DWELLINGS. 15 1 



hunger, and return as soon as they can in case of 

 danger. 



A large number of animals also hollow out shelters 

 for their eggs, with the double object of maintaining 

 them at a constant temperature and of concealing 

 them. Most reptiles act in this manner. The way 

 in which a Tortoise, the Cistudo lunaria, prepares its 

 nest is extremely curious. When the time for this 

 labour arrives, the tortoise chooses a site. It com- 

 mences by boring in the earth with the end of its tail, 

 the muscles of which are held firmly contracted ; it 

 turns the tail like a gimlet and succeeds in making a 

 conical hole. Gradually the depth of the hole becomes 

 equal to the length of the tail, and the tool then 

 becomes useless. The Cistudo enlarges the cavity 

 with the help of its posterior legs. Using them 

 alternately it withdraws the earth and kicks it away, 

 then piles up this rubbish on the edge of the hole, 

 arranging it so as to form a circular rampart. Soon 

 the posterior members can take nothing more from 

 the too distant bottom. The moment for laying has 

 now come. As soon as the egg arrives at the cloaca 

 one of the feet seizes it and lowers it gently into 

 the nest, while the second foot seizes another egg, 

 which during this time had appeared at the orifice. 

 This manipulation lasts until the end of the opera- 

 tion, when the tortoise buries all its family, and 

 to flatten the prominence which results she strikes 

 it repeatedly with her plastron, raising herself on 

 her legs. 



It is not only land animals which adopt this custom 

 of living in the earth, and there sheltering their off"- 

 spring. Fish also make retreats on the bank or at 

 the bottom. To mention only one case, the Bullhead 



