ACTINIA. 45 



again expands, the central opening at the top gradually widens, the margin 

 slowly rolls back, and the tentacles it concealed begin to show their tips. As 

 the expansion goes on, the tentacles continue to enlarge, and the margin to 

 spread outwards, until, finally, the disk with the mouth in the centre is fully 

 displayed, and the tentacles,'like petals, fringe it round. 



FIG 40. ACTINIA. 



In the species delineated in the next figure, Actinia gemmacea, there is an instinct 

 displayed of a very admirable character. Such individuals as have taken up their resi- 

 dence on the half-submerged rocks, where the daily recess of the tide exposes them to 

 observation, are covered with rough warts, and blotched with dusky brown and dull 

 orange ; and still further to insure their concealment, cover themselves with fragments 

 of shells, sea-weed, and gravel, which adhere to their skin so strongly as not to be 

 washed off ; and being thus veiled, the animals are quite concealed from observation. 

 On the other hand, those species which inhabit deep water, as if aware that the neces- 

 sity for concealment no longer existed, use no such precaution : their skins are smooth 

 and naked, and adorned with the vivid- tints which make the species so beautiful. These 

 Actinias are easily procured, and may be kept alive in sea-water for a long time without 

 difficulty : in a glass vessel their beauty is displayed to advantage. They are capable 

 of very long fasts, although sufficiently voracious when food is to be obtained. 



Although the Actiniae are usually affixed to the rocks by their 

 broadly-expanded bases, many of them can detach themselves, 

 and float through the water to a new resting-place ; or they will 

 slide along slowly over the ground, by the action of their base or 

 foot, and some are said to turn themselves over and walk upon 

 the extremities of their tentacles. There is, indeed, a small group 

 of Actiniae (Actineta) fitted expressly for an ocean life, by means 

 of an air-cavity in the base, containing a vesicular or spongy disk 

 made up of air-cells, which serves as a float Thus provided, the 

 animal lies on the water with its base uppermost and its mouth 

 and tentacles below the surface, and in this position it is carried 

 about by winds and currents. 



The tentacles of the Actiniae are not always simple tubes : in 

 the A. alcyonoides, represented on the left hand of our engraving 



