484 Mr. G. H. Grosvenor. [Nov. 3, 



identical. Also the nematocysts from the faeces of an ^Eolid, 

 which are generally admitted to be derived from their food, are 

 always identical with at least some of the nematocysts from the 

 cnidosacs. 



4. Those ^Eolids (Janidae, Fionidoe and Calma glaucoides) which 



habitually feed on animals other than Coelenterates have no 

 nematocysts. 



5. Though several have tried, no one has succeeded in giving even 



a plausible account of the development of nematocysts in 

 ^Eolids. 



6. This view affords a satisfactory explanation of the function of 



the ciliated canal through which nematocysts and other indi- 

 gestible bodies have been observed to pass from the gastric 

 diverticulum to the cnidosac. 



7. A repetition of Strethill Wright's experiments gave entirely 



confirmatory results. In one case three R. peregrinas, having 

 only small pip-shaped nematocysts in their cnidosacs, were fed on 

 Pennaria Cavolinii, the nematocysts of which, after a month, had 

 almost entirely replaced the original pip-shaped ones. 



In Section III the functions of the nematocysts and cnidophores of 

 ^Eolids are discussed : 



There is little doubt that the borrowed nematocysts are used as 

 defensive weapons by the Solids ; they are discharged after extrusion 

 from the cnidosacs; they are usually so arranged in the cnidocysts 

 that the threads are shot out radially ; the cerata are brightly coloured 

 and are erected and elongated on the approach of a foreign body. 

 The nematocysts are, however, very seldom ejected indiscriminately 

 into the surrounding water, but usually only when the individual cerata 

 are squeezed or plucked off. 



The original and still important function of the terminal openings 

 of the cerata is probably to allow of the escape of the nematocysts 

 and other indigestible bodies, which, owing to the diffuse nature of 

 the digestive cavity, it is impossible to eliminate entirely through the 

 anus only. 



Mechanism of Nematocysts. The behaviour of nematocysts in Solids 

 proves that their discharge must be independent of the direct action of 

 living tissue. The explanation most in accordance with the facts 

 observed both under natural conditions and in experiments with various 

 liquids and solutions, seems to be that the eversion of the nematocyst 

 thread is due to the contents taking up water by osmosis from the 

 surrounding sea water, but being unable to take it up from more con- 

 centrated solutions, such as the liquid surrounding the nematocysts in 

 the Ccelenterate nematocyte or the digestive juices of ^Eolids. 



