12 



THE CRUSTACEA 



The mandibles, like the antennae, are, in the nauplius, bira- 

 mous swimming-limbs with a masticatory gnathobase arising from 

 the basal segment of the protopodite. This form and function are 

 retained with little alteration in some adult Copepoda (Fig. 8, A). 

 In most cases, however, the exopodite is lost and the endopodite 

 (with the distal part of the protopodite) forms the "palp" (Fig. 

 8, B) or may disappear altogether (Fig. 8, C), while the " body " 

 of the mandible is formed by the coxopodite (or perhaps by the 

 precoxa), and has a masticatory edge armed with tubercles, teeth, 

 or spines. In parasitic forms with suctorial mouth -parts the 

 mandibles may take the shape of piercing lancets enclosed in a 

 tubular beak formed by apposition of the labrum and metastoma. 



A. 



Fio. 8. 



A, mandible of Copepod (Calanus) (after Sars) ; ex, coxopodite (or precoxa, according to 

 Ilansen), forming the " body " of the mandible ; bs, basipodite ; en, endopodite ; ex, exopoditr. 

 B. mandible of Crayfish (after Huxley) ; letters as above. In both cases the basipodite 

 and the segments distal to it form the " palp." G, mandible of LepMnrus (after Sars). 



In Ostracoda the mandibular palp aids in locomotion, and in a few 

 cases the masticatory part is greatly reduced. 



The maxillulae and maxillae (or, as they are often termed, the 

 first and second maxillae) are nearly always foliaceous appendages 

 having gnathobasic lobes or endites borne by the segments of the 

 protopodite (Fig. 9). The endopodite is reduced to a "palp" or 

 is absent. On the outer side, lobes representing the exopodite 

 and epipodite may be present. These appendages undergo great 

 modifications in the different groups and exact comparative in- 

 vestigations on their morphology are still wanting. 



The post-cephalic or trunk appendages vary greatly in number. 

 In some Branchiopoda there are more than 60 pairs, while in 

 some Ostracoda it is uncertain whether there are any. They 

 present great diversity of form and function in the various groups 



