LESSON 77.] NERVOUS SYSTEM IN THE MOLLUSCA. 253 



1147. From their remarkable minuteness, we have no direct 

 testimony in relation to the condition of the nervous system in the 

 Mites / but in the Scorpion we find the principal masses or ganglions 

 concentrated around the oesophagus in the cephalo- thorax. 



1148. From the small bilobed cephalic mass are sent upwards 

 the optic filaments, in front the nerves of the large forcipated claws, 

 and, backwards, the sympathetic nerves ; the sub-ossophageal gan- 

 glionic columns distribute nerves to the great under jaws, and to the 

 four pairs of thoracic legs ; two slender continuations of the double 

 columns are continued along the jointed abdomen, or tail, and seven 

 small ganglions are developed upon them, from which nervous fila- 

 ments are distributed to the surrounding parts. 



1149. In Spiders the central masses of the nervous system are 

 wholly, or in great part, concentrated in the cephalo-thorax. The 

 brain is a bilobed gan- FIG 373 



glion (Fig. 373, c), send- 

 ing forwards and up- 

 wards the optic nerves 

 (o) from its anterior 

 angles, and below these, 

 the two large nerves (m) 

 to the mandibles; a short 

 and thick collar encloses 

 the narrow gullet, and Nervous system of Spider, 



expands into a second very considerable stellate or radiated ganglion 

 (s) situated below the stomach ; it sends off five principal nerves on 

 each side : the first (p), to the feelers of the under jaw ; the second 

 (Z), to the feelers of the under lip ; the three posterior nerves supply 

 the remaining legs. 



LESSON LXXYII. 



NERVOUS SYSTEM IN THE MOLLUSCA. 



1150. The nervous system in these animals is arranged on a to- 

 tally different plan from what we have seen in the Articulata. 



1151. In contradistinction to the uniform system of arrangement 

 of the nervous system, so constantly displayed in the latter sub- 

 kingdom, the Mollusca are remarkable for the unsymmetrical form of 

 its development. The cephalic ganglia, when two in number, are 



