THE MOLLUSCOIDA AND CGELENTERATA. 83 



Annuloida, to recognise them as a separate division of the 

 Animal Kingdom. 



The next two classes the Actinozoa and the Hydrozoa con- 

 stitute one of the most natural divisions of the Animal Kingdom 

 the CCELENTERATA of Frey and Leuckart. In all these 

 animals, the substance of the body is differentiated into those 

 histological elements which have been termed cells, and the 

 latter are primarily disposed in two layers, an external and an 

 internal, constituting the " ectoderm " and " endoderm." 



Among animals which possess this histological structure, the 

 Ccelenterata stand alone, in having an alimentary canal, which 

 is open at its inner end and communicates freely, by means of 

 this aperture, with the general cavity of the body. In a large 

 proportion of these animals the prehensile organs are hollow 

 tentacles, disposed in a circle around the mouth, and all (unless 

 the Ctenophora should prove to be a partial exception to the 

 rule) are provided with very remarkable organs of offence and 

 defence, termed " thread cells " or " nematocysts." These, 

 when well exhibited, as, for example, by the common freshwater 

 polype (Hydra), are oval, elastic sacs, containing a long coiled 

 filament, barbed at its base, and serrated along the 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. These fine filaments readily penetrate any delicate 

 animal tissue with which they are brought into contact, and 

 cause great irritation in the human skin when they are of large 

 size. Nor can it be doubted that they exert a similarly noxious 

 influence upon the aquatic animals which are seized by, and 

 serve as prey to, the Actinozoa and Hydrozoa. Characteristic as 

 these organs are of the Coelenterates, however, it must not be 

 imagined that they are absolutely peculiar to the sub-kingdom ; 

 for some nudibranchiate Mollusca, such as Eolis, are armed with 

 similar weapons, and the integument of certain Turbellaria, and 

 even of some Infusoria, is provided with bodies which seem to 

 be of a not altogether dissimilar character. 



No Coelenterate possesses any circulatory organs, unless the 



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