CLASSIFICATION. 



The sub-kingdom Annulosa, to which the Crustaceans 

 belong, is, according to the commonly accepted 

 arrangement, one of the primary divisions, and in the 

 number of its members by far the largest division, of 

 the animal kingdom. Its most conspicuous character 

 is that which is expressed by the term annulose, its 

 members being composed of a variable number of more 

 or less distant rings or body-segments (technically 

 termed somites), arranged in longitudinal series, one 

 behind the other. Besides the Crustacea, a group of 

 which forms the subject of the present memoir, it 

 contains several other important classes, the most 

 familiar of which are the Annelida (worms and leeches), 

 Arachnid a (mites and spiders), and the Inseota. 



The Crustacea, in their typical forms, are very 

 distinctly segmented, the number of somites being 

 variously reckoned at twenty or twenty-one ; some of 

 this number of segments are, however, in almost all 

 cases suppressed, and not recognisable except theo- 

 retically. Some or other of the somites almost always 

 carry appendages adopted for mastication, locomotion, 



