THE SUB-KINGDOM C(ELENTERATA. 5 



cell the ecthoraeum lies twisted in many irregular 

 coils round its sheath; the barbs of the latter 

 being closely ajopressed to its sides, while it com- 

 pletely fills up the open end of the inner sac, into 

 whose interior it projects. Under pressure or 

 irritation, the cnida suddenly breaks, its fluid 

 escapes, and the delicate thread is projected, still 

 remaining attached to the sheath. So quickly is 

 this done that the eye can by no means follow the 

 process, but, in all probability, a complete ever- 

 sion of the cell's contents takes place. In some 

 cnidae the presence of a sheath has not yet been 

 discovered. 



Thread-cells vary much both in form and size. 

 They are unusually large in the Portuguese Man- 

 of-war (Physalia), where they are spherical in 

 ligure and attain a diameter of -003 of an inch. 

 The relative dimensions of the thread and cell 

 also vary. Sometimes the ecthorasum is scarcely 

 longer than the sac; in other cases itt* length is 

 nearly iifty times as great. 



The disagreeable stinging sensations experienced 

 when the human skin is brought into contact with 

 the bodies of some Coelenterata is, by most zoolo- 

 gists, attributed to the influence of the thread-cells. 

 It is supposed that the irritation is in part me- 

 chanical, arising from the friction of the fl lament 

 or its sheath, and in part chemical, from the 

 assumed poisonous nature of the fluid contained 

 within the cell. The ease with which many 

 Coelenterate animals seize and, as it were, para- 

 lyze their struggling prey, is also ascribed to the 

 same agency. These stinging propensities were 

 evidently known to Aristotle, who refers to dif- 

 ferent forms of the present group imder the name 



B 3 



