THE BRANCHIOPODA 



45 



Nervous System. As has been mentioned above, the nervous 

 system of most Branchiopoda shows very primitive characters in 

 the ladder-like form of the ventral nerve-chain (Fig. 11, p. 17, and 

 Fig 27). In some Cladocera, however (Polyphemidae and Lepto- 

 dora), the ventral ganglia are more or less coalesced into a single 

 mass. In all cases, the nerves to the antennae arise not from the 

 supra-oesophageal ganglia, but from the first post-oesophageal pair 

 of ganglia. This pair of ganglia is quite distinct from the next 

 succeeding pair, which innervate the muscles of the mandibles, 

 and, like all the ventral pairs of ganglia (with the occasional 

 exception of the mandibular), it has a double transverse com- 

 missure. It is closely connected with the origin of a " visceral " 

 nerve-ring which encircles the oesophagus and bears an unpaired 

 ganglion in the region of the labrum (Figs. 27 and 28, n.lbr). 

 So far as their nerve-supply is concerned 

 the antennae are, in the Branchiopoda, 

 unmistakably post-oral. In the Noto- 

 straca the nerves of the antennules (Fig. 

 28, n. a) arise from the oesophageal 

 connectives just in front of those of 

 the antennae. This was supposed by 

 Lankester to be a primitive condition 

 indicating the post-oral origin of these 

 appendages. Pelseneer showed, however, 

 that the fibres of these nerves pass 

 forward to a pair of nerve-centres in the 

 supra-oesophageal ganglion. In the other 

 Branchiopoda the corresponding nerves 

 arise from the posterior part of the brain, 

 and there can be little doubt that the 

 condition in Apus is a secondary one due 

 to the brain having been shifted forwards 

 and upwards in company with the paired 

 eyes which lie on the dorsal surface of 

 the head, while the antennules are in- FIG. 28. 



Sorted On its lower Surface. In ApUS Anterior part of the central 



,, 1 . T ,-, nervous system of Arum, from below, 



there is no ganglion corresponding to the semi -diagrammatic. r,- r , brain; 



reduced maxillae and these limbs receive gnM "' arvtennal ganglion ; ,(.', nerve 

 ui.i.MiitU., ami tuebe eive to an tennule ; n.a", nerve in 



a pair of nerves arising from the longi- antenna ; n.f, nerves to first trunk- 

 1 . -n i limb ; n.Ior, nerve-ring encircling 



tudinal connective between the maxillular oesophagus; .>./. nerves to man- 



i . ,1 T /T71- r, dible ; n,maf. nerves to maxillula ; 



and first thoracic ganglia (Fig. 2 8 r1M x rf nerve to mariUa;o, portion 



a. in. /'). In the Cladocera, also, where the aL^pt^'nmo (Aft r Lankestt ' r 



maxillae are quite rudimentary or absent, 



there is no ganglion corresponding to this somite. In Branchipus, 



however, there is a distinct pair of ganglia with double transverse 



commissures. 



