3. Porpita.
Body membranous, regular, circular, depressed, el ightly convex above ; internal cartilaginous support circular, with its surface marked by con centric strive crossing radiated strife, cover ed I On US upper sur face by a delicate membrane merely. The body is concave below, and the infe rior surface is fur nished with a great s. number of tentacula, 4. of which the exterior ones are the longest, and furnished with small cilia, each ter minated by a glo bule: theysometimssi contain air; and the internal ones are the shortest, the most simple, and the most fleshy. In the centre of those tentacula is the mouth, in form of a small proboscis, which leads to a simple stomach, sur rounded by a some what glandular sub stance.
Cuvier, from whom a great portion of the above description is taken, says, in the last edition of the ' ItI?gue that there is but one species (Porpita gi pantea) of a beautiful blue colour, from the Mediterranean, and other warmer seas Lamarck, who esta blished the genus for an animal which had been placed among the Jfecbsiee by Linumus, gives four species; but De Blainville and MM. Chamisso and Eisenhardt coincide with Cuvier in believing that they are all referable to one, though the former admits that the fact is still somewhat doubtful He observes that Bosc's species, Holothuria appendiculata, (Porpita appendicu lata, Lam.) was evidently established on an impaired animal. Eschscholtz, under the name of Porpita Mcditer ranea, conjoins three of Lamarck's species, and describes three new ones, ;eking for his character the proportion of the cartilaginous disk, and especially that of the marginal cirrhi.
Geographical Distribution.—Like that of Velella, very wide. Bose, who met with them at sea, says the animal has the appearance of a 24-sous piece borne along by the waves. Examples, Porpita gigantea, and P. glandifera.
4. Polybrachionia. (Guilding).
Dorsal support (sustentaculum) cartilaginous, naked, flattened, rounded, radiated, concentrically striated; mantle (pallium) narrow, free, surrounding the support; arms numerous, parallel, of various lengths, elon gated, affixed beneath, with a power of taking a de clining posi tion for the purpose of taking . prey. Mouth below, central, purse shaped, exten sile. Tentacula many, varying in form, sue tonal, cover ing the whole ventral sur face. Eggs, very small, innumerable, nestled among the tentacula. Example, Po lybrachionia Linnaana.
This genus has been este blished by Mr. Guilding, who describes the support as broad and vitreous, the body as ceru lean, tho ten tacula as pal lid, and tho arms, which are in a triple series,•glandu bons, the glen dules being pedunculated. The diameter of the mantle, exclusive of the arms, is stated to be eleven and a half lines. Mr. Guilding states that the animal is wonderfully beautiful, swimming, or rather floating on the serene surface of the Caribean Sea in calm weather, and embracing its prey by the sudden downward application of some or all of its arms, which are easily broken by attrition.
We think there is hardly enough to warrant a generic separation in this case; the species bears a strong resemblance to the Porpita carulea of Eschscholtz. Mr. Guilding observes that the Medusa porpita of the 'Arnxnitates Academicm ' seems nothing more than the central disk of some species deprived of all the organs of the body.
The fourth order is the PflYSOORADX. The structure of the creatures belonging to this order were but little known till the publication of Mr. Huxley's researches, in ' Philosophical Transac tions' for 1849. They are allied to the preceding order both in the conformation of the gastric organs and in their means of locomotion. The principal difference between this and the preceding families is, that we here lose the radiate form, and observe in the creatures which belong to it a lateral symmetry. This order is now made to include the genus Physalia and its allies, which are possessed of an air-bag, by means of which they float through the ocean, and also the various forms of Diphydce. These two forms were included by Cuvier in his division of Hydrostatic Acalephce. Before referring to their arrange ment, we shall give the principal results of Mr. Huxley's researches into the anatomy of these creatures, as given by him in the 'Report of the Twenty-First Meeting of the British Association for the Advance ment of Science.' We shall speak first of the Diphydce. If one of these creatures is examined, it will be found to consist of two trans parent crystalline pieces, which look, when taken out of the water, like morsels of cut-glass. One or both of these pieces contains a wide
cavity, lined with a muscular membrane, by the contraction of which the animal is propelled through the water. The attachment of the posterior piece to the anterior is very alight, and when detached it will swim about independently for hours together. It was this cir cumstance which led Cuvier to suppose that the two pieces were two independent animals, and in this he has been followed by the majo rity of zoologists. He describes the two individuals as always together, one including itself in a hollow of the other (l'uu s'emboltant dans un creux de l'autre), an arrangement which nevertheless permits their separation without the destruction of life. They are, he observes, gelatinous, transparent, and move very nearly like the Medusce. Tho including individual (Femboltant) produces from the bottom of its hollow a chaplet (chapelet) which traverses a demi-canal of the inclu ded individual (l'emboite), and would seem to be composed of ovaries and of tentacula and suckers like those of the preceding genera. Cuvier then goes on to state the divisions established by MM. Quoy and Gaimard, according to the relative forms and proportions of the two individuals. Thus in the Diphyes, properly so called, the two individuals are nearly alike, pyramidal, and with some points round their opening, which is at the base of the pyramid. In the Calpes, the included individual has still the pyramidal form, but the inclu ding individual is very small and square. In the Abyles, the included individual is oblong or oval, and the including rather smaller and bell shaped. In the Caboides, it is the included individual which is small and bell-shaped ; the including individual is much larger and square. In the Naricules, the included individual is bell-shaped; the including individual large also, but slipper-shaped (en forme de sabot). Cuvier concludes by remarking that there are many other combinations. There are two prominent forms of Diphydce, the Monogastric and the Polygastric. In the former a single polype is developed in a special cavity of the anterior piece. In the polygastric a long chain of such polypes, each enveloped in a little transparent bract, occupies a similar position. These polypes have no oral tentacles; but a long thread-like tentacle, bearing lateral branches, which are terminated by small sacs, is developed from the base of every polype. The small prehensile sac has a peculiar form, but is only a dilatation of its pedicle. It is much thickened on one side, and contains a great number of the stinging hairs to which we have before alluded. The repro ductive organs are medusiform bodies, which are developed by gemmation from the pedicle of the polype.
In the Polygastric Diphydce new polypes are continually being produced by gemination at the attached extremity of the polype chain, and in all the species the same gemmation is continually going on among the prehensile and reproductive organs.
The structure of the other forms of Physograda are modifications of a common type, in the main identical with that of the Diphydcr. The great difference is in the absence of the air-organ, or float. The same continual multiplication of parts by germination goes on among the Physsophoridce as among the Diphydce, and the structure and mode of development of the young organs are the same. Great variety is presented by the reproductive organs, from the form of mere sacs to that of free-swimming bodies precisely resembling Medusa', and developing the generative elements only subsequently to their liberation. In Physalia the female organs are free-swimming medusi form bodies, while the male organs are simply pyriform sacs.
As a general conclusion it may be stated that the Physograda are essentially composed of two membranes, an outer and an inner, which are called by Mr. Huxley 'foundation membranes,' since every organ is formed by the modelling into shape of one or other or both of these ; commencing as a simple process, or diverticulum, and assuming its perfect form by a gradual change of development. The stomach has no walls distinct from those of the general varieties. The reproductive organs are always developed externally. The stinging hairs, or thread cells, are found in all the species in the greatest abundance.
The following is the arrangement of these creatures according to M. de Blainville