PHYSIOLOGY OF THE INVERTEBRATA. 



327 



in the abdomen. The brain gives off nerves to the sense 

 organs (eyes, antenna)), tlie sub-oesopliageal ganglion supplies 

 the mouth, and the other ganglia the rest of the body. 

 The visceral nervous system is 

 well developed in the Tnseda, 

 (Fig. 6z, 4). 



In the Inseda^ " the ner- 

 vous system varies very much 

 in. the extent to which its com- 

 ponent ganglia are united to- 

 gether. In most Orthoptcra 

 and Neuroptcra, and in many 

 Cohoptcra, the thoracic and 

 abdominal ganglia remain dis- 

 tinct and are united by double 

 commissures as in Blatta (Feri- 

 planctct). In the Lepidoptera, 

 the thoracic ganglia have coa- 

 lesced into two masses united 

 by double commissures; while 

 in the abdomen there are five 



ganglia, with single or partially separated commissural cords. 

 The concentration goes furthest in some Diptcra and in the 

 StrcjJsiptera, in which the thoracic and abdominal ganglia are 

 fused into a common mass." In many insects there are respira- 

 tory nerves, whose branches are distributed to the muscles of 

 the stigmata. The inner ends of these nerves form a plexus, 

 which is situated " over the interval between two of the 

 ganglia of the central nervous cord, and they are connected 

 by longitudinal cords with one another, and with these 



g 



Fig. 64. 



Nervous System ok Crab. 



a = optic nerve. /> = antennary 

 nerve. c — circum-oesophageal 

 nerve. ^ = fused thoracic ganglia. 

 e — nerve-cord of abdomen. 



ranglia. 



The Arachnida. 



In the Artlirorjastrn^ there is a bilobed cerebral ganglion 

 or brain connected by commissures with the sub-cesophageal 

 ganglion : from this passes a nerve-trunk (consisting of two 



