THE ACTINOZOA. 23 



coral, and those creatures which are known to us under the names 

 of Beroe, Cydippe, Pleurobracliia, &c., transparent, beautifully 

 symmetrical, free-swimming animals, provided with eight rows 

 of longitudinally-disposed large cilia. In all these animals we 

 find a great uniformity of structure, 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 Aetinozoa have their mouths surrounded by ten- 

 tacles ; and there is the same primary distinction of the body 

 into two cellular layers the ectoderm and the endoderm 

 though, in the adult forms of the more highly organized Aetino- 

 zoa, these primitive layers become further differentiated into 

 bundles of definitely disposed muscular fibres, and even into 

 nerves and ganglia. 



As in the Hydrozoa, again, the alimentary canal communi- 

 cates freely, and by a wide aperture, with the general cavity of 

 the body ; but the whole of the Aetinozoa, polype-like as they 

 are in external appearance, differ from the Hydrozoa by a very 

 important further progress towards complexity. We found that 

 in the Hydrozoa the digestive cavity was completely outside the 

 general cavity of the body, the digestive portion of the organism 

 being continued into, and not in any way contained within, the 

 part which surrounds the general cavity. But if you make a 

 vertical section of a sea-anemone (Fig. 7), you will find that 

 the alimentary cavity as freely open at the bottom as in the 

 Hydrozoa is enclosed within a part of the body which contains a 

 prolongation of the general cavity. If you could suppose the 

 stomach of a Hydrozoon thrust into that part of the body with 

 which it is continuous, so that the walls of the body should rise 

 round it and form a sort of outside case, containing a prolon- 

 gation of the general cavity, the Hydrozoon would be converted 

 into an Actinozoon. 



The prolongation of the general cavity thus produced, which, 

 as it surrounds the chief viscus, may be termed the " perivisceral 

 cavity " (d), receives the products of digestion mixed with much 

 sea-water ; and the nutritive fluid, which fills the perivisceral 

 cavity and its ramifications, plays the same part as the blood of 

 the more highly organized animals. The gastric chamber of 



