ACTINIA. 73 



the still membranous upper part of the tube extends itself horizon- 

 tally outwards around the aperture (fig. 33, 2, b), doubling itself so as 

 to form a circular fold: this part of the membrane is no longer 

 irritable ; its internal surfaces unite, so as not to interrupt the con- 

 tinuity of the tube ; carbonate of lime is gradually deposited within it ; 

 and soon a prominent partition, composed of two lamellae, soldered 

 together through almost their entire extent, surrounds the tubular 

 cell. Generally many polyps of the same polypary form these par- 

 titions at the same time and upon the same plane. In this case the 

 gelatinous margins of the folded membrane unite, no space is left, 

 and they ultimately become most intimately soldered together, and the 

 solid plane or stage (fig. 32) is formed. If the animal construct its 

 partition against a tube already perfect and solidified, it fixes its collar 

 to its sides, so that the point of junction is imperceptible ; but when 

 it is quite insulated, as at b, fig. 32, the horizontal collar is still formed, 

 and it then assumes somewhat of an octagonal shape. The tube-form- 

 ing membrane exhibits no appearance of vessels or other traces of 

 organization. 



(160.) When the polyp is withdrawn within its cell, its tentacles 

 farm a cylindrical fasciculus (fig. 33, 2, c), the papillae which partially 

 cover them being laid upon each other like the leaflets of some Mimosce 

 when asleep. 



(161.) The protrusion of the creature from its tube is accomplished 

 by the contraction of the membrane, b, inserted into its neck. 



(162.) The germs, during the first period of their development, have 

 no organs distinguishable, not even the rudiment of a tube ; each ap- 

 pears to consist of a simple gelatinous membrane folded upon itself 

 (fig. 33, 4, c), and forming upon the stage upon which it is fixed a 

 little tubercle resembling a small Zoanthus or other naked zoophyte. 

 This tubercle gradually elongates, and assumes the form of a polyp, 

 provided with all its organs ; but the sac which encloses it is still 

 gelatinous at its upper part and membranous near the base (fig. 33, 4, b), 

 where it gradually diminishes in thickness, and, becoming calcareous, 

 gives to the animal the general appearance of its original. 



(163.) An extensive and important group of the ANTHOZOA, from the 

 fibrous character which the substance of their bodies assumes, have 

 been named by zoologists " Fleshy Polyps ; " nevertheless, although the 

 genera composing this division are exceedingly numerous, and vary 

 much in their external characters, they will be found to conform, in 

 the essential points of their organization, with the corals above de- 

 scribed. The subject we have selected as the type of these beautiful 

 zoophytes is a well-known Actinia, which being common upon our 

 own coasts, the reader will have little difficulty in procuring specimens 

 for examination, or for preservation in a marine aquarium, of which 

 they will form conspicuous ornaments. 



