EXCRETORY ORGANS. 21 



Primordial centralized nervous 

 system. These rings afford the earliest 

 example of a central nervous system in the 

 animal world. 



As to muscles, the whole inner surface of 

 the bell is lined by fine cancellated muscular 

 fibres. Muscular tissue also occurs in the 

 "veil." 



Primordial specialised excretory 

 organs. In some species of these medusoid 

 free buds, " pores " occur leading out of the 

 marginal canal. These pores subserve excre- 

 tion of whole matter and mark the earliest 

 occurrence of special organs for this purpose 

 amongst animals. 



In this free bud, which we have 

 cursorily described, with its " bell :> and 

 trailing tentacles, we are introduced to 

 particular forms of creatures, which are well- 

 known as Jelly-fish, but whose life-history 

 is not so fully understood.* 



*A distinction must, however, be made between the 

 naked-eyed and covered-eyed Jelly-fish ; the hydroid offspring 

 corresponding to the former only, the latter being the offspring 

 of the higher hydrozoans. 



