Actinian Zoophytes.
The great family of the Actinian zoophytes abounds in genera and species. The common Sea Anemone, or Actinia, of which there are more than seventy species on the British coasts, is the model of the minute polypes which inhabit the stony corals, and build the coral reefs and atolls of the tropical Pacific.
The Sea Anemone has a cylindrical body, attached at one end by a sucker to rocks or stones at no great depth, and a flat circular disk at the other, with the mouth in its centre: the mouth, which is surrounded by a series of tubular, smooth-edged, radiating tentacles, resembles a blossom. The soft smooth body consists of two layers, as may be seen in the sections of an Actinia ([fig. 129]). The outer layer generally contains red matter, the inner one is of muscular fibre, and contains a great cavity, in which a somewhat globular bag or stomach is suspended. The space between the stomach and the cylindrical body of the animal is divided into chambers by perpendicular radiating partitions, consisting of thin plates or lamellæ. The mouth, which opens at once into the stomach, imbibes sea-water; and the hollow tentacles surrounding it being perforated at their extremities, and in communication with the chambers immediately below them, also imbibe the sea-water and convey it into the chambers; and the vibrations of the innumerable cilia, with which all the cavities of the animal are lined, keep them perpetually bathed with the respiratory medium mixed with nutrient juices from the coats of the stomach.[[27]]
Fig. 129. Actinian polype.
The Sea Anemone is monœcious and oviparous; the eggs are formed and fertilized in the lower parts of the perpendicular lamellæ or radiant plates; but they are hatched within the visceral cavity, and the larvæ issue from the mouth. The Actiniæ are also propagated by buds. They have as great a power of repairing injuries as the Hydræ, and like them too, though generally fixed, they can creep about by means of their expanded suctorial disk, and even float on the surface of the water. In many species the tentacles, as well as the body, are brightly coloured. The Actinia sulcata, an inhabitant of the British Channel, is of a deep crimson, with from 100 to 200 grass-green tentacles. The tints are owing to coloured particles in minute globules, that lie under the transparent skin of the animal and its tentacles.
With the exception of some of the Acalephæ, the thread-cells of the Sea Anemone are more highly developed than in any other animals. They not only differ in the various Actinian zoophytes, but sometimes even in the same individual. The complicated structure and action of this warlike apparatus was unsuspected previous to the microscopic observations of Mr. Gosse on the Actiniæ in general, and especially on the little scarlet fringed Sagartia miniata, a native of the British coasts. Like all the Anemones, it is highly sensitive; on the slightest touch it draws in its scarlet blossom, and shrinks into the form of a hemispherical bulb. While in the act of contracting, white filaments like ribbons shoot out from various parts of its surface, and new ones appear on every fresh effort, streaming out to the length of several inches, irregularly twisted and tangled. As soon as the contraction is finished, these fine white filaments begin to be recalled, and gradually retire in small irregular coils into the interior chambers between the stomach and the wall of the body, where they are stored up when not in activity.
Each filament makes its egress and ingress through an almost imperceptible transverse slit, discovered by Mr. Gosse, in the middle of an oval depression in the wall of the animal’s body. The slits, which are called cinclides, are very numerous, and resemble a pair of inverted eyelids, which can be opened and shut at pleasure. When the animal is irritated it contracts, and the water which fills the perpendicular chambers is forced in a stream through the slits, and carries with it the white filaments lodged within them; and then these quivers, which are full of deadly weapons, are ready for action.
Under the microscope, the white filaments are like narrow flat ribbons with their edges curled in, and thickly covered with cilia. They have not the slightest trace of muscular fibre, even when viewed with a microscopic power of 800 diameters; yet they extend, contract, bend, and coil in every direction; they bring together the margins of the ribbon so as to form a tube, and open them again; and the filaments perform all these motions even when severed from the animal, no doubt by the contractile nature of the clear jelly or sarcode, of which their bases are composed, as in the tentacles of the Acalephæ.
Innumerable oblong dart or stinging-nettle cells, closely packed together, lie under the folded edges of the ribbons, throughout their whole length, especially at their tips.[[28]]