8 CRUSTACEA 



CHAP. 



of structure, but these variations can be reduced to at most 

 two, and possibly to one fundamental plan. In a typical 

 Crustacean, besides the paired eyes, which may be borne on 

 stalks, possibly homologous to highly modified limbs, there are 

 present, first, two pairs of rod -like or filamentous antennae, 

 which in the adult are usually specialised for sensory purposes, 

 but frequently retain their primitive function as locomotory 

 limbs even in the adult, e.g. Ostracoda : while in the Nauplius 

 larva, found in almost all the chief subdivisions of the Crustacea, 

 the t\vo pairs of antennae invariably aid in locomotion, and 

 the base of the second antennae is usually furnished with sharp 

 biting spines which assist mastication. Following the antennae 

 is a pair of mandibles which are fashioned for biting the food 

 or for piercing the prey, and posterior to these are two pairs 

 of maxillae, biting organs more slightly built than the 

 mandibles, whose function it is to lacerate the food and prepare 

 it for the more drastic action of the mandibles. So far, with 

 comparatively few exceptions, the order of specialisation is 

 invariable ; but behind the maxillae the trunk-appendages vary 

 greatly both in structure and function in the different groups. 



As a general rule, the first or first few thoracic limbs are 

 turned forwards toward the mouth, and are subsidiary to 

 mastication ; they are then called maxillipedes ; this happens 

 usually in the Malacostraca, but to a much less extent in the 

 Entomostraca ; and in any case these appendages immediately 

 behind the maxillae never depart to any great extent from a 

 limb-like structure, and they may graduate insensibly into the 

 ordinary trunk-appendages. The latter show great diversity in 

 the different Crustacean groups, according as the animals lead 

 a natatory, creeping, or parasitic method of life ; they may 

 be foliaceous, as in the Branchiopoda, or biramous, as in the 

 swimming thoracic and abdominal appendages of the Mysidae, 

 or simply uniramous, as in the walking legs of the higher 

 Decapoda, and the clinging legs of various parasitic forms. 



Without going into detailed deviations of structure, many 

 of which will be described under the headings of special groups, 

 it is clear from the foregoing description and from Fig. 1 (p. 10), 

 that three main types of appendage can be distinguished: first, 

 the foliaceous or niultiramous ; second, the biramous ; and, third, 

 the uniramous. 



