1903.] On the Nematoeysts of Solids. 475 



position, directed backwards along the sides of the body ; moreover, it 

 is doubtful whether the nematocysts which, in the ordinary course of 

 events, would have been derived from the very species being devoured, 

 would be very effective for paralysing the same. 



2. The second alternative is the one most generally accepted, and I 

 think it is undoubtedly the correct one. The experiments of Herdmant 

 and of Garstang prove that ^Eolids are unpalatable to many fish 

 (apparently not to all, for Mclntosh states that the cod eats ^olis 

 papillosa, but according to the same authority the most deadly bait for 

 cod are bright coloured anemones, and young flat fish fill their stomachs 

 with Edwardsia ; so that for these fish nematocysts evidently have no 

 terrors). No one can have witnessed the reaction of an ^Eolid to 

 various stimuli (touch of a foreign body, agitation of the water or, in 

 some cases, a shadow), without being convinced that the cerata are 

 used as a means of defence. The body is contracted, the head being 

 often nearly telescoped into the trunk, whi-le the cerata are erected and 

 waved about, especially in the direction of the foreign body, and are 

 often considerably extended (in R. peregrina the cerata when excited 

 are nearly double their length when at rest). 



But though the nematocysts are undoubtedly used defensively, I 

 think the actual method employed is generally misunderstood. The 

 ^Eolid is often described as " threatening " the foreign body with its 

 cerata, the supposition being that nematocysts are shot out against it. 

 It is certainly true, as stated above, that the cerata are turned towards 

 the foreign body which has touched the animal, but in experiments on 

 upwards of 40 individuals I have never seen nematocysts extruded 

 under these circumstances; and the mass of nematocysts extruded 

 from a cnidosac is quite visible to the naked eye. I have, indeed, only 

 once witnessed the emission of nematocysts for cerata still attached to 

 the body of an ^Eolid, and not themselves individually squeezed or 

 otherwise ill-treated. On this occasion a Sp. neapolitana about 3 '5 cm. 

 in length, in a small beaker of sea-water, was somewhat violently 

 stimulated with a glass tube. It erected its papillae, which in this 

 species are very numerous and usually carried curled up along the 

 sides of the body, and lashed them about in all directions, at the same 

 time emitting quite conspicuous little masses of nematocysts from its 

 cnidosacs. These were carried by the currents in the water and adhered 

 as a flaky white deposit to the bottom of the vessel. On examination 

 they were found to consist almost entirely of discharged nematocysts. 

 It is, I think exceedingly doubtful whether this promiscuous discharge 

 would have much effect on such an enemy as a fish, for 



(1) A very small proportion of nematocysts would come in contact 



with the fish, 



(2) Since eversion of the thread follows, as a rule, very rapidly on 



