478 Mr. G. H. Grosvenor. [Nov. 3, 



nematocytes or other cells, and the nematocyst itself is thrust out into 

 the water, the latter enters the basal portion of the inverted thread, 

 diffuses through its thin walls and causes the gelatinous substance to 

 swell up. He mentions in support of this the staining properties of 

 the nematocyst contents; this, when undiluted, i.e., in an undischarged 

 nematocyst, stains darkly with picric acid or methylen blue; in 

 successive stages of the eversion of the thread it stains successively 

 lighter. 



Von Lendenfeld, in reviewing this explanation of Iwanzoff' s, says : 

 " So schon diese Theorie auch ist, so erscheint sie doch ganz unhalt- 

 bar," the objections which he considers insurmountable being two : 



1. The thin probably chitinous wall of the capsule could not 



possibly prevent the access of water to the intensely hygroscopic 

 contents. 



2. The fact that the contents of undischarged nematocysts are 



stained proves that water can enter without discharging them. 



Also when a nematocyst is extruded from a cnidosac of an ^Eolid, as 

 stated above, there is nothing there capable of removing the operculum. 

 But, though the second objection seems fatal to Iwanzoff' s theory as it 

 stands, I think a slight modification of the same will not only avoid 

 these difficulties but explain a number of facts inexplicable on any other 

 hypothesis as yet propounded. 



When we examine the conditions antecedent to the discharge of 

 nematocysts in Ccelenterates and Nudibranchs, we find that apparently 

 the only one common to the two cases is a change of the medium 

 immediately surrounding the nematocyst. In Ccelenterates the nemato- 

 cysts before discharge are partially protruded from the surface of the 

 nematocytes into the surrounding water ; in Solids the nematocyst is 

 extruded from the cnidosac into the water. The nematocysts of the 

 faeces also discharge themselves on bemg extruded from the rectum into 

 water. In each case it seems to be the passage from a solution of 

 greater to one of less concentration which causes the eversion of the 

 thread. This idea is confirmed by the following observations : 



1. In connection with the earlier part of this paper I had occasion to 



examine a large number of ^Eolid cerata for the identification 

 and comparison of the nematocysts. For this purpose I found 

 Calberla's fluid a very convenient examination medium ; but 

 though large numbers of nematocysts were usually extruded on 

 placing the ceras in this liquid, the threads were never everted 

 except when a large proportional amount of sea water was 

 carried over with the ceras., 



2. Similarly, if a ceras be examined in a fairly strong solution of 



sugar or salt, the nematocysts are not discharged, but if the 



