POLYPI. 87 



their functions in relation to the development of the new progeny, 

 drop off like the seed-capsules of plants. This phenomenon afforded 

 to the early botanists an additional argument in favour of the relation 

 of these ramified and rooted animals to the Vegetable Kingdom. 

 The Anthozoa {Jig. '^6.), or polypes of the second great class, cha- 

 racterised by a distinct abdominal cavity in which their 

 simple digestive sac is suspended, constitute the most nu- 

 merous and important part of the whole race, and in- 

 clude the largest individuals. They are principally the 

 inhabitants of the warmer or tropical seas. 



They are subdivided according to the number of their 

 oral tentacula. Most of the species have only eight of 

 these radiated prehensile oi-gans : the rest have a greater 

 number. To this latter group belong the soft-bodied 

 and solitary species called Sea-Anemonies or ActinicB, 

 which are common upon our own coasts. 



In the species here dissected, you will see that the skin is thick 

 and opake : in the living Actinia, it is lubricated by a mucous 

 secretion : the disposition of the muscular fibres by which it is 

 acted upon, is indicated by the superficial striae. In the middle 

 of the circle of the tentacles is situated the mouth, from which a 

 short oesophagus leads to a large gastric cavity, the parietes of which 

 are connected by a great number of membranous vertical folds with 

 the external wall of the body. The tentacula are tubular ; they are 

 perforated at their free extremity, and communicate with the inter- 

 spaces of the mesogastric lamellae. They absorb the sea-water into 

 these spaces, and are elongated by the injection of that water into 

 tJieir interior. The extended surface of the abdominal cavity is 

 beset with innumerable minute cilia, through the action of which 

 it is bathed by a constant current of the admitted medium of re- 

 spiration, the sea-water. 



The ova are formed within the mesogastric folds : beneath the 

 folds is situated an equal number of sacs or bodies, composed 

 of convoluted tubes which contain granules and spermatozoa, de- 

 monstrating the androgynous nature of the Actinia. The impreg- 

 nated ova are developed into ciliated gemmules in the abdominal 

 reservoir of sea-water : then make their way by the small inferior 

 aperture of the stomach into that cavity, and 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 lamel- 



G 4 



