Fig. 89.—Cydippe pileus
In order to observe the form and habits of the Beroe we transfer it to a vessel of sea water, when it immediately displays its regular spheroid form, and its eight rows of comb-like plates which form the meridians before alluded to. Its mouth is situated on the little tubercle at what we may call the lower pole, for it is the habit of the Beroe to swim in an inverted position, and the digestive cavity may be seen through its glassy body.
At first no appendages of any kind are visible, but soon the animal protrudes two long and exceedingly slender arms, fringed with slender gelatinous threads, from two cavities, at opposite sides of the body, into which they can be withdrawn. A close examination will also reveal the rapid movements of the cilia of its combs, and it is remarkable that these do not always work together, the animal being able to move any of its plates independently, and to reverse their motion when occasion requires. It has no tentacles corresponding with those of jelly-fishes and anemones, but is assisted in the capture of its prey by its two long arms, the chief use of which, however, seems to be that of a rudder for steering.
If the Beroe is left out of water for some time, the water which forms such a large proportion of its body evaporates, leaving an almost imperceptible residue of solid matter; and if left in water after it is dead, its substance rapidly dissolves away, leaving not the slightest trace of its presence. There seems to be no satisfactory way of preserving this beautiful form of animal life. If placed in strong spirit the water is rapidly extracted from its body, and its animal substance shrivelled to a minute, shapeless mass; while in weak spirit and in other fluid preservatives it becomes more or less distorted, and deprived of its beautiful transparency, or else it disappears altogether.
We now come to the great favourites among the cœlenterates—the beautiful anemones-the animated flowers of the ocean, remarkable not only for their lovely flower-like forms, but also for the great variety of colour and of habits which they display. These, together with the corals, form the division of the cœlenterates known as the Zoantharia, characterised by the possession of simple tentacles, the number of which is a multiple of either five or six. The latter differ from the former mainly in the power of secreting a calcareous skeleton which remains attached by its base after the animal substance has decayed.
The expanded anemone exhibits a more or less cylindrical body, attached by a suctorial base to a rock or some other object, and a broad circular disc above. In the centre of this disc is the mouth, surrounded by the tentacles, often very numerous, and arranged in one or more whorls. When the animal is inactive the tentacles are usually completely withdrawn, and the body contracted into a semiglobular or pear-shaped mass which is very firm to the touch.
The general internal structure of an anemone may be made out by simple dissections, and the examination conducted with the specimen submerged in water. A longitudinal section will show that the body is a double tube, the outer being formed by the body-wall, and the inner by the wall of the stomach. Thus there is a body-cavity distinct from that of the stomach, but the two will be seen to communicate below, since the stomach-wall does not extend as far down as the base. It will be seen, too, that the body-wall is made up of two distinct layers—an outer one, that is continued inward at the mouth to form the inner wall of the stomach, and an inner one that lines the whole of the body-cavity. The latter contains the muscular elements that enable the anemone to contract its body.
When the animal is expanded, the whole interior is filled with sea water, as are also the tentacles, which are hollow tubes, really extensions of the body-cavity, and formed by prolongations of the same two layers that constitute the body-wall. As it contracts this water is expelled, partly through the mouth, and partly through small openings that exist at the tips of the tentacles.