Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 33G COMPARATIVE ANATOMY. primitively superficial position, although, they become covered over by folds of other regions of the integument (mantle), and so come to be placed in a special cavity—the branchial cavity. The function of respiration is part of the duty of the integu- ment, but it does not seem to be always localised in homologous regions, so that we cannot regard all the organs which appear to be gills as morphologically identical. As a rule the gills of the Mollusca are processe
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 33G COMPARATIVE ANATOMY. primitively superficial position, although, they become covered over by folds of other regions of the integument (mantle), and so come to be placed in a special cavity—the branchial cavity. The function of respiration is part of the duty of the integu- ment, but it does not seem to be always localised in homologous regions, so that we cannot regard all the organs which appear to be gills as morphologically identical. As a rule the gills of the Mollusca are processes which are placed at the sides of the body, and when least metamorphosed arise between the foot and the mantle (cf. Fig. 167, A B br). They vary very greatly, not only as to the extent of the body which they occupy, but also in the way in which they are connected with different parts. In the Placophora they merely form a series of folds or lamella?, which encircle the body between the foot and the mantle, and appear to be formed from the epipodium (epipodial gills). In the Lamellibranchiata they form lamellar organs, which project between the mantle and the visceral sac, which ends with the foot, into the cavity enclosed on either side by the mantle (Fig. 176, br br'). Their free edge is directed towards the ventral surface. Almost all the Lamellibranchiata have two pairs of these gills on either side, an inner pair, which are placed mediad, and an outer pair at the sides of these. The former are often the larger. Ex- cept in Anomia, where there are a large number of other adaptive modifications, the gills are arranged symmetrically. Each gill-lamella is developed from a row of pro- cesses which bud out close to one another; in many forms these pro- cesses remain separate from one another, and form separate branchial filaments, parallel with each other (Mytilus, Avicula, Area, Pectun- culns, Pecten, Spondylus). In most, however, the gills lose this embryonic condition, owing
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