THE CRUSTACEA 



1 1 



Flo 6 



Early nauplius larva of Copepod (Ci/dops s]>.) 

 from below. ', antennule; a", antenna; gn, piatho- 

 basic processof ante nna ; tor, labrum ; wd, mandibl.-. 



as true appendages, although they dift'er 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 uniramous in the 



nauplius larva (Fig. 6, 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 



Apus, 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 Malacostraca 



the antennules are often 



Viiramrmt: /Fio- 7\ Vint tVi^rn 

 5 V ri S- '/I DUt tnere 



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 sensory in 

 function, but they may also be natatory 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 nauplius larva (Fig. 6, 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 oj: th'e 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 flagellum, while the . exopodite may 

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

 or may disappear altogether. 



Fio. 7. 



Antennulo 

 of Crayfish. 



(After 

 Huxl.'y.) 



