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



occurrence of these cylinders which leads one to suppose they must 

 serve some useful end. Certainly, the evidence of the use by the 

 Solids of their stolen nematocysts is very much stronger. 



In the first place, the nematocysts discharge themselves, after 

 extrusion from the cnidosac. I mention this, because the fact might 

 be considered improbable, though it has been recorded by many 

 observers. It is true chat, in many cases, the cnidae are so minute, 

 and the discharge is effected with such rapidity that the act itself 

 is difficult or impossible to follow. But tips of the cerata are so trans- 

 parent that the cnidse can usually be seen lying in the walls of the 

 cnidosac undischarged ; whereas a second or two after they have been 

 extruded into sea water, the threads of the majority are everted. 

 Sometimes the actual eversion itself can be seen, as when the thread is 

 very conspicuous (e.g., large Actinian nematoeysts) or when, for some 

 reason, the eversion takes place slowly. For instance, I have seen the 

 large bean-shaped nematocysts of Eudendrium from a ceras of E. peregrina 

 evert the distal portions of their threads with great deliberation ; the 

 part already everted lengthened with a curious sinuous motion of the 

 tip, while the part still within the capsule uncoiled itself at a corre- 

 sponding rate. (I need hardly say that I cannot confirm M. Hecht's 

 opinion that the nematocysts of Nudibranchs differ from those of Cnidaria, 

 in that only the basal portion of the thread of the power is inverted, 

 the distal part being coiled up inside the inverted base ; on the contrary, 

 in all cases, in both Nudibranchs and Cnidaria, in which I could clearly 

 distinguish the thread inside the capsule, it was coiled up more or less 

 regularly, between the inverted base and the wall of the capsule, and 

 must, therefore, have been inverted itself.) 



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 Tubularia- 

 like nematocysts lie much more indiscrimately, and that even the long 

 Actinian nematocysts are sometimes reversed, but as a general rule the 

 arrangement is as described. 



The fact that those species which have no cnidosacs are more richly 

 provided with glands on the cerata, as if in compensation for their 

 absence (see Hecht on Calma glaucoides and Krembzow on Fiona), seems 

 to point to the same conclusion. 



But if the nematocysts are used as weapons, they may be either 



1. Offensive weapons, used against prey, 



2. Defensive weapons, used against enemies. 



1. This view was suggested by Bergh in his paper of 1861 (according 

 to the abstract in the ' Microscopical Journal '). But I have often watched 

 ^Eolids feeding, and noticed that the cerata are held in the normal 



