328 PHYSIOLOGY OF THE INVERTEBRATA. 



closely-applied commissural cords) to the three ganglia 

 situated in the region of the twelfth to the fourteenth somites 

 of the body. The abdomen contains four ganglia, from the 

 last of which leads two nerves terminating in the extremity 

 of the body. The cerebral ganglion, as in the Insecta, gives 

 ofE nerves to the eyes and other sense organs ; while branches 

 from the sub-CESophageal ganglion are distributed to the 

 maxillae and following somites. 



The visceral nervous system is well developed in these 

 animals. 



In the Araneina, the nervous system is more concentrated 

 than in the last-mentioned order. It consists of cerebral and 

 sub-cesophageal ganglia with branch-nerves, which proceed to 

 the organs of sense and other parts of the body. In fact it 

 will be observed that in the Araneina the ganglia are con- 

 centrated round the oesophagus. The same arrangement 

 occurs in the Acarina. 



The Crustacea. 



As a representative of the lower Crustacea we describe the 

 nervous system of Gyclestheria hislopi, belonging to the 

 Phyllopoda. The nervous system of this animal has been 

 recently worked out by Dr. Gr. 0. Sars.* The cerebral 

 ganglion or brain (see Pig. 1 1) is located within the pre-oral 

 part of the head, posterior to the compound eye and im- 

 mediately below the anterior part of the alimentary canal. 

 It is rather large and of a somewhat irregular form, but very 

 difficult to examine minutely on account of its being to a 

 great extent concealed by the scape of the antennae. From 

 the upper part of this ganglion, and somewhat in front, the 

 strong optic nerves originate. These nerves are not united, 

 but quite separate throughout their whole length, each giving 

 rise, at the end, to a ganglion, lying at a short distance 

 posterior to the eye and sending off to this organ numerous 



* Ohristiania Vidensledbs-SeWkahs Forhandlinger, 1887. 



