THE GASTROPODA 81 
pulmonary or for branchial respiration. On the right side of the 
pallial opening the mantle border sometimes bears a tentacle, as in 
Valvata (Fig. 132), Oliva, Strombus, Acera, and Gastropteron. In 
Aldeorbis there are two such tentacles (Fig. 133). In many Tecti- 
branchs the mantle edge at the right side of the pallial opening 
bears a large inferior pallial lobe (Fig. 148, I), which forms the 
“balancer” in the Thecosomata. 'Fhis lobe is also found in the 
basommatophorous or aquatic Pulmonates, and in some species of 
this group it is converted into a pallial branchia (Figs. 89 and 175). 
The dorsal surface of the mantle secretes a shell, formed of a 
single piece, which necessarily reproduces the form of the mantle, 
or rather of the visceral sac contained in the mantle. As the 
visceral sac is always coiled (even in forms with conical shells like 
the Patellidae and Fissurellidae and in the various Gastropods which 
are naked when adult the visceral sac is coiled during develop- 
ment), it follows that the shell is also coiled. The curvature of the 
coil, or conchospiral, is, generally speaking, a logarithmic spiral. 
The spire, that is to say, the totality of the whorls, with the excep- 
tion of the last formed, may be excessively prominent, as, for 
example, Terebra, Turritella, Turbonilla, certain Cerithiidae, etc., or 
may exhibit every possible disposition, until the prominence dis- 
appears and the shell becomes discoidal as in Planorbis, Atlanta 
(Fig. 141), ete. 
The various whorls of the spire are normally contiguous, but 
it occasionally happens that, after a certain number of turns, the 
visceral mass and the shell appear to unroll more or less completely, 
and to continue their course either in a much looser spiral or in a 
slightly curved line, or even in a nearly straight line (Vermetus, 
Fig. 45, Magilus, Cyclosurus, Caecum, Fig. 68). The extremity of 
the last whorl may also form a certain angle with the direction of 
the preceding whorls, as, for example, in certain helicomorphous 
Pulmonates (A nostoma). 
The coil, commencing from the initial point of formation or 
summit, is dextral when the shell, held with the summit towards 
the observer, has the mouth or aperture below and to the right. 
It is sinistral when, under the same conditions, the aperture is to 
the left. Dextral shells are much more common than sinistral. 
This direction of the coil, when it is not obscured by “ hyper- 
strophy,” is conformable with that of the asymmetry of the organ- 
isation ; that is to say, a sinistral coil corresponds completely to the 
situs inversus viscerum of a dextral Gastropod. This sztus inversus 
may be seen in the genera 7’riforis, Laeocochlis, Actaeonia, Blauneria, 
Clausilia, Physa ; in certain species of the genera Fulgur, Neptunea, 
Bulimulus, Helicter, Vertigo, Ariophanta (Nanina), Ancylus, Diplom- 
matina ; and in some teratological individuals of Bucconwm undatum, 
Littorina littorea, Neptunea antiqua, Limnea stagnalis (in which the 
6 
