EXCRETORY ORGANS. '21 



Primordial centralized nervous 

 system. These rings afibrd 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. INLuscular 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, Avith 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 dLstinction muBt, however, be made between the 

 naked-ej-ed and covered-eyed Jelly-fiah ; the hydroid offspring 

 corresponding to the former only, the latter being the olfwpriug 

 of the higher hydrozoans. 



