I 



C(ELENTERATA I HYDKOZOA. 67 



of the body, and the tubular tentacles, and is termed the 

 1 endoderm.' These membranes correspond with the primitive 

 serous and mucous layers of the germinal area, and become 

 differentiated in opposite directions, the ectoderm growing 

 from within outwards, the endoderm from without inwards. 

 Each consists of numerous nuclear bodies, or * endoplasts,' 

 imbedded in a granular 'intercellular substance' or 'peri- 

 plast ; ' and each may be rendered more or less complex by 

 vacuolation or fibrillation. 



In connection with the integument of the Ccdenterata, 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 Jelly-fishes) 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 retro version 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.) The Ccelenterata 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, 6.) 



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 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 Jelly-fishes (Medusae), and 

 the Portuguese man-of-war (Phy solid). Owing to the great 



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

 Ccelcnterata, 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. 



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