POLYPI. 



135 



63 



Actinia, dissected. 



are perforated at their free extremity, and communicate with the 



interspaces, h, of the meso- 

 gastric lamellae, g. They ab- 

 sorb the sea- water into these 

 spaces, and are elongated by 

 the injection of that water 

 into their interior. The ex- 

 tended surface of the abdo" 

 minal cavity is beset with 

 innumerable minute cilia, 

 through the action of which 

 it is bathed by a constant 

 current of the admitted me- 

 dium of respiration, the sea- 

 water, mixed with the chyle 

 from the stomach. These re- 

 spiratory streams pass from 

 one compartment to another 

 by the orifices marked g', g' : the arrows show their direction in the 

 opened tentacule, h. 



The ova are formed within the mesogastric folds : beneath which 

 are situated an equal number of bodies composed of convoluted tubes 

 which contain sperm-cells and spermatozoa, demonstrating the an- 

 drogynous nature of the Actinia. The impregnated ova are developed 

 into ciliated gemmules in the abdominal reservoir of sea-water; they 

 sometimes enter and distend the hollow tentacles ; then make their 

 way by the small inferior aperture of the stomach into that cavity, 

 and finally escape by the mouth of the parent. 



Many of the large actiniform polypes of the tropical seas com- 

 bine with a structure which is essentially similar to our own sea- 

 anemonies*, an internal calcareous axis or skeleton, which, pene- 

 trating the interior of the mesogastric folds, presents the lamellated 

 and radiated structure recognisable in the enduring support 

 of the large Fungice and in the polype-cells of the skeletons of the 

 Caryophyllece, MadreporeB, &c. This skeleton consists of carbonate 

 of lime, which in the Madreporince is in the condition of organised 

 corpuscles compacted into a closely reticulated mass, on a basis of 

 gelatinous matter. This animal constituent exists in such a propor- 

 tion in some CaryophyllecB (C sinuosa, e. g.) as to preserve the form 

 and in some degree the foliated character of the skeleton after solu- 

 tion of the earthy part in acid.f 



The species of polypes which take the most important share in the 



* CXXn. p. 176. pL>Tii. 



E 4 



t See Prep. No. 82. c. 



