192 



THE POPULAR SCIENCE MONTHLY. 



ing open, unprotected apertures (Helix clausa)* The former 

 were found to be alive after several days, but the unprotected 

 snails were eaten from their shells during the first night. 



From tropical America we will now follow a great circle half 

 round the globe, pausing in India or Ceylon. Here, too, the 

 snails are exposed to the conditions of free competition for life in 

 a tropical climate. What structures have been evolved in this 

 totally dissimilar snail -fauna, corresponding to the evolution- 

 products of American life under similar climatic conditions ? As 

 we would expect on a priori grounds, the protective structures, 

 while strictly analogous, are in no way homologous, having arisen 

 wholly independently in the two hemispheres. The Asiatic snails, 

 instead of developing projecting teeth upon the edge of the 

 aperture, have a system of calcareous blades or folds situated a 

 distance within the shell, behind which the animal retreats when 

 needful. The figures tell, better than any description, the ex- 

 treme degree of complication which has been attained by the 

 more highly organized forms. Beetles have occasionally been 

 found sticking in the interstices of the folds, unable either to 

 force their way into the interior or to extricate themselves and 

 retreat. 



The culminating point in the series of obstructive structures 

 is perhaps reached by the narrow-throated snail of China (Stego- 

 dera angusticollis) . In this bizarre form, the last of the spiral 



whorls is distorted and crowded 

 against the preceding volution, 

 producing an extremely narrow 

 passage into the more spacious 

 interior, as shown in the figure. 



But, in spite of these various 

 expedients for the protection of 

 the snail, they have some ene- 

 mies able to overcome or to 

 evade all obstacles. It is sad to 

 learn that in this case, too, civil 

 wars are the bloodiest ; the most 

 deadly of the " malacophagi " are 

 brother snails of the genera Se- 

 lenites and Glandina. When the hungry Selenites discovers a 

 temptingly juicy snail, a Helicina perhaps, the victim retreats 

 into his shell, barring the entrance with his strong door or oper- 

 culum. The Selenites thereupon sets to work cutting a hole 

 through the large whorl of the Helicina, in order to gain en- 

 trance behind the barricade. The tongue-like odontophore with 



Fig. 6. — Chinese Narrow-throated Snail. 

 The narrow throat is shown by the dotted 

 line. 



* The Helix uvulifera is a Southern, the other a Northern species. 



