CCELENTERATA : HYDROZOA. 87 



body and opens on the surface by a mouth at one end and an 

 excretory aperture or anus at the other. 



Though of the true " radiate " type, some Coelenterates show 

 traces of bilateral symmetry. Thus, in some Sea-anemones 

 one of the tentacles is larger than, or differently coloured 

 from, the others; and in some corals two of the primary 

 septa, opposite one another, are larger than the rest, and 

 divide the animal into two halves. 



With regard to the fundamental tissues of the Ccelenterata, 

 there exist two primary membranes, of which one forms the 

 outer surface of the body, and is called the " ectoderm ; " 

 whilst the other lines the alimentary canal, the general cavity 

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

 " endoderm." These membranes correspond with the primi- 

 tive 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," 

 embedded in a granular " intercellular substance " or " peri- 

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

 vacuolation or fibrillation. Between the ectoderm and endo- 

 derm there is sometimes a third layer, which appears to be 

 muscular. 



In connection with the integument of the Cozlenterata, 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. 18, d.) Many beautiful modifications 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, comparatively speaking, 



* 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 selicornis}. There likewise exist analogous organs 

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



