STRUCTURE OF THE MOLLUSCA. 201 



but sometimes approximated so as to seem single, are re- 

 laxed. All the species are aquatic, some residing in fresh 

 water, but the greater number in the sea. They feed on 

 organic particles carried to the mouth by the water through 

 the lower siphon. Their generative system consists of an 

 ovary enveloped in the visceral mass, and the individuals do 

 not impregnate each other. 



This class may be divided into several great families; but 

 the species are so nearly allied in structure that it has not 

 been found expedient to arrange them into orders. 



The Ostracea have no foot, or a very small one, and for 

 the most part live fixed to submarine bodies, either by the 

 shell, or by their byssus ; their mantle is open behind as well 

 as before, and its two lobes nowhere unite to form particular 

 apertures, as is the case with all the other Lamellibranchiata. 

 Some of them have a single adductor muscle, while others 

 have two. 



Among the former are the Ostrece, or oysters, having a 

 coarse laminated shell ; and the Pectines, or clams, having 

 a divergently ribbed shell. To the bimuscular series belong 

 the MeleagrincE, or pearl-oysters, and several other genera. 



The Mytilina have the mantle open beneath, coherent 

 behind, forming a single orilice ; the foot slender, tongue- 

 shaped, with a byssus at its base behind ; the shell regular, 

 equivalve, very inequilateral. To this family belong the 

 genera Mytilus, Modiola, Pinna, and others. 



The Unionina have the mantle open beneath, coherent 

 behind, forming two orifices, the lower incomplete and bar- 

 bate ; the foot large ; the shell regular, equivalve, inequi- 

 lateral, with a strong epidermis. They are fresh water ani- 

 mals. Unio, Alasmodon, Anodon. 



The Arcina have the mantle open in its whole length, 

 and destitute of tubes ; the foot very short, thick, and trun- 

 cated ; the shell equivalve, inequilateral, with numerous 

 small teeth along the hinge. Area, Nucula, Pectunculus. 



The Chamacea have the mantle closed, with three aper- 

 tures, one for the foot, the two posterior for respiration and 

 the passage of the excrement, but without tubes; the valves 



