Invertebrate Gallery of the Indian Museum. 63 



rical swimming "tail": in the Crabs the abdomen is 

 rudimentary, is carried tucked up out of sight beneath 

 the thorax, and lodges only the hinder end of the intes- 

 tine : while in the Hermit-crabs the abdomen is in an 

 intermediate condition, being useless as an organ of locomo- 

 tion, asymmetrical, soft, and liable to injury, but being of 

 considerable size, and lodging several important organs. 



In all the Decapod Crustacea the body is made up of 

 twenty-one segments, if we include the region that carries 

 the eye-stalks and the telson as such, and the first fourteen 

 are almost always immovably fused together to form a 

 cephalothorax, the back of which is known as the carapace, 

 and the sides of which are expanded outwards and down- 

 wards to form a gill-chamber. Of the paired appendages 

 of these fourteen segments, the first are eyes borne on long 

 stalks, the second are antennules fringed with hairs which 

 are probably organs of smell (also in the base of the 

 antennules the organ of hearing is lodged), the third are 

 long antennae or feelers, the fourth are mandibles or bit- 

 ing-jaws, the next five pairs are closely crowded together 

 on each side of the mouth and are modified to form chew- 

 ing-jaws, or maxillae and maxillipeds, while the last five 

 are many-jointed crawling feet, some or all of which may 

 end in powerful nippers, or chelae. The first pair of thoracic 

 legs usually end in nippers of great size and are known as 

 "chelipeds". In a few forms the one or two last pairs of 

 crawling legs are rudimentary or absent. 



The abdomen consists of seven segments, the structure 

 of which will be subsequently considered. 



In connexion with the appendages the breathing-organs, 

 or gills, must be noticed : they are sometimes feather-like, 

 but much more commonly brush-like structures attached 

 to or near to the bases of a certain number of the thoracic 

 appendages on each side, and enclosed in a gill-chamber 

 formed by the lateral extensions of the carapace already 

 mentioned. The water with its dissolved oxygen usually 

 passes into the gill-chamber from behind, and after bath- 



