502 CONCHIFERA. 



for, seeing that it is always fixed and firmly adherent at all points of its 

 attachment. In order to understand the circumstances 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 connexion between the 

 two, and, like the rest of the pallial 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. 



(1322.) We have as yet limited ourselves almost exclusively to a de- 

 scription of the simplest forms of CONCHIFEHA, namely those belonging 

 to the Ostracean family, which, being generally incapable of locomotion, 

 are deprived of a foot, and are recognizable by having the two lobes of 

 the mantle unconnected with each other around their entire circum- 

 ference. 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 

 degrees enclosed by the pallial membranes, and seem, as it were, saccu- 

 lated. Moreover, sometimes the mantle is prolonged into membranous 

 tubes of considerable length called si/phons, through which the water is 

 conveyed to the gills, and excrementitious matters expelled from the 

 body. In the Mussels (Mytilacea) the edges of the mantle are partially 

 joined, so as to present two apertures, through one of which the foot is 

 protruded, while the other, the smaller of the two, gives issue to the 

 excrement. A third family (Chamacea) has the circumference of the two 

 divisions of the mantle still more intimately united, leaving three di- 

 stinct fissures one for the passage of the foot, another for the entrance 

 of water to the branchiae, and a third for the ejection of matter from the 

 rectum. Of these, some are of gigantic dimensions, and fix themselves 

 by a strong byssus. One species, indeed (Tridacne gigas), is so enormous 

 in its size, that its shells alone not unfrequently weigh upwards of two 

 hundred pounds, and hatchets are employed to chop its thick and tendi- 

 nous cables from the rock to which it holds. 



(1323.) The Cockle family (Cardiaced) is recognized by having the 

 mantle open anteriorly, but prolonged at one extremity into two tubes, 

 one of which admits the water for respiration, while the other discharges 

 effete matter. In the Cockle (Cardium) the tubes are short, and scarcely 

 reach beyond the shell (fig. 252, a) ; but in other genera, as, for example, 

 Mactra (fig. 250, 6, c), they are of such length, that, when extended, 

 they protrude to a considerable distance. We at once perceive the use of 

 the tubular arrangement of the mantle here referred to, when we reflect 



