270 Martin, A Note on the Occurrence of Nematocysts and Similar Structures etc. 



As is well known from the researches of Hadzi and Boulenger, 

 the Nematoblasts have the power of carrying the Nematocysts from 

 the point at which they are first laid down to the point at which 

 they are used and there arranging the Nematocyst so that finally 

 the pole of the capsule from which the thread is to be discharged 

 always points towards the periphery. The same phenomenon is 

 shown by the wandering cells which carry the ingested Nemato- 

 cysts of Microstomum from the gut to their definitive position under 

 the skin, cf. Martin, "The Nematocysts of Turbellaria", Q. J.M. S., 

 Vol. 52, 1908. 



"At a later period nematocysts can be found lying just outside 

 the gut, sometimes free, but usually surrounded by three or four 

 cells. These cells seem to be mesenchymatous phagocytes, though 

 I am not quite sure whether it is not possible for the cells of the 

 gut itself to become free and take up a wandering existence in 

 the body cavity. Finally the nematocyst is transported to a posi- 

 tion directly under the ectoderm; here it lies in the vacuole (vide, 

 figs. 2 4) (which is not an artifact, since it can be seen in the 

 living animal) surrounded by about six cells. The wall of the 

 vacuole after a time becomes thinner and denser. There is one 

 point of great interest as regards the orientation of the nematocysts 

 under the skin. The large barbed nematocysts in their final posi- 

 tion, always lie so that the thread, when it is discharged, will pass 

 out of the animal, although they may lie pointing in any direction 

 while they are still in the gut cells of the body cavity. This rule 

 does not seem to hold good in the small cylindrical nematocysts, 

 which, as far as I can see usually lie almost parallel to the 

 surface." 



Something of the same kind has been observed byGrosvenor 

 in Aeolids, p. 474, Proc. Roy. Soc., Vol. 72, "On the Nematocysts 

 of Aeolids". 



"The arrangement of the Nematocysts within the cnidosac 

 seems to indicate that they are used as weapons, for they usually 

 lie with the aperture, through which the thread will be everted, 

 turned towards the periphery of the 'cnidoblast'. It is true that 

 the 'round Turbellaria-like nematocysts lie much more indiscrim- 

 ately, and that even the long Actinian nematocysts are sometimes 

 reversed, but as a general rule the arrangement is as described'.'' 



It is very hard to see how this arrangement of Nematocysts 

 can be effected. Probably in the case of the Cleptocnids the original 

 efforts of the animal were directed towards the expulsion of amass 

 of undigestible capsules through the skin. Only two possible ex- 

 planations of the arrangement of a Nematocyst have occurred to me 



(1) that the Nematocysts exercise a stimulus on the cell con- 

 taining them, which in some extraordinary way compels that cell 



