CHAP. V CBTJSTAGE A— NERVOUS SYSTEM 347 



Fio. 23(3,— Central nervous systems of various Crustaceans. A, Of Euphausla pelluoida 

 (after G. O. Sars). B, Of Astaous fluvlatilis (after Vogt and Yung). C, Of Apseudes Latreillii 

 (comtined from several figures ty Claus). D, Of Limnadla (after Klunzinger), anterior portion. 

 E, Of Asellus aquations (after G. 0. Sars). F, Of Maja sqninado (after Milne Edwards). G, Of 

 Argulus Corregoni (after Clans). H, Of SappMrina Edwardsii (after Haeolcel). gg, Brain ; an, 

 nerves of the paired eyes ; ua, unpaired eye -with its nerve ; go, ganglion opticum ; «i, nerve of the 

 1st antenna; a-,, of the 2d antenna; sc, oesophageal commissures; y, post-oesophageal tranverse 

 commissure of the same (commissure of the antennal ganglia of 2d antenna ? ) ; a^g, ganglion of tlie 

 2d antenna (in D) ; md, mandibular ganglion ; i/i^i, mx2, ganglia of the 1st and 2d pairs of maxillee ; 

 I-VIII, thoracic ganglia ; 'bg, sub-oesophageal ganglion, consisting of several fused ganglia ; 1-6, 

 abdominal ganglia ; s, sympathetic nervous system ; sg, ganglion of the same ; eg, commissural 

 ganglion ; m, stomach (in F) ; ft&, fused abdominal ganglia (in E). In G, g^ signifies 2d ventral gan- 

 glion ; ?ii-m4, nerves for the 4 pairs of limbs ; fc/, nerve for the clinging foot. In H, gm signifies the 

 ganglionic mass (brain fused with ventral chord) broken through by the oesophagus ; cl, corneal lens ; 

 I, lens ; bn, posterior strong nerves, the lateral pairs of -which go to the limbs. 



has remained separate behind the cesophagus and in front of the 

 mandibular ganglia. The ganglia of all the pairs of limbs, even 

 those of the oral limbs, have Femained separate on the ventral 

 chord, which has 17 ganglionic swellings. "We thus find, counting 

 from before backward: (1) a mandibular ganglion, (2) and (3) 2 

 maxillary ganglia; (4) to (11) 8 thoracic ganglia, and (12) to (17) 

 6 abdominal ganglia. It is a significant fact that in the larva the 6 

 abdominal ganglia are followed by a small 7th swelling, belonging to 

 the limbless 7th abdominal segment ; this, however, entirely disappears 

 at a later stage. This fact is rightly interpreted to mean that there 

 must originally have been more than 6 pairs of pleopoda. 



In contradistinction to the ladder-like nervous system of the Phyllopoda, the 

 ganglia in Nebalia, are fused in the middle line to form a double ganglion, and the 

 longitudinal commissures are moved nearer each other. The latter are very much 

 shortened in the thoracic region in correspondence Avith the abbreviation of the 

 thoracic segments. 



II. Arthrostraea, Anisopoda. — The richly segmented nervous 

 system of Apsendes (Fig. 236, C) is closely connected with that of 

 Nehalia. The brain and oesophageal commissures are followed by an 

 infra-oesophageal section, in which we can clearly distinguish the 

 separate ganglia for the mandibles, the two pairs of maxillse, and the 

 pair of maxillipedes. It is important to note that the maxillipedal 

 ganglion, which corresponds with the first thoracic ganglion of Nehalia, 

 is here more closely connected with the preceding ganglia, in keeping 

 with the commencing transformation of the first pair of thoracic feet 

 into a pair of maxillipedes. The 4 infra -oesophageal ganglia are 

 followed by 7 double ganglia for the thorax and 6 for the abdomen, 

 the last and largest of which is probably composed of two or more 

 fused together. The ganglion for the posterior antennse is moved 

 towards the brain, but the transverse commissure corresponding with 

 it is clearly visible behind the oesophagus, in front of the mandibular 

 commissure. The ganglia of the ventral chord are clearly double, and 

 connected by two separate longitudinal commissures. 



In Tanais fusings and displacements seem already to have taken place in the 

 ventral chord. The ventral chord here has, it is said, only 12 ganglia. 



