Martin, A Note on the Occurrence of Nematocysts and Similar Structures etc. 249 



general usage of these terms, e. g. (Delage, Sedgwick, Bourne, 

 Fowler, etc.) although Chun and Bedot have attempted to confine 

 this word to that part of the Nematoblast cell which is actually 

 converted into the Nematocyst, and Grosvenor has I think unfortun- 

 ately used the word Nematoblast in Aeolids for the cells in the 

 cnidosacs which ingest the Nematocysts (vid. p. 482 of his paper 

 "On the Nematocysts of Aeolids"). 



"It is round this basal end of each cnidoblast that the mem- 

 branous cyst is first secreted; the cnidoblast itself is drawn away 

 from this point towards the lumen of the cnidosac, and does not 

 apparently take part in the secretion of the cyst, but in the mean- 

 time ingests nematocysts at its opposite end, which remains naked 

 and amoeboid." 



It will not be necessary to give here a full history of the 

 earlier theories of the mechanism of the Nematocyst explosion, as 

 this has already been done by Lend enf eld t in his two valuable 

 summaries of Nematocyst work published in the Biologisches Central- 

 blatt for 1887 and 1897. 



In nearly all these early theories the main cause of the explosion 

 of the Nematocyst was thought to be a contraction either of a 



*/ 



muscular wall surrounding the Nematocyst, e. g. Chun or of the 

 capsule wall itself, and as Lendenfeldt remarks, p. 522, ,,Es ist 

 selbstverstandlich, dass alle diese Erklarungen der mechanischen 

 Schussursache, eine Verkleinerung des Volumens der Kapsel beini 

 Schusse, wie sie Zoja (s. o.) auch tatsachlich beobachtet haben will, 

 voraussetzen." 



Now it is the great merit of Iwanzoff, 1896, that he seems 

 to have been the first to regard the Nematocysts as absolutely 

 dead structures the mechanism of which could be explained on 

 purely physical grounds. He was able to show quite clearly that 

 Nematocysts could explode under conditions which absolutely exclude 

 all possibility of a nervous, muscular or cytoplasmic factor (p. 141). 



Iwanzoffs explanation depends upon the fact that the cause 

 of the expulsion of the thread is the entrance of water into the 

 capsule. 



The whole of the factors involved in the discharge of a Nemato- 

 cyst in a Coelenterate are not, yet I believe, satisfactorily accounted 

 for. But the main fact that the process is, as far as the capsule 

 is concerned, a purely physical one, is I think abundantly clear. 



There seems to be no necessity to enter here into a detailed 

 account of the work, of later authors in this field, nor of the vast 

 literature that has grown up on the subject of the development of 

 the Nematocyst. There is however one point in the behaviour of 

 Nematoblasts which I should like to refer to here, and that is their 

 power of conveying in a hydroid the Nematocyst from the point 



