TRIBE I ASTR^EACEA. 123 



than the ordinary tentacles could do, for the process of aeration : and 

 these appendages may be either external and marginal, or they may 

 be scattered among the other tentacles, or may take the place of 

 all the ordinary tentacles, and cover a large part of the disk. Such 

 species are usually furnished with suctorial vesicles, either upon the 

 fringe-tentacles, disk, or sides, or both, which enable them to gather 

 around them the sand, pebbles, and shells, beneath which they lie 

 concealed. Nothing can exceed the beauty of the embroidery with 

 which these species are decorated.* 



Besides the above modifications, there are some species which 

 swim free in the ocean, and for this purpose have an air cavity in the 

 base of the animal, formed by drawing together the sides of the base; 

 and, within the cavity, there is a broad disk composed of air-cells, 

 which answers as a float for the animal. This structure was first 

 figured and accurately described by Lesueur. " This disk is formed 

 of a multitude of small membranes, placed one on another, the largest 

 at the upper part, the rest diminishing in size to the point where this 

 disk is in contact with the bottom of the cavity ; its membranes in 

 this place are solid and pressed together, particularly towards the 

 centre, where is a small conic, hard, opaque nucleus, whence these 

 membranes seem to have their origin, and form the disk. This air- 

 bladder disk is easily separated from the cavity which encloses it."f 



* It has been questioned whether these fringe-appendages, especially when marginal, 

 should be considered the analogues of tentacles. The relation which has been shown to 

 exist between the size of the animal, the number of its visceral lamella, and the number 

 and position of its tentacles, affords some ground for deciding upon this point. From the 

 facts stated in 25, it appears that the normal number of tentacles in the Actiniae is quite 

 large, and each corresponds to an interval between the numerous visceral lamella?, of 

 which there are, in the species referred to, about seven to a breadth of a fourth of an 

 inch. In species, therefore, which have, along with these fringe-appendages, compara- 

 tively few simple tentacles, as inner series, it is a fair inference, that the fringe is actually 

 an analogue of the outer tentacles. The same conclusion also follows with regard to the 

 tubercles of the margin, which characterize many species. 



The elongate lobes to the disk, which distinguish the Lucernarire, and which are fur- 

 nished with suctorial vesicles without proper tentacles, may arise in part from the pro- 

 duction of the tentacles in sets, instead of their separate developement. And in the 

 Alcyonaria, which have but eight tentacles, and appear to be related to the Actiniae 

 through the Lucernarire, it is quite possible that each fringed tentacle should be viewed 

 as the analogue, generally, of more than a single tentacle in an Actinia ; this would be 

 inferred from the usual relation between the size of the animals and the number of these 

 organs. 



t Lesueur, Jour. Acad. Nat. Sci., Philad. i. 182, and figure 6, pi. 7. 



