The .fifth order, A.Lcyos.IDE-E, is characterised by the " organization being gelatinous, or like that of a tremella, of a dirty olive red colour, becoming darker on exposure to the air." The Alcyonidea evidently approach very near to the zoophytes of the genus Alcyonium ; but neither polypi, nor the habitations of polypi, are to be ob served in them. A transverse section of these plants ex hibits large hexagonal or irregular reticulations, which di minish in size towards the sides of the stem. In a longi tudinal section, these reticulations appear lengthened, and form at the exterior a very fine epidermis. The fructifi cation of this order is conspicuous enough. It consists of yellowish capsules, situated in the epidermis, and full of small black seeds. The Alcyonidea have considerable affinity to the genus Dumontia of the Floridem, above de scribed. They decompose rapidly when left by the tide on the shore. Their texture is so loose, that when once dried and compressed, they never resume their original form.
The only genus is .ellcyoniclium : " Capsules containing seeds, innate in the fleshy or gelatinous substance of the plant." The principal species is the Ulva diaphana of former writers, (Eng. Bot. t. 263), and which is not un common on our shot-es. It is remarked by M. Lafoy, that, at certain times of the year, the Ulva diaphana is phos phorescent; but this fact has not been accurately ascer tained, and it is possible that the phosphorescence alluded to, may depend merely on numbers of a minute shining nereiii adhering to the plant, when newly cast ashore, or drawn from the sea. We have frequently observed a bril liant phosphorescent appearance produced by this cause, in specimens of Fueus dentatus, and in various species of Sertularia, recently taken from the sea, the phosphore scence being renewed upon moving the specimens, or dis turbing the minute animals.
The sixth ortltr, SroNoonEx, has this character : " Or ganization spongy, colour green, tarnishing on exposure to the air." This order, as published by Lamouroux, con sists only of a single genus, which Olivi, a distinguished Italian naturalist, proposed in his Zoology of the .thiriatic Bea, under the name of Lamarckia. This name, however, having been pre-occupied by a genus of phauogamous plants, Stackhouse suggested Codium. Lamouroux has preferred Sjeongodium, on account of the general resem blance of the plants which constitute the genus to some kinds of sponges, and because, like them, they imbibe wa ter. It has this character : " Seeds scattered through the substance of the plant, but most abundant at the ex tremities of the filaments which clothe its surface." One of the principal species is Spongodium dichotomum, the Focus tomentosus of former authors, figured in Engteh Botany,t, 712, and in the History of the t. 135. The substance of the plant consists of a collection of interwoven fistular tubes, full of a transparent fluid. The surface, as
noticed in the character, is covered with small capillary filaments, which scent to serve for the absorption of water, and among which vesicles and capsules are to be looked for. These filaments are best observed while the plant remains in the sea : when it has been withdra•n for some time from that element, they nearly disappear. The co lour is a dark grass-green, which becomes paler by drying. It may be proper to add, that the Focus fungosus, found on the coast of Barbary by Destontaines, and described in the Flora ?lilantica, is to be considered as only a variety of F. toruentosus. Another species of Spongodium is Fu cus bursa of our shores, (Turn. t. 136.) the Alcyonium bursa of Linnaeus. It may here be mentioned, that since the publication of Lamouroux's system, Turner has de scribed and figured (Hies Fuc. t. 175) a new spongodium by the title of Focus siinfilicitisezthis, a name which is to be understood as applicable only in a relative sense, being meant to distinguish it from either the dichotomous rami fication of F. tomcntosus, or the spherical shape of F. bursa. Instead of having a soft velvety exterior like these, it is papillose ; but its internal structure is quite similar. Tur ner remarks, that F. simpliciusculus, in external appear ance, closely resembles the caulerpa, having the same co lour, the same glossy surface, and the same kind of sub stance, intermediate between the submersed alga and the zoophytes, or even approaching more to the nature of the latter than of the former. The spongodea appear to last more than one year. They inhabit places never complete ly uncovered by the tide. They are nearly allied to the Ulvacea, and might perhaps commodiously follow them.
In Lamouroux's arrangement of sea-plants which has now been detailed, no fewer than 273 ascertained species are referred to by the author. There are, besides, near 200 unpublished species alluded to by him : of 20 of these, figures are given in the 4nnales, but no descriptions ; and when, or in what form, the remainder are to be brought before the public, does not appear. There are nearly 100 varieties of different species mentioned ; but in no tribe of plants is there greater difficulty in determining the limits between species and varieties ; it will be nowise surpriz ing, therefore, if several of these varieties he hereafter es tablished as species ; and, on the other hand, a few of the species degraded to the rank of varieties. Marine plants are not only liable to very considerable diversity of form, but, as already noticed, it is nearly impossible to cultivate them with a view to observe their germination, and ex tremely difficult even to watch their progress, on their native submarine rocks, from infancy to maturity. Of the ascertained species, 148 are found on the shores of France.