THE JOINTED-FOOT ANIMALS 143 



tough and leathery, and covered with many reddish-brown 

 dots ; it is so transparent that the stomach and intestine can 

 be seen clearly while the animal is at rest. 



The appendages show some interesting adaptations. The 

 antennas are as long as the body, very slender, and during 

 forward locomotion are constantly waving through the water. 

 One of the antennules on either side extends forward, and the 

 other backward, over the eye. At times, in sea-water aquaria, 

 the recurved antennule appears to be clearing the surface of 

 the eye of dirt. The cheliped is only about two thirds as 

 long as the first walking-leg ; it has the curious habit of 

 bending backward frequently at a middle joint like a knee. 

 The first walking-leg also has a nipper, and is much used 

 for grasping food. The remaining three appendages of the 

 thorax are about the length of the cheliped, and em.1 in a sin- 

 gle point. When at rest the prawn is usually found clinging 

 easily to the under surface of seaweeds. 



Crabs. The species represented in Fig. 71 (Eupagu'rus 

 pollica'ris] is called the hermit-crab, probably because it lives 

 in a " house " by itself. The house consists of the shell of a 

 dead snail which may have been washed to the beach, or may 

 have rested on the bottom of the bay. The hermit-crab in its 

 early life is pelagic, but at a certain stage of its development 

 it sinks to the bottom, finds a snail-shell, and backs into it ; 

 from that time on, the general shape of the body and the 

 special modification of certain organs are determined by the 

 form of the snail-shell. The abdomen is soft, and all the abdom- 

 inal appendages except the terminal ones are misshapen and 

 useless. The terminal appendages extend laterally like flanges, 

 and prevent the body from being drawn forcibly from the 

 snail-shell by an enemy. The chelipeds are abnormally devel- 

 oped and, besides being of use in capturing prey, serve the 

 important function of closing the aperture of the shell in 

 times of danger. 



