244 



ANIMAL PHYSIOLOGY. 



[LESSON 74. 



In the higher Articulata the segments first coalesce on the anterior 

 and posterior portions of the trunk, and hence the enlarged form 

 presented by their cephalic and caudal ganglia, independent of the 

 great size often attained by the appendices developed from the 

 terminal parts of the body. 



The cephalic ganglion is shown at a, and this, together with 

 the ganglia which succeed it, down to b, is of larger size than the 

 abdominal ganglia, with the exception of the last (c),) which in this 

 form of the nervous system always attains increased size. 



1108. The most concentrated form of the nervous system met 

 with in the Crustacea, is that found in the short and broad trunks 

 of the Crabs (Fig. 369), where all the symmetrical ganglia of the 

 columns are collected into two masses, the one in the head, and 

 the other in the centre of the cephalo-thorax ; the sympathetic nerves 



are confined to a nervous band around 

 the wide ossophagus. The supra-oeso- 

 phageal ganglion, or brain, is compara- 

 tively small in the Crabs, from the 

 smallness of the cephalic appendices, 

 which it supplies with nerves. 



1109. The infra-cesophageal mass 

 is of great size, consisting of the whole 

 chain of ganglia, which was originally 

 extended along the body behind the 

 oesophagus, fused into one mass. It 

 sends out* numerous branches to the 



surrounding viscera, and to the five 

 Nervous system, Crab. paipg Q legg which radiate from around 



that point, and the columns are prolonged backwards, ramifying along 

 the short slender post-abdomen, as a simple nervous chord. 



FIG. 369. 



LESSON LXXIY. 



NERVOUS SYSTEM IN INSECTS. 



1110. In Insects the nervous system differs chiefly from that in 

 the Myriopoda in having its primary divisions more definitely devel- 

 oped, and in manifesting degrees of concentration corresponding with 

 the increase of bulk and strength in particular parts of the trunk, 



