76 MANUAL OF ZOOLOGY. 



In connection with the integument of the C&lenterafa t the 

 organs termed " thread-cells " (" cnidae," or " nematocysts ") 

 must be noticed. These are peculiar cellular bodies, of 

 various shapes, which probably serve as weapons of offence 

 and defence, and which communicate to many members of 

 the sub-kingdom (e. g., the Sea-blubbers) their well-known 

 power of stinging. In the common Hydra the thread-cells 

 consist of " oval elastic sacs, containing a long coiled filament, 

 barbed at its base, and serrated along its edges. When fully 

 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 uncommonly equal to many times the length of 

 the latter."* (Huxley.) (Fig. 12, d.) Many beautiful modi- 

 fications of shape are known in the thread-cells of different 

 Ccelenterates, but their essential structure in all cases is much 

 the same as in the Hydra. It is only in few cases, compara- 

 tively speaking, that the thread-cells have the power of piercing 

 and irritating the human skin ; but even in the diminutive 

 Hydta it is probable that they exercise some benumbing and 

 deleterious influence on the living organisms which may be 

 captured as prey. The Cozlenterata are divided into two 

 classes, termed respectively the Hydrozoa and the Actinozoa. 



CLASS I. HYDROZOA. 



The Hydrozoa are defined as Ccelenterata in which the walls 

 of the digestive sac are not separated from that of the general 

 body-cavity, the two coinciding with one another ; the reproduc- 

 tive organs are in the form of external processes of the body-wall. 

 (Fig. 12, a, b.) 



It follows from the above, that, since there is but a single 

 internal cavity, the body of a Hydrozoon on transverse section 

 appears as a single tube, the walls of which are formed by 

 the limits of the combined digestive and somatic cavity. 



The Hydrozoa are all aquatic, and the great majority are 

 marine. The class includes both simple and composite 

 organisms, the most familiar examples being the common 

 Fresh-water Polype (Hydra), the Sea-firs (Scrtularida), the 

 Jelly-fishes (Medusa), and the Portuguese man-of-war (Phy- 



* 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 Mollusca, as well as in 

 some Annelides (Spio seticornis}. There likewise exist analogous organs 

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



