476 Mr. G. H. G-rosvenor. [Nov. 3, 



the extrusion from the cnidosac, the few that did reach the fish 

 would probably be already discharged, 



(3) It is very doubtful whether nematocysts would penetrate the 

 scaly covering of a fish large enough to be dangerous to an 

 ^Eolid. 



It is, therefore, not surprising that this method of defence is not 

 more generally employed. 



With this single exception, in all the Solids with which I experi- 

 mented nematocysts were emitted only when individual cerata were 

 squeezed or plucked off. In this way a fish which had snapped at the 

 cerata, the loss of which, as is well known, does not seem to incon- 

 venience the Nudibranchs at all, would receive a discharge of nematocysts 

 into its mouth, where they would probably act with the greatest possible 

 effect. This is in complete agreement with Garstang's view as to the 

 meaning of certain features in the coloration of Solids. He suggests 

 that the localisation of the bright colours in the cerata " series to 

 direct the experimental attacks of young and inexperienced enemies to 

 the non-vital papillae and away from the vital and inconspicuously 

 coloured parts of the body," and " at the same time gives them (the 

 enemies) the needful experience of the unpalatable nature of their 

 intended prey " ((9) p. 175). The erection and elongation of the cerata 

 conduce to the same result (i.e., make them the most probable mouthful 

 for an enemy), even when the bright colours are absent or otherwise 

 disposed. 



But though the nematocysts are certainly used as defensive weapons, 

 their removal from the digestive system is an important, perhaps the 

 original, function of the cnidosacs. In molluscs other than the 

 cladohepatic Nudibranchs, the food is digested in the stomach, the 

 liquefied products being passed into the gastric gland, where absorption 

 takes place (see Simroth (20)). In Tritonia, therefore, the anus suffices 

 for the passage of nematocysts out of the body. But in the Clado- 

 hepatica part of the food is digested in the gastric gland, quite fresh 

 pieces of hydroid being found in the ducts and ceratal diverticula of a 

 recently fed ^Eolid. On examining the cerata of a Doto which had 

 been feeding on Pennaria Cavolinii, I found them crowded with large 

 nematocysts. How Dotonids, which habitually feed on hydroids and 

 have no apertures in their cerata, get rid of the nematocysts I cannot 

 say ; perhaps by throwing off their cerata, which as is well known they 

 do with great ease. When an aperture for the extrusion of nemato- 

 cysts had once been acquired, it would be obviously advantageous 

 that the distal end of the " hepatic " diverticulum should be modified 

 to form a cnidosac where the nematocysts might be stored to be used 

 in defence as required. 





