THE CLASS CRUSTACEA 311 



It is characteristic of Crustacea to have two pairs of 

 antennae, the anterior pair bearing the diminutive appellation 

 oi antennules ; but one pair, or even both, may be reduced to 

 a vestige, or be entirely suppressed. 



In most Crustacea the three pairs of appendages immedi- 

 ately behind the antennae lie close to the mouth and form 

 jaws. In many, the anterior appendages of the thorax are 

 also crowded beside the mouth and form accessory foot-jaws. 



In Crustacea, as in Arthropoda generally, the sexes are 

 separate ; but there are numerous sessile and parasitic forms 

 that are hermaphrodite; while among some of the smaller 

 forms — particularly those inhabiting fresh water — males 

 appear only at certain seasons or at long intervals, and in 

 their absence generation after generation is produced by 

 females alone. The ducts of the reproductive organs open 

 on the ventral surface, commonly in the after part of the 

 thorax, but occasionally near one or other end of the body. 



The spermatozoa are non-motile. The eggs are numerous 

 and are in most cases carried by the female — either attached 

 directly to some of the appendages, or lodged in a special 

 brood-pouch — until they are hatched, and sometimes even 

 longer. The new-hatched young, or larva (Lat. larva = Taz.sV) 

 is, in the majority of Crustacea, unlike the parent, and 

 acquires the adult form by a series of post-embryonic develop- 

 mental changes or metamorphoses. 



In the higher forms of Crustacea the segments of the 

 body are constantly 21 in number, and are grouped in three 

 regions — head, thorax, and abdomen — the head, however, 

 almost never being sharply separated from the thorax, and 

 often being enclosed with the thorax in a common fold or 

 process of the integument, known as the carapace. In these 

 higher forms, of which the prawn and lobster are types, the 

 appendages of the body number twenty pairs — a pair to each 

 segment except the last, which never carries appendages. 

 The Crustacea which conform to this definite type constitute 

 the subclass Malacostraca. 



In the more primitive Crustacea the number of segments 

 and appendages is quite inconstant ; there may be few, or 

 there may be a great many, or the segmentation may in 

 cases be quite indistinct. The segments, again, may be 



