THE NERVOUS SYSTEM OF CCELENTERA. 105 



the cells and fibres previously described as nervous from 

 histological appearances, 



It is not probable that we shall be able to prove in a 

 similar conclusive manner that the " nervenschicht " of the 

 Hydroids and of the xA.nthozoa has the same function, but 

 nevertheless these experiments give us much more con- 

 fidence in the assertions that are made as to its nervous 

 function. 



Mention was made at the commencement of this essay 

 that in sea-anemones and other Coelenterates the nerve 

 plexus is not confined to the ectoderm, but that similar 

 cells and fibres may be found in connection with the 

 endoderm. 



The brothers Hertwig have described and figured this 

 endodermic nervous system in the sea-anemones, but per- 

 haps the most favourable Coelenterate that can be readily 

 obtained for its investigation is the Alcyonium. 



This form consists of lobate colonial masses perforated 

 by tubes^ — ^the polype cavities — passing from the surface, 

 where they terminate in the mouths of the polypes, to the 

 base of the colony. These tubes are lined by endoderm. 

 The ectoderm is confined to the general surface of the mass 

 and to the body wall, tentacles and stomodaeum of the pro- 

 trusible portions of the polypes. It would be impossible 

 in such an essay as this to describe the details of the course 

 of these two layers, but it will be sufficient to state that 

 when compared with a Hydrozoan or a sea-anemone, the 

 endoderm is in Alcyonium a very extensive and important 

 tissue. The endoderm lining the tubes is provided with a 

 considerable muscular layer — the muscles forming a 

 cylindrical sheath immediately subjacent to the epithelium. 



It is now fully recognised that when an Alcyonium is 

 removed from the water or irritated in any manner in the 

 water it is capable of contracting in bulk very considerably, 

 and this contraction must be brought about by the cylindri- 

 cal muscle sheaths expelling a portion of the water in the 

 tubes. 



The endodermic nervous system which, we may pre- 

 sume, governs these contractions, may be seen quite clearly 



