THE ACTINO/OA. 



23 



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

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

 body ; but the whole of the Adinozoa, polype-like as they are in 

 external appearance, differ from the Hydrozoa by a very im- 

 portant 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 contains the general cavity. But if you make a 

 vertical section of a sea anemone (Fig. 6), 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. 



Fio-. 6. 



Fig. 6. — Perpendicular section of Actinia holsatica (after Frey and Leuckart) ; a, mouth ; 

 6, alimentary cavity ; c, common cavity; d, intermesenteric chambers; e, cord con- 

 taining thread-cells at the edge ; /, the mesentery ; g, reproductive organ ; h, tentacle. 



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

 the perivisceral cavity and its ramifications, plays the same part 



