TYPE CRUSTACEA. 



411 



the ventral surface. Arteries pass off from both ends of the 

 heart. Otocysts are always developed in the basal joints of 

 the antennules. 



1. Suborder Macrura. 



In the Macrura the abdomen is well developed and usu- 

 ally as long as the cephalothorax, and is provided with its 

 full complement of appendages, the sixth pair forming with 

 the telson a tail-fin. Exceptions to these ■ arrangements oc- 

 cur ; in the Hermit-crabs, Eupagurus, which inhabit the empty 

 shells of Gasteropod Mol- 

 lusks, the abdomen is gener- 

 ally soft and unsymmetrical, 

 since it is coiled around the 

 columella of the shell, but 

 terminates in a movable tail- 

 fin which serves, together with 

 the remaining pleopods and 

 the last (and sometimes also 

 the penultimate) pereiopod, 

 which is bent dorsally, to re- 

 tain the animal in the shell. 

 The chelae of the anterior 

 pereiopods are generally un- 

 equal in size, serving to oc- 

 clude the mouth of the shell, 

 and occasionally the abdomi- 

 nal appendages of only one 

 side are developed. In the 

 genus Hippa too the abdomen, 

 though with a well-developed 

 and calcified cuticle, is short, 

 the terminal half being bent 

 up under the thorax, the Fig. 187. 

 condition characteristic of 

 the Crabs being thus ap- 

 proached. In some forms, such as Sergestes and Zicdfer, 

 the fourth and fifth pereiopods may be rudimentary or even ab- 

 sent, but more usually all these appendages are well devel- 



A, A TOUISTG Lucifer (adapted 

 from Brooks); B, Eupagurus bern 

 hardus (after Leunis). 



