ACTINIA OR SEA-ANEMONE. 481 



mals are always compound, and the polypidom almost invariably 

 consists of a stony or horny axis supporting a fleshy rind, in 

 which the individual Polypes are enclosed. 



ORDER L HELIANTHOIDA 



1 121. Of this Order of Polypes the Actinia, so abundant on 

 our shores, may be taken as the types, not only on account of the 

 ease with which they may be examined, from their being amongst 

 the commonest and hardiest of the inhabitants of the marine 

 Aquaria now so much in fashion, but also because they exhibit 

 in themselves the characters of a Helianthoid Polype in full per- 

 fection, unmasked by any social peculiarities, at the same time 

 that their usually large size renders the investigation of their 

 structure comparatively easy. 



1122. An Actinia of the ordinary form constitutes a short 

 fleshy cylinder, the walls of which present a very firm texture. 

 At its lower extremity this cylinder is as it were abruptly cut 

 off, forming a flat muscular disc by which the creature adheres 

 firmly to rocks ; the upper extremity is closed by a membrane of 

 lar greater delicacy than the leathery skin of the cylinder itself, 

 and forms a second disc, in the centre of which is the opening of 

 the mouth, surrounded by a great number of tubular tentacles ? 

 arranged in several rows. When the Actinia closes its mouth, 

 folds in its tentacula, and draws together the upper part of its 

 body, it presents an almost hemispherical form the flat side 

 being attached to the rock, and the mouth being just visible at 

 its summit ; but when the animal expands itself, the body ac- 

 quires the more or less cyclindrical form above alluded to, the 

 oral disc becomes nearly as wide as the base, and the fringes of 

 tentacula display their brilliant colours to the light of day. The 

 arrangement and appearance of these so much correspond \vith 

 that of the petals of double flowers, still more with that of the 

 florets of the Composite tribe, that it is by no means surprising 

 that the uninformed observer should almost always regard this 



