TTNITALVE SHELL AND ITS OPERCULUM. 207 



tudinal direction with I'espect to the animal ; while the shells of 

 Conchifera, being applied to the sides of the body, necessitate tlie 

 transverse direction of the adductors. But we shall have little 

 occasion to disturb the latter from their position while we attempt 

 to follow those transitional steps by which, in theory, we may trace 

 the conversion of the physical condition of the Conchifer (in har- 

 mony with the slight metamorphosis of its testaceous covering) 

 into that of the Gasteropod. If, then, while the shell-metamor- 

 phosis is going forward we suppose the body of the Conchifer, now 

 occupying its left valve, to revolve, irrespective of the adductors, 

 from left to right on its longitudinal and transverse axes, in both 

 cases moving through a quarter of a circle, the foot and oral 

 extremity at the same time undergoing certain changes (the latter 

 especially, by the acquisition of new parts), the characters of the 

 resulting being wiU closely resemble those of Navicella, to which 

 we have already alluded. 



All the external parts of the Gasteropod or the Conchifer 

 appear to be superimposed, as it were, upon each other in a definite 

 order, and so disposed as to efiect a more or less perfect bilateral 

 symmetry. Proceeding from above downwards, they admit of 

 arrangement into four distinct systems, as in the following Table. 

 The principal openings are the Oral, Anal, and Generative, — the 

 Oral and Generative between the ''basijwdial^^ and ^^ einpodial''* 

 systems, and the Anal between the gills in the Gasteropod, in which 

 therefore all are in advance of the retractors. In the Conchifer, 

 on the other hand, the epipodial system being absent, the oral 

 orifice lies between the gills, or rather their anterior appendices, 

 below the anterior adductor, the anal orifice opens between the two 

 adductors, and the generative openings are below the posterior 

 adductor. The particulars just mentioned constitute the more 

 essential points of difference between the Gasteropod and the 

 Conchifer ; but the subject wiU be better understood by reference 

 to the Table. [See next page.] 



Although an abdomen may be said to exist in all cases, it can 

 scarcely be regarded as a distinct part of the animal, being merely 

 the space included by all the external organs and containing the 

 viscera ; and the " soma " or body is formed by the union of all these 

 organs enclosing the abdamen. In the case of the Gasteropod 

 therefore, the viscera are contained in the left-mantle pedicle, pro- 

 tected by the left valve, which becomes concave to receive them. 

 The right-mantle pedicle, bearing the operculiun, from its usual 

 position has hitherto been mistaken as a part of the foot, and called 



