SEA-ANEMONES. 83 



Let us inquire what is the difference between this group 

 of polypes called " Helianthoid^^ and that which contains 

 the Hydra and the other ^' HyclfoicV polj^es. It is this : 

 that while the former are composed of all the distinct parts 

 which we have been enumerating, — the outer covering, the 

 basal and upper discs, the vertical plates, the fleshy sub- 

 stance between them, the central stomach, and the lipped 

 mouth; the latter is a simple bag, open at one end and 

 fringed with tentacula, — mouth, stomach and all in one, — - 

 so simple indeed that it may be turned inside out and yet 

 perform the same functions. 



When describing the fresh- water Green Hydra, we shall 

 have occasion to speak of experiments made on that won- 

 derful creature, in the way of cutting up in pieces to ob- 

 serve its power of reproducing parts. Similar experiments 

 have been made, although not to so great an extent, with 

 ActinicB, and with a similar result. An Actinia, for instance, 

 has been cut across the centre, and instead of forming, as 

 might be expected, a new basal disc, it put forth, at the 

 severed part, a new set of tentacula, surrounding a new 

 mouth, so that the creature seized and devoured food at 

 both ends. New tentacles are soon supplied for any that 

 are accidentally or designedly mutilated. 



