v CRUSTACEA NERVOUS SYSTEM 345 



shows several degrees of centralisation, from a more or less decentralised 

 condition to the almost complete fusion of brain and ventral chord to 

 form one ganglionic mass, pierced through by the esophagus. The 

 segmentation is most complete in the free-swimming Copepoda, from 

 which all the others are to be derived. In the Calanidce, for example, 

 we have (apart from the brain) a ventral chord consisting of 7 ganglionic 

 swellings which stretches more or less far into the abdomen. 



In other free-living Copepoda the number of ganglia is reduced, and the abdominal 

 ganglia become small or disappear. But in the Corycceidce (Fig. 236, H) we already 

 have only one single ganglionic mass surrounding the cesophagus, from which nerves 

 radiate to the sensory organs, extremities, the musculature of the body, etc. The 

 nervous system in various delicate parasitic Copepoda shows a similarly con- 

 centrated, though partly also reduced, condition. 



The nervous system of the Carp-lice (Argulidce, Fig. 236, G), which 

 are closely connected with the true Copepoda, is relatively highly 

 developed. The concentrated ventral chord consists of 6 ganglia with 

 much -shortened longitudinal and transverse commissures. The 4 

 posterior ganglia supply the 4 pairs of limbs, the 2 anterior the jaws, 

 maxillipedes, and clinging feet. At the points at which the cesophageal 

 commissures join the brain there are 2 ganglionic swellings, from which 

 nerves go to the 2d antennae. 



Among the Cirripedes the nervous system of the Lepadidce is the 

 most richly segmented. They possess a brain, long oesophageal com- 

 missures, and 5 or 6 ventral ganglia. The nervous system of the so- 

 called Cypris-like larvae, that is, of those which develop into herma- 

 phrodite individuals, is similar. 



The so-called complementary males of the Lepadidce, on the contrary, and their 

 Cypris-like larvce, only possess a cerebral ganglion (supra-oasophageal ganglion) and a 

 thoracic ganglion, which alone represents the whole ventral chord. In the Balanidce 

 the ventral ganglia are fused into one large ventral ganglionic mass. Degeneration 

 goes very far in the whole nervous system of the Rhizocephala (Sacculina, Peltogaster), 

 which are so much degenerated by parasitism ; we here find it in the form of one 

 single ganglion, from which various nerves radiate (cf. Fig. 248, p. 373). This 

 ganglion is said not to correspond with the larval supra-oasophageal ganglion, but to 

 arise anew in the development of the adult animal. 



Malaeostraea. 



I. Leptostraea. The interesting genus Nebalia, which of all living 

 Malaeostraea stands nearest to their racial form, possesses an extra- 

 ordinarily richly segmented nervous system. If this pronounced 

 segmentation recalls, on the one hand, the nervous system of the Pliyl- 

 lopoda (Branchiopoda), it shows on the other (especially in the structure 

 of the brain) a decidedly Malacostracan character. The ganglia for the 

 posterior antennae are moved far forward on the cesophageal commissures, 

 and form, as in all Malaeostraea, the most posterior division of the brain. 

 The transverse commissure, which corresponds with them, however, 



