CRUSTACEA OUTER ORGANISATION 



311 



ally typically biramose, but appear in various ways transformed in 

 consequence of having undertaken the function of mastication. It 

 is always the basal joint of the protopodite, as that lying nearest the 

 mouth, which changes into a variously formed, hard, masticatory 

 portion (corpus mandibulare), and is often toothed on the side turned 

 towards the mouth ; the rest of the limb is more or less degenerated. 



Entomostraca. In the Phyllopoda (Fig. 211, G) the mandible is reduced to the 

 horny masticatory portion. The mandibles of the Ostracoda (H} have retained the 

 typical form. The strongly developed masticatory joint is followed by a segmented 

 "feeler," whose first joint (corresponding with the distal joint of the protopodite) 

 may carry a little fan-like plate. This represents the exopodite, while the feeler, 

 with the exception of its first joint, represents the endopodite. The mandibles in 

 the free -living Copepoda (E, F) are toothed masticators (hence Gnathostomata) 



FIG. 211. Mandibles of various Crustaceans. A, Lucifer, Nauplius (after Brooks). B, Nebalia 

 9 (after Glaus). C, Campy laspis nodulosa, Cumacean (after Sars). D, A larva of Branchipus, 0.8 

 mm. long (after Glaus). E, Notodelphys Almannii (after Thorell). F, Cyclops tenuicornis (after 

 Glaus). G, Apus lucasanus (after Packard). H, Xestoleberis aurantia, Cytherid Ostracod (after 

 Dahl). I, Astacus fluviatilis (after Huxley) ; I, proximal, II, distal joint of the protopodite ; ex, 

 exopodite ; en, endopodite (feeler) ; fc, masticatory part or ridge. 



and carry feelers. The first joint of the feeler (2d joint of the protopodite) may 

 carry a segmented exopodite. In most parasitic forms the mandibles are changed 

 into stylet-shaped organs for sucking and piercing (Siphonostomata). Among the 

 Cirripedes the mandibles are wanting in the Ehizocephala, and in other groups are 

 developed as masticatory portions without feelers. 



Leptostraca (B) and Malacostraca (A, C, /). The exopodite is everywhere 

 w r anting ; it is only present in the Nauplius stage of a few Malacostraca. The 

 mandible consists of the basal masticatory or cutting joint, and a frequently 3- 

 jointed feeler, whose first joint belongs to the protopodite, while the last two 

 represent the joints of the endopodite. The feeler may here and there be wanting ; 

 it is entirely wanting in the Cumacea (C]. 



We see from the above review that among all Crustacea only the Ostracoda and 



