io8 



MANUAL OF ZOOLOGY. 



developed the sacs are tensely filled with fluid, and the slightest 

 touch is sufficient to cause the retroversion of the filament, 

 which then projects beyond the sac for a distance, which is not 



Fig. 36. Thread-cells of Ccelenterata, greatly magnified. A and B, The thread-cell of 

 Caryophyllia Smithii, in the everted condition, and in two varieties ; C and D, The 

 thread-cell of Corallimorphus profundus, in a quiescent and active condition, en- 

 larged about 500 times ; E, The thread-cell of Hydra, in an everted condition. (After 

 Gosse and Moseley.) 



uncommonly equal to many times the length of the latter " * 

 (Huxley). Many beautiful modifications of shape are known 

 in the thread-cells of different Coelenterates, but their essential 

 structure in all cases is much the same as in the Hydra. It is 

 only in few cases, comparatively speaking, that the thread-cells 

 have the power of piercing and irritating the human skin ; but 

 even in the diminutive Hydra it is probable that they exercise 

 some benumbing and deleterious influence on the living organ- 

 isms which may be captured as prey. Besides the thread- 

 cells, the tentacles of some Hydroids are furnished with rigid 

 hair-like processes, which are probably tactile in function, and 

 which are known as " palpocils." 



The Ccelenterata are divided into two classes, termed respect- 

 ively the Hydrozoa and the Actinozoa. 



* Thread-cells, though very commonly, if not universally, present in the 

 Ccelenterata, are nevertheless not peculiar to them. Similar organs have 

 been shown to exist in several of the Nudibranchiate Molhtsca, as well as 

 in some Annelides (Spio seticornis). There likewise exist analogous organs 

 trichocysts) in several of the Infusoria, and in the Planarida. 



