1903.] On the Nematooysts of Solids. 477 



IV. The Mechanism of the Discharge of Nematocysts. 



To understand how an ^Eolid is able to swallow Crelenterate nemato- 

 cysts undischarged, pass them in this condition through its alimentary 

 canal, and then discharge them in its own defence, we must know 

 something of the conditions and causes of this discharge. As far as 

 I know, this question has never been discussed as regards Nudibranchs, 

 but the theories brought forward to explain the action of the nemato- 

 cysts in Ccelenterates are innumerable. In 1887 von Lendenfeld 

 considered the solution of the problem in sight but ten years later 

 he confesses that the increase of knowledge has only multiplied hypo- 

 theses, and an adequate explanation seems further off than ever. I 

 think a consideration of the behaviour of nematocysts in ^Eolids throws 

 some light on the question. 



When nematocysts are extruded from a cnidosac and subsequently 

 evert their threads, not only have the original Crelenterate nematocytes 

 been digested, but the nematocysts are often quite free from any trace 

 of protoplasmic substance, even the " cnidoblast " in which it was con- 

 tained in the cnidosac. The discharge, therefore, cannot be directly 

 due to the contraction of the nematocytes or surrounding tissues of the 

 Coalenterate. 



Nussbaum and others have suggested that the elasticity and tension 

 of the capsule itself aids in causing the eversion of thread, while 

 Grenacher considers this tension sufficient in itself to evert the thread 

 but for the resistance of the operculum (" Deckelchen ") covering the 

 opening any slight access of pressure from outside suffices to over- 

 come this resistance and causes the discharge. But in this case the 

 nematocysts would be at least as likely to be discharged while being 

 swallowed by the ^Eolid, as when lying undisturbed in the sea water 

 after extrusion from the cnidosacs. 



The theories so far discussed rely on the assertions of H. Frey, 

 Clarke, Graffe, and others that the capsule of a nematocyst is smaller 

 after than before discharge. Iwanzoff, on the other hand, from a series 

 of measurements and calculations, reaches the conclusion that while 

 the capsule itself may be smaller, the total capacity of the nematocyst 

 is larger after discharge. For in a resting nematocyst the volume of 

 the thread must be subtracted from that of the capsule ; while in a 

 discharged nematocyst, the thread being hollow, its capacity must be 

 added to that of the capsule. The substance contained in the capsule 

 has therefore increased in volume, and Iwanzoff considers this to be 

 the cause of the eversion of the thread. He supposes the original 

 contents of the nematocyst to consist of a highly hygroscopic gela- 

 tinous substance ; this is protected from the access of water by the 

 impervious capsule wall and the operculum over the aperture of the 

 inverted thread. When this operculum is removed by the action of the 



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