.