THE FUNCTION OP NETTLEOELLS. 397 



or between tide-marks, bury the body in the sand and expand 

 their tentacles in the surface of the sand, the waves move the 

 sand and it is evident that masses of sand must be continually 

 falling on the tentacles. I have often observed a species of 

 Actinia exceedingly abundant in the " sands " of Port Phillip, 

 Victoria, and I know that they do not retract their tentacles 

 when the water moves and the sand drops on them. 



According to the mechanical theory, each sand-grain which 

 came into contact with the tentacles would cause the explosion 

 of a great number of Nettlecells. It is evident that this 

 cannot be so. 



Further, there are Nettlecells embedded in the jelly of the 

 umbrella of some Medusae (discovered by me, 1. c, over the 

 marginal bodies of Crambessa mosaica) which explode if 

 the surface of the body is touched with acetic acid instan- 

 taneously and long before the acid could have got to where 

 they are situated. 



This will show that touch is by no means sufficient to cause 

 the explosion of the Nettlecells ; nor is it the only possible 

 cause. 



It would now seem possible that the cnidoblasts were 

 exploded at the will of the animal by a contraction caused 

 consequent on a centrifugally acting nervous irritation in the 

 plasmatic mantle surrounding the capsule. Chun (6) has 

 observed muscular diflferentiations in this plasmatic mantle in 

 Physalia, which is one of the severest stinging Coelenterates. 

 Others have been inclined to consider the peduncle as 

 muscular and contractile. 



If we were to assume this we should not be able to see the 

 use of the cnidocil. 



It seems, therefore, that there can be but one explanation of 

 the mode of action of the cnidoblasts — of the large kind, at least 

 — which is the following : 



1. The structureless peduncle is a support and may contract 

 so as to withdraw the cnidoblast with its lid from the surface 

 under certain circumstances, particularly when the parts where 

 the Nettlecell is situated are to be contracted. The animal has 



