MOLLUSCA — CLASS I. TROPIOPODA. 31 



turn amplectente, lobos duos laminiformes forraante, laminis vel 

 liberis vel antice coadunatis. Branchiae externa? intra corpus et 

 pallium reconditae. Circulatio simplex corde uniloculari. 



Testa semper bivalvis, modo libera, modo affixa ; valvis cardine 

 cum ligamento vel externo vel interno conjunctis ; partibus testa- 

 ceis accessoriis, valvis alienis, testam interdum amplificantibus. 



The Tropiopoda or keel-footed Mollusca, the Acephala of Cuvier, the 

 Conchifera of Lamarck, constitute a numerous and well-defined class of 

 soft inarticulated animals, having the body enveloped in a large mantle 

 forming two laminiform lobes, each of which is protected by a separate 

 testaceous valve, hinging either externally or internally by means of 

 a strong dorsal horny cartilage or ligament. The animal is entirely 

 destitute of head or eyes, but is provided with a mouth, situated behind 

 the union of the lobes of the mantle ; and the lower part of the bodv is 

 dilated into a keel-shaped foot, giving it full power of locomotion, when 

 not attached to any marine substance ; it is obvious, however, that in so 

 extensive a series of animals, this organ passes through a considerable 

 modification of form. The branchiae are large, vascular and crescent- 

 shaped, placed on each side between the body and the bilobed mantle ; 

 and there exists a perfect system of circulation, performed by a small 

 heart composed of a single ventricle. The lobes of the mantle are fringed 

 round the edge with numerous tentacular filaments, very sensitive and 

 irritable to the touch, and in constant activity, drawing a current of 

 water within for the capture of prey. There appears to be no distinction 

 of sex, the animal being hermaphrodite. 



The shell of the Tropiopoda is always essentially bivalve, composed of 

 two opposite pieces or valves, joined internally or externally near their 

 dorsal edges, by means of a strong coriaceous cartilaginous ligament, the 

 elasticity of which tends to open the valves, in opposition to the con- 

 tractile action of the adductor muscle, by which they are internally 

 attached to the animal. They are equal or unequal, close or gaping, and 

 generally furnished on the dorsal margin with numerous denticulations 

 closely interlocking with each other. These denticulations are techni- 



