THE CRUSTACEA 



Fig. 0. 



Early nauplius larva of Copepod [Ctjdops s]).) 

 fioni below, o', antennule ; a", antenna ; gn, gnatlio- 

 basic process of antenna ; Ihr, labrum ; miX, niandibli'. 



as true appendages, although they differ from all the other append- 

 ages in the fact that they are always innervated from the brain (or 

 supra- oesophageal ganglia), and that they are uniranious in the 

 nauplius larva (Fig. 0, a) 

 and in the adults of all the 

 sub-classes except the Malaco- 

 straca. As regards the in- 

 nervation, an apparent excep- 

 tion is found in the case of 

 Apns, where the antennular 

 nerves arise, behind the 

 brain, from the oesophageal 

 connectives. This is un- 

 doubtedly a secondary 

 position, however, and the 

 nerve-fibres have been traced 

 forward to centres in the 

 brain. In the [Nlalacostraca 

 the antennules are often 

 biramous (Fig. 7), but there 

 is considerable doubt as to 

 whether the two flagella correspond to the endopodite and exopodite 

 of the other limbs. In most cases the antennules are sensor}' in 

 function, but they may also be natatoiy or prehensile, and in the 

 Cirripedia they form organs of attachment. 



The antennae (or second antennae) are of special interest on 

 account of the clear evidence that, although preoral in position in 

 all adult Crustacea, they were originally postoral appendages. In 

 the naui)lius larva (Fig. G, a") their position is beside 

 rather than in front of the mouth, and they may bear 

 hook-like masticatory processes {gn) which assist the 

 similar processes of the mandibles in seizing the food. 

 In the Branchiopoda and less distinctly in some other 

 groups, the nerves to the antennae arise not from the 

 brain, but from the oesophageal connectives, and the trans- 

 verse commissure of the corresponding ganglia can be traced 

 behind the oesophagus, even in those forms in which the 

 ganglia have moved forward into the brain (Fig. 2, p. 5). 

 The functions of the antennae are very varied. As 

 already stated, they act as jaws in some nauplius larvae. 

 In many cases they are important organs of locomotion, 

 and they may serve as sexual " claspers," or as organs of attach- 

 ment in parasites. In the Malacostraca they are mainly sensory, 

 the endopodite being a long flagcllum, while the exopodite may 

 form a flattened " scale " probably used as a balancer in swimming, 

 or may disappear altogether. 



Fig. 7. 



Antennule- 



of Cravtisli. 



(After 



lluxl.y.) 



