

NERVOUS SYSTEM. INVERTEBRATA. 29 



ventro-lateral border six pairs of pedal nerves, distributed 

 respectively to the five pairs of walking appendages 

 and to the operculum. A pair of fine nerves for the chilaria 

 arise from the ventral surface of the "post-oral" ganglion. 

 The ring is united posteriorly by connectives to a chain of 

 six pairs of transversely concentrated ganglia, situated in 

 the abdomen. From each of these, except the last, two 

 pairs of nerves are given off one to the appendages 

 (gills) of the segment proper to the ganglion, the other to 

 the integument. The latter are united on either side 

 external to the bases of the limbs by a longitudinal con- 

 necting cord, parts of which only are to be seen in the 

 specimen. The three posterior ganglia are fused together 

 to form a single mass, which sends nerves to the last two 

 pairs of gills, the hinder extremity of the abdomen, and 

 the post-anal spine. 



The central nervous system, together with the motor 

 and some few sensory nerves, is enveloped by a large 

 arterial blood-space; a pair of arteries branches of the 

 anterior aorta open into the space on either side above 

 the anterior part of the nerve-ring. Experiments upon 

 this nervous system show that, although parts of it are 

 much concentrated, each segmental centre is entirely inde- 

 pendent in its actions, the regulation and orderly sequence 

 of the movements of different segments being due to the 

 transmission of stimuli from centre to centre, and not to 

 the influence of any one specialized centre of co-ordination. 



Viallanes, Ann. Sci. Nat., ser. 7, t. xiv. p. 405 (Anat.). 



Hyde, Journ. Morph., vol. ix. 1894, p. 431 (Physiol.). 



ARACHNIDA. 



St. Remy, Arch. Zool. Exp., t. v bis, 1887, p. 1. 

 D. 21. The nervous system of a Scorpion (Pandinus imperator), 

 exposed from the dorsal aspect. The main part of the 

 central system lies in the cephalothorax and is concentrated 

 around the oesophagus in a compact mass that represents the 

 cerebral and first nine ventral-chain ganglia. The cerebral 

 part of the mass is bilobed ; it innervates the median 

 and lateral eyes and the chelicerse. On either side of the 

 oesophagus it is continuous with the anterior region of the 



