336 



COMPARATIVE ANATOMY. 



primitively superficial position, althougli 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 MoUusca 

 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 hr). 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 lamellte, 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. 1 76, hr hr') . 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 ; iu many forms these pro- 

 cesses remain separate from one another, and form separate branchial 

 filaments, parallel with each other (Mytilus, Avicula, Area, Pectun- 

 culus, Pecten, Spondylus). In most, however, the gills lose this 

 embryonic condition, owing to the concrescence of the gill-filaments. 

 Owing to this union of the flattened filaments or lamellse, which have 

 their surfaces directed towards one another, a gill-plate is formed; 

 this is either effected by the mere adhesion of the filaments, or by con- 

 crescence ; in the latter case pad-like projections appear on each of 

 the gill-filaments at regular distances from one another, and these 

 unite together. As fine clefts are left between these junctions, by 

 which the water passes through the gills, each plate forms a kind of 

 lattice-work. These filaments are not simple prolongations, but loops, 

 so that they enclose a space (intrabranchial space) ; when the gill- 



Fig. 176. Vertical section through an 

 Anodonta. m Mantle, hr Outer; 

 br' Inner gill-lamella. / Foot, v Van. 

 tricle. a Auricle. 2W' Pericardial 

 cavity, i Enteric canal. 



