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, eg. Ostracoda; while in the Nauphus 
larva, found in almost all the chief subdivisions of the Crustacea, 
the two 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 multiramous; second, the biramous; and, third, 
the wniramous. 
