TERRESTRIAL AIR-BREATHING MOLLUSKS. 



The number of eggs produced varies in the genera and species in the 

 same proportion as the dangers to which they are exposed are greater 

 or less. Thus, in the naked genera, whose means of protection and 

 whose chances of preservation are much less than of those protected by 

 an external shell, the number is much greater than in the latter. The 

 number of eggs produced by two individuals of Limax agrestis kept in 

 confinement by Dr. Leach was, in the course of rather more than a year, 

 seven hundred and eighty-six. It usually amounts to at least three 

 hundred per annum. The other species, though not equally prolific, 

 multiply greatly ; and each pair of the various shell-bearing species pro- 

 duces, annually, from thirty to one hundred eggs, and perhaps more. 

 The young of the Limaces complete their growth and reproduce their 

 kind sometimes within the year of their birth, and always as soon as 

 the second year ; and the species of the other families are believed not 

 to require a much longer time to attain maturity. This rapid increase 

 replaces the numbers annually destroyed, and maintains the species in 

 their relative importance. 



Their extreme tenacity of life is manifested in every stage of growth, 

 from the egg to the mature animal. The eggs of Limax have been so 

 entirely desiccated that their form has disappeared, and there remained 

 only a thin skin, friable between the fingers. In this condition they 

 have been kept for years ; and yet a single hour's exposure to humidity 

 was sufficient to restore their form and elasticity. 1 They have been 

 dried in a furnace eight successive times, until they were reduced to an 

 almost invisible minuteness, yet in every interval have regained their 

 original bulk in a moist situation. 2 In all these instances the young 

 have been developed in the same manner as in other eggs not subjected 

 to the experiment, In the northern part of the United States we have 

 frequently observed the eggs of tiie shell-bearing genera in the forest 

 covered with snow, protected only by a single leaf, where they had 

 remained through the winter months, constantly exposed to a tempera- 

 ture much below the freezing-point. The shell-bearing species them- 

 selves withstand the cold of the severest winters in the same situations ; 

 and Snccinea has been frozen in a solid block of ice, and yet escaped 

 unharmed. Helices, when frozen in a state of confinement, though they 

 sometimes recover so far as to move about with some activity, usually 

 survive but a short time. 



The power of reproduction of parts of the body is more astonishing 



1 Bouchard-C'hantereaux. 2 Leuchs. 



