v CRUSTACEA NERVOUS SYSTEM 343 



the posterior antennae is not yet securely established. Such a ganglion would have 

 to be compared with the infra-cesophageal ganglion, and the segment corresponding 

 with it to the first trunk segment of the Annulata. In most Crustaceans, however, 

 the nerves for the posterior antennae do not arise from a distinct ganglion but from 

 the cesophageal commissures ; their places of origin in the higher Crustacea have 

 indeed moved as far forward as the brain. There are, however, many important 

 reasons, to be brought forward later, in favour of the assumption of an originally 

 distinct pair of ganglia and a special segment for the posterior antennae. 



All the more important and striking deviations from the typical 

 Crustacean nervous system can be classed under the following 

 heads. 



1. Approximation of the two ganglia of a double ganglion 

 by the shortening of the transverse commissure which unites them, 

 and finally the fusing of these two ganglia into one ; its composition 

 out of two lateral ganglia can, however, often be seen externally, and 

 always internally, on examination of its finer structure. 



2. Approximation and contact of the longitudinal commissures 

 connecting the consecutive ganglia. 



3. Approximation and contact of the consecutive ganglia by the 

 shortening of the longitudinal commissures. This may lead to the 

 fusing of the consecutive ganglia into one ganglionic mass, in which 

 the original composition out of several ganglia can sometimes be clearly 

 recognised ; at other times, however, this is very indistinct, or even 

 quite unrecognisable. 



4. Longitudinal displacement and shifting of the ganglia, 

 generally from behind forward. Ganglia may be displaced from one 

 segment into another. 



5. Shifting of the points at which nerves leave the ventral 

 chord. These displacements, however, apparently never affect the 

 real origin of the nerves in the centres of the nervous system. 



6. Entire disappearance of ganglia. This is perhaps only to be 

 proved with certainty of the most posterior ganglia of the body. 



All these changes go hand in hand with transformations of the 

 whole organism, especially with modifications in the segmentation of 

 the body and in the development of the extremities. In the young 

 stages of many Crustaceans it often happens that certain ganglia are 

 still separate which in adult animals are fused. We see from this how 

 important the knowledge of ontogeny is for a right comprehension of 

 the morphology of the nervous system. 



The concentration of the nervous system (the fusing of originally 

 distinct ganglia to form larger ganglionic masses) can be observed in 

 most of the natural divisions of the Crustacea. 



For the sake of clearness we shall in each group of the Crustacea 

 place the most conspicuously segmented nervous system in the fore- 

 ground, while the description of the deviating, concentrated, or 

 simplified nervous systems will be distinguished by the use of small 

 print. 



