332 DESCRIPTION OF THE PLATES. 



down the centre of B. The dilatation is armed near its distal end with 

 three spines. This form is characteristic of the larger nematocysts of 

 Hydrozoa, and is only found in that group. In some Anthozoa the basal 

 dilatation is of great extent, and is armed with spines disposed in a spiral. 



Nematocysts are found in all Anthozoa and Hydrozoa, but in no 

 other Coelenterata. The trichocysts of Infusoria are formed in the same 

 manner, as are the structures found on the dorsal processes of Aeolidia 

 among Mollusca. In the Turbellaria very similar structures are met with 

 in the rhabdocysts or rod-cells. 



