composed of a slender thread coiled up in a very close and regular 

 spiral, bearing a resemblance to the shell of a Cerithium. The 

 ecthorceum is discharged reluctantly, and the wall of the cnida is 

 very subtile. 



4. Globate cnidse (Cnidce globatce)1 These are globose vesicles 

 found in the acontium of S. parasitica, which have some characters 

 in common with the cnidce y but of whose real nature I am doubtful. 

 In the indubitable cnidce the emission of the ecthorceum is a pro- 

 cess of eversion. This is proved by many circumstances, such as 

 the order in which the portions are evolved, the basal portion first ; 

 as well as by direct observation, the terminal part of the ecthorceum 

 being occasionally detected in running out through the centre of the 

 portion already evolved. 



The cnidce are filled with a fluid, which holds organic corpuscles 

 in suspension, and these are seen driven rapidly through the ectho- 

 rceum in the process of eversion. I conclude that in this fluid resides 

 the expansile force, which, on the excitement of a suitable stimulus, 

 distends and projects the tubular portion of the wall that has hitherto 

 been inverted. 



All of the four kinds of cnidce enumerated have been at various 

 times seen surrounded by a membranous investiture, which I distin- 

 guish as the peribola. This coat must be ruptured before the cnida 

 can emit the ecthorceum. 



Several experiments show that the ecthorceum has the power of 

 penetrating the tissues of other creatures, and even of the Verte- 

 brata. In some of these experiments shavings of human cuticle, 

 presented for an instant to the tentacles of B. crassicornis, and to 

 the acontium of S. parasitica, were found on examination to be 

 pierced through with numerous cnidce. 



Experiments with blue vegetable juices were instituted, with a 

 view to test the acid or alkaline properties of the poisonous fluid 

 supposed to be ejected on the discharge of the ecthorceum ; but with 

 no definite result. The existence of such a poisonous fluid is in- 

 ferred, however, with a degree of probability amounting to moral 

 certainty, and that of such concentrated power as, under certain cir- 

 cumstances, to destroy life with great rapidity, even in vertebrate 

 animals. 



Admitting the existence of a venomous fluid, it is difficult to 



