Palæontology--Invertebrate

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1902 Excerpt: ...foot (/). This is a laterally flattened muscular organ, frequently hatchet-1 or ploughshare-shaped, and is used for crawling, or for burrowing in sand or mud. Sometimes, as in the case of Trigonia, by means of a rapid movement, it enables the 1 Hence the name Pelecypoda used by some authors for this class. Fig. 84. Mya arenaria. The left valve and mantle and half the siphons have been removed, a, anterior adductor muscle; a', posterior adductor; b, visceral mass; c, cloacal chamber into which the anus opens; /, foot; g, branchiae; h, heart; m, cut edge of the mantle; o, mouth; p, edge of mantle; s, branchial siphon; s', anal siphon; t, labial palps; v, anus. (From Woodward.) animal to jump to a considerable distance. In the genus Mytilus the foot is very much reduced; in others which have lost the power of locomotion (e.g. Ostrea) it is absent altogether. On the posterior part of the foot there is in some genera (e.g. Mytilus, Avicula, Area) a gland which secretes a bundle of horny fibres, known as the byssus, by means of which the animal moors itself to foreign objects. On each side of the foot, between it and the mantle, and attached to the body dorsally, are the gills or branchiae (fig. 84, g); these consist of filaments which usually become connected so as to form leaf-or plate-like bodies, whence the name Lamellibranchia. In some forms, the margins of the two mantle-lobes although in contact are not united, and when this is the case there are usually at the posterior margin two openings leading from the exterior to the mantle-cavity; these are produced by adjoining excavations or notches in the two lobes of the mantle. A current of water, caused by the cilia on the gills and mantle, flows in through the ventral opening, and provides the animal with foo...