Invertebrate Gallery of the Indian Museum. 55 



I, ARTHROPODA CRUSTACEA. 



\(^mt% 37—42 northern txxis 48-55 SEcston toali, anb 

 64—69 (Southern enb]. 



The Crustacea are Arthropods that breathe by means 

 of gills. 



A majority of them live in the sea, but many species 

 inhabit fresh-water, and some live on land. 



The ringed or segmented nature of the body is gener- 

 ally very apparent, and most of the rings or segments 

 carry a pair of jointed appendages which are variously 

 modified in different regions of the body for feeling, for 

 biting and chewing, for prehension, for crawling, and for 

 swimming. 



A good general idea of the external structure of a 

 typical Crustacean can be gained by examining the large 

 Prawn in Case 49, in which all the segments of which the 

 body is made up are, as far as is possible, disjointed from 

 one another, and laid out in succession with their ap- 

 pendages. The Crustacea are usually encased in a stout 

 " sheir, or exoskeleton, formed by the hardening of the 

 superficial layer of the integument : this shell is periodically 

 cast off or moulted en masse, to be again soon renewed. 



In many, or in most, Crustacea the main develop- 

 mental changes, instead of being completed within the 

 ^§&j are continued long after the young one has left the 

 egg and has started an independent life, the final form 

 being reached only after a succession of moults or " meta- 

 morphoses" : so that the young when first hatched is quite 

 unlike the adult. 



The Crustacea fall into two divisions — the Entojuo- 

 straca which are for the most part small lowly organised 

 aquatic species, with a body made up of an inconstant 

 number of segments or somites ; and the Malacostraca 

 which are for the most part large, highly-organised species, 

 both aquatic and terrestrial, with a body made up of a 

 constant definite number of somites, each one usually with 



