TEETH IN APERTURE OF SNAILS 



63 



enabled, by closing their valves, to baffle the assault of their 

 smaller foes, and the operculum of univalves, both marine and 

 land, serves a similar purpose. Many land Mollusca, especially 

 Helix and Pupa^ as well as a number of Auriculidae^ have the 

 inside of the aperture beset with teeth, which are sometimes so 

 numerous and so large that it is puzzling to understand how the 

 animal can ever come out of its shell, or, having come out, can 

 ever draw itself back again. Several striking cases of these 

 toothed apertures are given in Fig. 24. Whatever may be the 



Fig. 24. — Illustrating the elaborate arrangement of teeth in the aperture of some 

 land Pulmonata. A. Helix {Labyrinthus) hifurcata Desh., Equador. B. H. 

 {Pleurodonta) picturata Ad., Jamaica. C. H. {Dentellaria) nux denticulata 

 Chem., Demerara. D. Anostoma carinatum Pfr., Brazil; a, tube communicating 

 with interior of shell. E. H. {Stenotrema) stenotrema Fer., Tennessee, X f. 

 F. H. {Polygyra) auHculata Say, Florida, X f. G. H. (Plectopylis) refuga 

 Gld., Tenasserim (a and b x 2). 



origin of these teeth, there can be little doubt that their extreme 

 development must have a protective result in opposing a barrier 

 to the entrance, predatory or simply inquisitive, of beetles and 

 other insects. Sometimes, it will be noticed ((7), the aperture 

 itself is fairly simple, but a formidable array of obstacles is 

 encountered a little way in. It is possible that the froth emitted 

 by many land snails has a similar effect in involving an irritating 

 intruder in a mass of sticky slime. The mucus of slugs and 

 snails, on the other hand, is more probably, besides its use in 

 facilitating locomotion, a contrivance for checking evaporation, 

 by surrounding the exposed parts of their bodies with a viscid 

 medium. 



Some species of Lima shelter themselves in a nest constructed 

 of all kinds of marine refuse, held together by byssiferous threads. 



