388 CONCHIFEllA. 



of the shelly valves until the period when the included mollusks 

 arrive at their mature size, the adductor muscle or muscles have 

 been of necessity perpetually changing their position, advancing 

 gradually forward as the enlargement of the shells was accomplished, 

 so as to maintain in the adult precisely the same relative situations 

 as they originally did in the young and as yet minute animal. 

 Taking the Oyster for an example, it is quite obvious that the 

 adductor muscle, which at first was connected with the thin and 

 minute lamellae forming the earliest shell, has, during the entire 

 growth of the animal, become further removed from the hinge, and 

 transferred from layer to layer as the shell increased in thickness, 

 till it arrives at the position occupied by it in connection with the 

 last-formed stratum that lines the interior of the ponderous valves 

 of the full-grown oyster. The manner in which this progressive 

 advance of the adductor muscle is effected is not at first easily 

 accounted for, seeing that it is always fixed and firmly adherent at 

 all points of its attachment. In order to understand the circum- 

 stances connected with its apparent removal, it is necessary to 

 premise that a thin layer of the mantle itself is interposed between 

 the extremities of the muscle and the inner surface of the shell, 

 forming the bond of connection between the two, and, like the rest 

 of the pallia! membrane, assisting in increasing the thickness of the 

 shell by adding layers of nacre to its inner surface. Particle after 

 particle is laid on by a kind of interstitial deposit between the 

 mantle and the extremity of the adductor muscle, but so gradually, 

 that the firm attachment between the muscle and the shell is not 

 at all interfered with ; and as the animal grows the transference of 

 the muscle from layer to layer is thus slowly and imperceptibly 

 effected. 



(424.) We have, as yet, limited ourselves almost exclusively to 

 a description of the simplest forms of CONCHIFERA, namely, those 

 belonging to the Ostracean family, which, being generally inca- 

 pable of locomotion, are deprived of a foot, and are recognisable 

 by having the two lobes of the mantle unconnected w^th each other 

 around their entire circumference. On turning our attention to 

 the organization of the mantle in other families, we find that 

 in them it no longer offers the same simple arrangement ; but, 

 the two lobes becoming gradually more and more completely 

 united along their edges, the bodies of the mollusks are by de- 

 grees enclosed by the pallial membranes, and seem, as it were, 

 sacculated ; moreover, sometimes the mantle is prolonged into 



