The Radiata are so named on account of the ray-like form generally observable in the structure of the creatures; in some the ray-like divisions give such a speciality to the structure as to distinguish them at once as members of this division; as in the star-fishes, for instance, in which the intestinal canal branches out from the body into the several rays which form the star, and in the anemones, in which the relation to the tribe is at first sight perceptible in the tentacles which surround the mouth, and which render it so exquisitely beautiful as a marine representative of a true flower.
But though the term Radiata is applied to an extensive division, in which the members have many characteristics in common with each other, the ray-like form is not equally distinguishable in all. In some tribes there is a tendency to associate into groups, in which each individual has a certain degree of connection with the rest, as in the infusoria common in our brooks, and indeed most of the polypes which thus live in community. The resemblance to vegetable forms is, however, common to a great portion of the Radiata, and those in which this resemblance is the strongest are grouped together under the general designation of Zoophytes. In Zoophytes, the leading feature of a radiate animal is very distinctly observable, and that leading feature is the arrangement of the vital organs around a centre, the organs composing the rays of the imaginary star, or the petals of the imaginary flower, to which the mouth or stomach is the centre.
EDWARDSIA VESTITA, ÆSOP PRAWN, ENTEROMORPHA COMPRESSA, ULVA LATISSIMA.
Among the Zoophytes we meet with many of the creatures which have the greatest attraction for the student of the Aquarium. The brooks supply him with the curious hydra, the seven-headed monster that perpetuates one of the triumphs of Hercules—withal a beautiful and wondrous creature, that may be cut in pieces, turned inside out, or even thrust one animal within the other, and still remain the same. The sea supplies the madrepores, the builders of ocean-reefs, and the founders of islands and continents; as it also supplies the sea anemones of more than a hundred species, from the curious Edwardsia vestita, here figured, from the first seen in this country, at present in the collection of Mr. Alford Lloyd, to the familiar members of the genus Actinia, obtainable everywhere on our coasts.
The true Zoophytes have all, more or less, the plant-like form, and they readily separate into two great classes, namely, the Anthozoa, or flower-life, and the Polyzoa, or many-life, in which the individuals are associated together in numbers. They are all inhabitants of water, are all destitute of joints, lungs, nerves, and proper blood-vessels; but in the place of nerves possess what naturalists call an irritable system, in obedience to which they expand or contract at will. At the upper part of the body is situated the mouth, which is usually surrounded with tentacles, which are mostly used in securing prey. There is no alimentary duct, for the stomach has the form of a simple sac, the aliment being injected and ejected by the same orifice.
The Anthozoa comprise animals which are perfect in themselves, and these are mostly soft bodied, having no shelly covering, and are protected only by the leathery integument which surrounds them, and the thousand weapons of offence and defence which they expand in the form of tentacles. Among the Polyzoa we meet with creatures that encase themselves in horny shells, or calcareous coatings, such as the Madrepores, which, like submarine masons, elaborate the carbonate of lime which the sea supplies them with, into shelly retreats; and the tubed Hydrioda, which construct winding galleries and convoluted tubes, from the mouths of which they protrude their fans and tentacles in search of prey.
Among the higher orders of the Radiata we meet with the strange Sea Cucumbers and the Sea Urchins, and the Star fishes; and among the lower orders the Sea Anemones, many forms of which are described and figured in these pages.
A Sea Anemone, then, is a Zoophyte belonging to the class Anthozoa, or flower-life, and the order Helanthoida, or sunflower-like creatures. The central disk of the sea flower is composed of the lips, which open into a mouth which communicates with the simple sac which constitutes the stomach, and the petals and fringes which surround it—now like the anemone, now like the sunflower or the mesembryanthemum, or the richest carnation that ever won for a florist a golden prize. The further subdivision is dependent on the details of individual structure; and a large section—that of Actinia—comprehends most of those on which the aquarian bestows his patience in the work of domestication.