STINGING CELLS 



173 



the gelatinous substance to swell up. According to others, the 

 cnidoblast contracts as a whole. The action of the threads is 

 mechanical and chemical. They fix, e.g. by the stilets, into the victim, 

 and the secretion poisons the wound, paralysing or killing small 

 animals, and sometimes acting as a solvent. Many seem to be pre- 

 hensile threads rather than weapons. 



The nervous system of the Coelenterates is of a primitive 

 type : a network of nerve-fibres runs diffusely and almost 

 uniformly through the body ; thus, as Romanes showed, 

 it is possible to cut a jelly-fish into a fantastic pattern, a 

 long ribbon for instance, without preventing the conduc- 



FiG. 86. — Diagram of stinging-cells or cnidoblasts, the one to the 



right undischarged. 



I, Nucleus ; 2, cytoplasm ; 3, lasso or nematocyst, with barb-like 

 processes at its base ; 4, the fluid-containing cavity of the cell in 

 which the undischarged nematocyst lies coiled up ; 5, the trigger or 

 cnidocil. 



tion of nervous impulses from one end to the other ; 

 whereas in Vertebrates an injury to the spinal cord at once 

 cuts off the lower part of the body from all nervous com- 

 munication with the brain. The Coelenterates have no 

 central nervous system, but only a nerve-net ; but in the 

 Anthozoa and Scyphomedusae there may be regions over 

 which the nerve-net does not extend, there may be differ- 

 entiated organs of special sensitiveness, and there are 

 usually certain nerve-tracts in which conduction of the 

 impulse takes place rapidly and in a determined direction — 

 hinting at the definite localised nerves of higher phyla. 



