NERVOUS SYSTEM OE AKTHROPODA. 255 



Crustacea, and these also are often explicable as being adaptations to 

 changes in the form of the body. We find this concentration in the 

 Copepoda, where the Calanidie possess a ventral chain, formed of 

 ganglia, whilst in the Corycse'fdas this is concentrated into a mass 

 fused with the cerebrum. So too, among the Cirripedia, the 

 Lepadiche have a series of 4-5 ganglia in their ventral chord ; in the 

 Balanidas this is represented by a single ganglionic mass (Fig. 129, 

 B, gi). Similar phasnomena are seen in the Arthrostraca, but as a 

 rule they have a large number of ganglia (10-12 in the Amphipoda, 

 7-13 in the Isopoda). 



§ 107. 



In the Protracheata the nervous system remains in a lower con- 

 dition. A highly developed and closely united pair of cerebral 

 ganglia surround the mouth, and give off lateral nerve- chords down- 

 wards. They are closely approximated below the oesophagus, and 

 then pass ventrally to the hinder end of the body, diverging some- 

 what in their course. These nerve-chords are united at their ends. 

 Along their whole length they are connected by fine transverse 

 commissures (in Peripatus Edwardsii) ; the most anterior of these 

 are the most distinct. There are no swellings on the ventral chord, 

 and the ganglionic cells are equally distributed in it. This 

 arrangement is that of the more indifferent condition of the ventral 

 ganglionic chain ; the chain is formed by the separation of the 

 ganglionic cells in the longitudinal trunks into separate parts, 

 corresponding with the metameres. 



Since in the Branchiata the ventral ganglia are always differen- 

 tiated, the arrangement in Peripatus is a lower one, and so far 

 justifies us in regarding the Tracheata as an independent phylum. 



§ 198. 



The nervous system of the Myriapoda indicates a well-marked 

 step forward, for there is a ventral chord which traverses the whole 

 length of the body with scarcely any variation in character, provided 

 with ganglia so arranged as to correspond exactly with the metameres. 

 The first ganglion which supplies the gnathites sometimes indicates 

 distinctly that it is composed of a number of ganglia. The suc- 

 ceeding ones differ in size according to the extent to which their 

 appendages are developed ; they are arranged at regular distances 

 from one another, and in the Diplopoda succeed one another in pairs. 

 When the longitudinal commissures are shortened they form closely 

 applied swellings (Julidae). An approximation of this kind tending 

 to concrescence is ordinarily seen in the last ganglia, even when 

 the others are distinctly separated. These ganglia correspond in 

 number to the metameres, so that there may be even as many as 

 140 (Geophilus). In these arrangements we see a condition which 

 is most like the typical form of the higher Tracheata. 



