METHODS OF ATTACK AND DEFENCE 107 



poisonous type of weapon, Martin (19 14) proposes the 

 following classification : — 



1. Autocnidag : true nematocysts confined to the 

 Coelentera. 



2. Cleptocnidae : Ccelenterate nematocysts which have 

 been ingested with the food (Solids, Turbellaria, etc.). 



3. Pseudocnidae : often confused with true nemato- 

 cysts, but not homologous. This group includes the 

 nematocysts of certain Nemertines from which a spiral 

 thread can be discharged and those of Turbellaria in which 

 no trace of a thread has been discovered. 



4. Polar capsules : strictly analogous as regards develop- 

 ment, structure, and mechanism to true nematocysts, but 

 confined to the Sporozoa. 



In Echinoderms {e.g. the sea-urchin) the place of nemato- 

 cysts is taken by the gemmiform pedicellariae. These, the 

 largest of the pedicellariae, are about 10 mm. long. The 

 " head " is composed of 3 valves or jaws each provided with 

 a poison gland which opens at the base of the tooth forming 

 the apex of the valve. Further, there is present at the 

 base of each jaw on its inner side a sense organ in the 

 shape of a small papilla formed of sensory cells, each of 

 which is provided with a stiff process or hair. 



When the sea-urchin is attacked by its inveterate 

 enemy the starfish, the chemical substance exuded by the 

 latter causes the gemmiform pedicellariae to gape. Upon 

 the enemy coming in contact with the sensory papillae the 

 blades close violently, inflicting a wound which causes the 

 poisonous juices of the starfish to exude. The effect of this 

 is a renewed stimulus, which causes the poison glands to 

 discharge their contents into the wound. 



Prouho (1890) has watched a combat in an aquarium 

 between the starfish Asterias glacialis and the sea-urchin 

 Strongylocentrotus lividus. As soon as the latter feels the 

 contact of the tube feet of the starfish it directs its spines 

 away from the part attacked. This discloses the pedi- 

 cellariae, which are seen to be stretching towards the starfish 

 with their jaws agape. No sooner does one of the tube feet 



