GASTROPODA ANISOPLEURA. 473 



podium of the latter group are flattened laterally, and the mesopodium 

 frequently carries a sucker, whilst the metapodium forms, as a rule, a large 

 swimming tail. The metapodium often bears upon its dorsal surface in many 

 Azygobranchia, even when not divided off from the mesopodium, a spiral 

 and calcareous, or chitinoid operculum, which closes the aperture of the 

 shell when the animal is completely retracted within it. This operculum is 

 sometimes present in the embryo Opisthobranch, but is always lost in the 

 adult. Lateral lobes of the foot, extending for a greater or less extent 

 along its sides, and known as epipodia, are present in some Azygobranckia 

 and many palliate and a few non-palliate Opisthobranchia ; and the cephalic 

 hood or fold above the head, seen in many members of the last-named 

 group, is perhaps an anterior extension of these folds. The foot is com- 

 pletely aborted in some of the non-palliate Opisthobranchia, e. g. Phyllirhoe. 

 The visceral dome is not distinctly marked off in the order just named and 

 in some Pulmonata. Its torsion is sometimes ' reversed/ and certain genera 

 or species, e. g. the Whelk (Buccinum undatum) often show a great tendency 

 to this reversal. The dome varies much in size and development, and is 

 most marked in the Reptant Azygobranchia and some stylommatophorous 

 Pulmonata. So too with the mantle fold. It maybe aborted altogether in 

 the adult of some Opisthobranchia, hence termed Non-Palliata, e. g. Doris, 

 Eolis, Phyllirhoe, or become insignificant in size, as in some Pulmonata 

 (Limacidae). It generally forms a low ridge, but is well developed when it 

 incloses the ctenidia in a branchial cavity, which opens either to the right 

 side or anteriorly. The aperture of this branchial cavity in most Pulmonata 

 is reduced to a small contractile pore by the concrescence of the free edge 

 of the mantle with the body wall. The edge of the mantle is often 

 produced into variously-shaped processes, and is sometimes reflected 

 over the edges of the shell when the animal is fully expanded temporarily, 

 as in many Streptoneura, or permanently as in Aplysia among palliate 

 Opisthobranchia. 



The primitive shell-sac of the embryo appears to persist in some 

 Pulmonata, and incloses in Arion a mass of crystals of Calcium carbonate, 

 or a small shell as in Limax. The shell of Clausilia is also stated to originate 

 within it in the first instance. But as a rule it disappears, or only con- 

 tributes a minute portion to the shell, which is formed by the edges and 

 surface of the mantle and visceral dome. The size of the shell is therefore 

 correlated with the size of these two regions. Its shape is variable : some- 

 times plate-like, e. g. Aplysia ; conical, as in the Limpets Fissurella and 

 Patella ; usually spiral. The axis of this spiral may be steep, or at various 

 degrees of obliquity : almost horizontal, forming a disc-like shell in Planorbis. 

 In some cases successive coils inclose one another more or less completely, 

 e. g. Conus or Cypraea. The spiral twist follows that of the visceral dome, 

 and is therefore usually right-handed or dextral : and it is rare for a left- 



