POLYZOA.
Among Polyzoa the animal"s body is coated with a membraneous covering, like that of the Tunicated Mollusca, but which is a continuation of the edge of the cell, which doubles back upon the body in such a manner that when the animal protrudes from its cell it pushes out the flexible membrane just as one would turn inside out the finger of a glove. This oneness of cell and polype is a distinctive character of the group. Another is the higher organization of the internal parts. The mouth, surrounded by tentacles, leads by gullet and gizzard through a channel into a digesting stomach, from which the rejectable matter pa.s.ses upwards through an intestinal ca.n.a.l till it is discharged near the mouth.
The tentacles also differ much from those of true Polypes. Instead of being fleshy and contractile, they are rather stiff, resembling spun gla.s.s, set on the sides with vibrating cilia, which by their motion up one side and down the other of each tentacle, produce a current which impels their living food into the mouth. When these tentacles are withdrawn, they are gathered up in a bundle, like the stays of an umbrella. Our Plate I. contains the following examples of Polyzoa.
VALKERIA CUSCUTA. PL. I. FIG. 3.
From a group in one of Mr. Lloyd"s vases. Fig. 3 A is the natural size of the central group of cells, in a specimen coiled round a thread-like weed. Underneath this is the same portion enlarged.
When magnified to this apparent size, the cells could be seen in different states, some closed, and others with their bodies protruded. When magnified to 3 D, we could pleasantly watch the gradual eversion of the membrane, then the points of the tentacles slowly appearing, and then, when fully protruded, suddenly expanding into a bell-shaped circle. This was their usual appearance, but sometimes they could be noticed bending inwards, as in fig. 3 C, as if to imprison some living atom of importance.
Fig. B represents two tentacles, showing the direction in which the cilia vibrate.
CRISIA DENTICULATA. PL. I. FIG. 4.
I have only drawn the cells from a prepared specimen. The polypes are like those described above.
GEMELLARIA LORICATA. PL. I. FIG. 5.
Here the cells are placed in pairs, back to back. 5 A is a very small portion on the natural scale.
CELLULARIA CILIATA. Pl. I. FIG. 7
The cells are alternate on the stem, and are curiously armed with long whip-like cilia or spines. On the back of some of the cells is a very strange appendage, the use of which is not with certainty ascertained. It is a minute body, slightly resembling a vulture"s head, with a movable lower beak. The whole head keeps up a nodding motion, and the movable beak occasionally opens widely, and then suddenly snaps to with a jerk. It has been seen to hold an animalcule between its jaws till the latter has died, but it has no power to communicate the prey to the polype in its cell or to swallow and digest it on its own account. It is certainly not an independent parasite, as has been supposed, and yet its purpose in the animal economy is a mystery. Mr. Gosse conjectures that its use may be, by holding animalcules till they die and decay, to attract by their putrescence crowds of other animalcules, which may thus be drawn within the influence of the polype"s ciliated tentacles. Fig. 7 B shows the form of one of these "birds" heads,"
and fig. 7 C, its position on the cell.
Fl.u.s.tRA LINEATA. PL. I. FIG. 1.
In Fl.u.s.trae, the cells are placed side by side on an expanded membrane. Fig. 1 represents the general appearance of a species which at least resembles F. lineata as figured in Johnston"s work.
It is spread upon a Fucus. Fig. A is an enlarged view of the cells.
Fl.u.s.tRA FOLIACEA. PL. I. FIG. 2.
We figure a frond or two of the common species, which has cells on both sides. It is rarely that the polypes can be seen in a state of expansion.
SERIALARIA LENDIGERA. PL. I. fig. 10.
NOTAMIA BURSARIA. PL. I. fig. 11.
The "tobacco-pipe"" appendages, fig. 11 B, are of unknown use: they are probably a.n.a.logous to the birds" heads in the Cellularae.
PLATE V.
CORALS AND SEA ANEMONES.
CARYOPHYLLAEA SMITHII. PL. V. FIG. 2. PL. VI. FIG. 3.
THE connection between Brainstones, Mushroom Corals, and other Madrepores abounding on Polynesian reefs, and the "Sea Anemones,"
which have lately become so familiar to us all, can be seen by comparing our comparatively insignificant C. Smithii with our commonest species of Actinia and Sagartia. The former is a beautiful object when the fleshy part and tentacles are wholly or partially expanded. Like Actinia, it has a membranous covering, a simple sac-like stomach, a central mouth, a disk surrounded by contractile and adhesive tentacles. Unlike Actinia, it is fixed to submarine bodies, to which it is glued in very early life, and cannot change its place. Unlike Actinia, its body is supported by a stony skeleton of calcareous plates arranged edgewise so as to radiate from the centre. But as we find some Molluscs furnished with a sh.e.l.l, and others even of the same character and habits without one, so we find that in spite of this seemingly important difference, the animals are very similar in their nature. Since the introduction of gla.s.s tanks we have opportunities of seeing anemones crawling up the sides, so as to exhibit their entire basal disk, and then we may observe lightly coloured lines of a less transparent substance than the interstices, radiating from the margin to the centre, some short, others reaching the entire distance, and arranged in exactly the same manner as the plates of Caryophyllaea. These are doubtless flexible walls of compartments dividing the fleshy parts of the softer animals, and corresponding with the septa of the coral. Fig. 2 A represents a section of the latter, to be compared with the basal disk of Sagartia.
SAGARTIA ANGUICOMA. PL. V. FIG. 3, A, B.
This genus has been separated from Actinia on account of its habit of throwing out threads when irritated. Although my specimens often a.s.sumed the form represented in fig. 3, Mr. Lloyd informs me that it must have arisen from unhealthiness of condition, its usual habit being to contract into a more flattened form. When fully expanded, its transparent and lengthened tentacles present a beautiful appearance. Fig. 3 A, showing a basal disk, is given for the purpose already described.
BALANOPHYLLAEA REGIA. PL. V. FIG. 1.
Another species of British madrepore, found by Mr. Gosse at Ilfracombe, and by Mr. Kingsley at Lundy Island. It is smaller than O. Smithii, of a very bright colour, and always covers the upper part of its bony skeleton, in which the plates are differently arranged from those of the smaller species. Fig. 1 shows the tentacles expanded in an unusual degree; 1 A, animal contracted; 1 B, the coral; 1 C, a tentacle enlarged.
PLATE VI.
CORALS AND SEA ANEMONES.
ACTINIA MESEMBRYANTHEMUM. PL. VI. FIG. 1 A.
This common species is more frequently met with than many others, because it prefers shallow water, and often lives high up among rocks which are only covered by the sea at very high tide; so that the creature can, if it will, spend but a short portion of its time immersed. When uncovered by the tide, it gathers up its leathery tunic, and presents the appearance of fig. 1 A. When under water it may often be seen expanding its flower-like disk and moving its feelers in search of food. These feelers have a certain power of adhesion, and any not too vigorous animals which they touch are easily drawn towards the centre and swallowed. Around the margin of the tunic are seen peeping out between the tentacles certain bright blue globules looking very like eyes, but whose purpose is not exactly ascertained. Fig. 1 represents the disk only partially expanded.
BUNODES CRa.s.sICORNIS. PL. VI. FIG. 2.
This genus of Actinioid zoophytes is distinguished from Actinia proper by the tubercles or warts which stud the outer covering of the animal. In B. gemmacea these warts are arranged symmetrically, so as to give a peculiarly jewelled appearance to the body. Being of a large size, the tentacles of B. cra.s.sicornis exhibit in great perfection the adhesive powers produced by the nettling threads which proceed from them.