THE CLASSIFICATION OF CRUSTACEA 61 



The Thalassinidea are small lobster - like animals 

 which burrow in sand and mud, and have generally 

 a more or less soft abdomen (see Fig. 38, p. 103). 



The tribe Paguridea includes the Hermit Crabs 

 (Pagurid<z) and their allies. The typical Hermit 

 Crabs (Plate VII.), which are familiar objects in 

 seaside rock-pools, live in the empty shells of Whelks 

 and other Gasteropod Molluscs, which they carry 

 about with them as portable shelters. The structure 

 of the animals is modified in adaptation to this 

 curious habit. The abdomen, which is protected 

 during life by the borrowed shell, is soft and 

 unarmoured, and is spirally twisted. The swim- 

 merets, which have only the function of carrying 

 the eggs in the female, are much reduced, and are 

 usually present only on one side of the body. The 

 uropods no longer form a tail-fan, but are adapted 

 for firmly wedging the hind part of the body into 

 the coils of the shell. One of the chelipeds is much 

 larger than the other, and serves to block up the 

 opening when the animal withdraws into its shelter. 

 In tropical countries certain Hermit Crabs (Cot.no- 

 bitidcE) have become adapted to a life on land, and 

 one of these, the well-known Coconut Crab, or 

 Robber Crab (Birgus latro), which is the largest 

 species of the tribe, has given up the habit of 

 protecting itself with a shell, and its abdomen has 

 again acquired a strong armour on the upper side. 

 The marine Lithodida to which the British Stone 



