22 ON CLASSIFICATION. 



A third distinctive character of the Hydrozoa is, that the 

 digestive cavity communicates directly, by a wide aperture, with 

 the general cavity of the body ; the one, in fact, passing by 

 direct continuity into the other. Furthermore, the digestive 

 sac is not in any way included in the substance of the rest of 

 the body, but stands out independently, so that the outer wall 

 of the digestive cavity is in direct contact with the water in 

 which the animal lives, and there is no perivisceral chamber. 

 The like is true of the reproductive organs, which may vary 

 very much in form, but have the common peculiarity of being 

 developed as outward processes of the body wall, so that their 

 external surfaces are directly in contact with the surrounding 

 medium. 



No nervous system has yet been discovered in any of these 

 animals. The majority of them seize their prey by means of 

 tentacula developed either around the mouth, or from the walls 

 of the digestive cavity, or from the body wall ; and these tenta- 

 cles, as well as other parts of the body, are provided with 

 those peculiar weapons of offence which have been termed 

 " thread-cells." 



The class of the Actinozoa contains those animals which are 

 familiar to us as Sea-anemones and Coral-polypes, by the latter 

 of which, in many parts of the world, those huge reefs, which are 

 so well known to navigators, are constructed. It embraces the 

 Sea-pens and the Red coral, and those creatures which are 

 known to us under the names of Beroe, Cydippe, Pleurobrachia, 

 &c, transparent, beautifully symmetrical, free-swimming ani- 

 mals, provided with eight rows of longitudinally-disposed large 

 cilia. In all these animals we find a great uniformity of struc- 

 ture, and their plan of construction is quite as readily definable 

 as that of the preceding class, with which they exhibit a close 

 affinity. Like the majority of the Hydrozoa, most Actinozoa 

 have their mouths surrounded by tentacles; and there is the 

 same primary distinction of the body into two cellular layers — 

 the ectoderm and the encloderm — though, in the adult forms of 

 the more highly organized Actinozoa, these primitive layers be- 

 come further differentiated into bundles of definitely disposed 

 muscular fibres, and even into nerves and ganglia. 



