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, Acer a, and Gastrupteron. In 

 Adeorbis 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. This 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 discoid al as in PlanorUs, Atlanta 

 (Fig. 141), etc. 



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, Cydosurus, 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 (Anostoma). 



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 correspords completely to the 

 situs inversus viscerum of a dextral Gastropod. This situs inversus 

 may be seen in the genera Trifwis, 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 Buccinum undatum, 

 Littorina littorea, Neptunea antiqua, Limnea stagnalis (in which the 



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