PHYSIOLOGY OF THE INVERTEBRATA. 299 



to the ectoderm, and consists also of nerye-fibres and 

 ganglion-cells. In these respects there is no difference 

 between the lower and upper nerve-rings ; but it may be 

 remarked that a difference is distinguishable between the 

 two. In the former there are many nerve-fibres of con- 

 siderable thickness, whereas in the latter the nerve-fibres are 

 exceedingly slender, and there are few ganglion-cells. " The 

 two nerve-rings are separated from one another by a very 

 thin membrane, which, in some species at all events, is bored 

 through by strands of nerve-fibres which, serve to connect the 

 two nerve-rings with one another.'' 



"The peripheral nervous system is also situated in the 

 ectoderm, and springs from the central nervous system, not by 

 any observable nerve-trunks, but directly as a nervous plexus 

 composed both of cells and fibres. Such a nervous plexus 

 admits of being detected in the sub-umbrella of all Medusoe, 

 and in some species may be traced also into the tentacles." 



This nerve-plexus is situated between the muscle-fibres and 

 the epithelium. " There are also described peculiar tissue 

 elements, such as, in the umbrella, nerve-fibres which pro- 

 bably stand in connection with epithelium-cells ; nerve-cells 

 which pass into muscle-fibres, similar to those which Kleinen- 

 berg has called neuro-muscular cells ; and in the tentacles 

 neuro-muscular cells joined with cells of special sensation. 

 No nervous elements could be detected in the convex surface 

 of the umbrella, and it is doubtful whether they occur in the 

 veil." (Romanes.) 



The nervous system of the covered-eyed- differs from that 

 of the naked-eyed Mediism. In the former the central 

 nervous system consists of separate centres unconnected with 

 ■commissural cords. There are eight, twelve, or sixteen (but 

 generally eight) of these nerve-centres situated in the 

 margin of the umbrella. They consist of cells of special 

 sensation and a thick layer of delicate nerve-fibres. These 

 nerve-fibres are merely prolongations of epithelial cells, as 

 true ganglion-cells are entirely absent. 



