• THE MOLLUSCOIDA AND CCELENTERATA. 81 



1 cannot doubt, then, that the Molluscoida form a natural 

 assemblage ; but, until the precise characters, it' any exist, which 

 unite them with the Mollusca proper can be clearly defined, I 

 am inclined to think it might be better, as in the case of the 

 Annuloicla, to recognise them as a separate diviskm 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 Frev and Leuckart. In all these ani- 

 mals, the substance of the body is differentiated into those histo- 

 logical 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 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. 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 Ccelenterates, however, it must not be 

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

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



G 



