110 CRUSTACEA. 



ORDER V. — ISOPODA. 



This order embraces the Linnsean genus Oniscus, with 

 the numerous sectional and generic groups separated 

 therefrom. It is distinguished by the hard envelope of 

 the body, the eyes not raised upon footstalks, the body 

 divided into a series of nearly equal-sized and generally 

 flattened segments, and the legs of nearly equal size, and 

 fourteen in number, the anterior pairs not being chelifer- 

 ous, or armed with large claws. The legs are terminated 

 by a short curved hook or nail : the under surface of the 

 abdominal portion of the body is fm*nished with flattened 

 appendages or plates, having the edges fringed with fine 

 hairs, serving, as is supposed, for respiratory organs, and 

 covered by a larger pair. The body is generally flattened, 

 and of an oval form, being broader than it is thick. The 

 females carry their eggs for a considerable period in a mem- 

 branous bag, under the breast or between the pectoral scales. 

 The young, when hatched, are very similar in form to their 

 parents, and undergo scarcely any other change than that of 

 an increase of size produced by the occasional shedding of 

 the outer envelope of the body. The greater number of 

 species inhabit the water, some of them being parasitic upon 

 fishes. The terrestrial species also require a certain degree 

 of moisture, in order to preserve their branchiae in a fit state 

 for respiration. 



Latreille divides this order into six sections, as follows : — 



1. Ejncarides. — Destitute of eyes and antennae, legs un- 



fitted for locomotion. This section comprises only a 

 single genus, Bopyrus (Latreille), which is often to be 

 found under the shell of the prawn. 



2. Cymothoidce. — Four antennae, terminated by a multi- 



articulate slender thread j fore legs generally furnished 



