344 COMPARATIVE ANATOMY CHAP. 



Entomostraea. 



The nervous system of the Phyllopoda (Fig. 236, D\ especially 

 that of the Branchiopoda (Branchipus, Artemia), best corresponds with 

 the scheme of the Crustacean nervous system given above. The 

 transverse commissures between the ganglia of the ventral chord are 

 tolerably long, and so the whole ventral chord has the character of a 

 ladder nervous system. The two ganglia of a double ganglion are 

 connected together by two transverse commissures. We must note as 

 specially important that the pairs of ganglia for the oral appendages, 

 i.e. for the mandibles and maxillae, have remained distinct. Behind 

 these 3 pairs of ganglia come (in Branchipus) the 1 1 pairs of ganglia 

 of the limb -carry ing segments, then the 2 pairs of ganglia in the 

 genital segments. Behind these, ganglia only occur as rudiments in 

 the two following segments. 



The nerves for the posterior antennae arise out of the cesophageal 

 commissures, which are provided with a layer of ganglionic cells ; this 

 layer can the better be considered the ganglion of the 2d antennae, 

 since in front of the mandibular ganglion, and behind the oesophagus, 

 the cesophageal commissures are connected by a double tranverse 

 commissure, corresponding with the double transverse commissures of 

 the other ganglia of the ventral chord. 



The nerves for the unpaired frontal eye, for the paired stalked 

 eyes, and for the anterior antennae spring from the brain. 



In correspondence with the greatly reduced segmentation of the whole body in 

 the Cladocera (Phyllopod] its central nervous system is also much less pronounced. 

 The ladder-like ventral chord consists of 7 pairs of ganglia, the foremost of which 

 (infra-cesophageal ganglion) innervates the mandibles and maxillse, and the other 6 

 the 6 pairs of limbs. In front of the infra-cesophageal ganglion we find here a 

 transverse commissure connecting the cesophageal commissures. The nerve for the 

 2d antennae arises from the infra-cesophageal ganglion. In Leptodom the 6 ventral 

 ganglia in the adult animals are fused into one ventral ganglionic mass, while in the 

 young animals they are still tolerably distinct. 



The nervous system of the Ostracoda deserves further investigation. 

 The ventral chord of Cythere which follows the brain and oesophageal 

 commissures is said to consist of an infra-oesophageal ganglion and 4 

 subsequent ventral ganglia. The infra-cesophageal ganglion is said to 

 show its composition out of two ganglia and to innervate the jaws, 

 while the 3 subsequent ganglia give off nerves to the limbs, and the 

 last ganglion nerves to the most posterior divisions of the body and 

 the genital apparatus. 



In contrast with the above, the ventral chord of Halocypris appears much con- 

 centrated. It consists of an infra-cesophageal ganglion with nerves to the jaws and 

 maxillipedes, and a small ventral ganglion. Out of the latter arise 2 pairs of nerves, 

 which probably innervate the musculature of the limbs and the abdomen. 



In the various divisions of the Copepoda the central nervous system 



