32+ PHYSIOLOGY OF THE INVERTEBRATA. 



at the base of the proboscis. Nerves are given off from this 

 gauglion to the proboscis, and through the retinacla to the 

 muscular wall of the body. 



The CHiETOGNATHA. 



This class contains only one genus — Sagitta. The nervous 

 system consists of a cerebral ganglion (brain) on which the 

 eyes are placed, and a ventral ganglion situated near the 

 middle of the body. These i\^o ganglia are united by 

 commissures. Near the mouth there are a pair of sub- 

 oesophageal ganglia, which are united to each other, and to 

 the cerebral ganglion by commissures which embrace the 

 oesophagus. 



The Prototracheata. 



The nervous system of Peripatus consists of two large 

 supra-oesophageal ganglia, and two imperfectly-ganglionated, 

 widely-separated nerve-trunks, which proceed to the posterior 

 part of the body. From these two trunks many lateral 

 nerves pass outwards and inwards; and, according to Grube,* 

 the latter act as commissures between the two nerve-trunks. 



The Myriapoda. 



The nervous system of these animals forms a ventral chain, 

 with a pair of ganglionic enlargements for each segment of 

 the body. The anterior pair is united by commissures 

 with the cerebral ganglia. The ventral chain gives off on 

 each side a number of lateral nerves. The nervous system of 

 the Myriapoda has been compared to that of the larvas of 

 the Insecta. The cerebral ganglia furnish nerves to certain 

 sense organs, such as the eyes. 



The ganglia are constituted of cells, and the cords of nerve- 

 fibres. 



* Archiv fur Anatomic, 1853. 



