50 NATURE STUDIES. 



A POISONOUS LIZAIW. 



BT DR. ANDREW TTILSOX, F.R.S.E., F.L.S. 



THE possession of a poison-apparatus is by no means 

 a common event in the animal world; although, in- 

 deed, very diverse animals are possessed of offensive 

 means of this kind. Low down in the animal scale we 

 find the jelly-fishes, sea-anemones, and their neigh- 

 bours, possessing these curious stinging organs called 

 " thread-cells," the virulence of which many an un-> 

 wary bather has experienced to his cost. Each thread- 

 cell is really a minute bag, tensely filled with fluid,, 

 and containing, coiled up in its interior, a thread 

 or filament. When, from any cause as by pressure,, 

 for example the cell is ruptured, the fluid escapes, 

 and if the thread and fluid together come in contact 

 with the tissues of any animal liable to be affected, the 

 animal in question will be paralysed, or even killed. 

 In this way the Hydra, or "fresh-water polype/' 

 captures its prey; and even in the lower deeps of 

 the animal world (as amongst the Infusoria, for 

 example), these thread-cells appear to be represented.. 

 Higher up in the animal series, we come upon the 

 poison-apparatus of insects, carried in their tails, as 

 also is the " sting " of the scorpion. The centipede's 

 poison-fangs are situated, on the contrary, in its 

 mouth. Amongst the shell-fish, or Molluscs, no poison- 

 secretions occur. In fishes, as the lowest Vertebrates, 



