Part 11

PLATE I. ZOOPHYTA. POLYZOA.

THE forms of animal life which are now united in an independent class, under the name Polyzoa, so nearly resemble the Hydroid Zoophytes in general form and appearance that a casual observer may suppose them to be nearly identical. In all but the more recent works, they are treated as distinct indeed, but still included under the general term “ZOOPHYTES.” The animals of both groups are minute, polypiform creatures, mostly living in transparent cells, springing from the sides of a stem which unites a number of individuals in one common life, and grows in a shrub-like form upon any submarine body, such as a shell, a rock, a weed, or even another polypidom to which it is parasitically attached. Each polype, in both classes, protrudes from and retreats within its cell by an independent action, and when protruded puts forth a circle of tentacles whose motion round the mouth is the means of securing nourishment. There are, however, peculiarities in the structure of the Polyzoa which seem to remove them from Zoophytology to a place in the system of nature more nearly connected with Molluscan types. Some of them come so near to the compound ascidians that they have been termed, as an order, “Zoophyta ascidioida.”

The simplest form of polype is that of a fleshy bag open at one end, surmounted by a circle of contractile threads or fingers called tentacles. The plate shows, on a very minute scale, at figs. 1, 3, and 6, several of these little polypiform bodies protruding from their cells. But the Hydra or Fresh-water Polype has no cell, and is quite unconnected with any root thread, or with other individuals of the same species. It is perfectly free, and so simple in its structure, that when the sac which forms its body is turned inside out it will continue to perform the functions of life as before. The greater part, however, of these Hydraform Polypes, although equally simple as individuals, are connected in a compound life by means of their variously formed polypidom, as the branched system of cells is termed. The Hydroid Zoophytes are represented in the first plate by the following examples.

HYDROIDA.

SERTULARIA ROSEA. Pl. I. fig. 6.

A species which has the cells in pairs on opposite sides of the central tube, with the openings turned outwards. In the more enlarged figure is seen a septum across the inner part of each cell which forms the base upon which the polype rests. Fig. 6 b indicates the natural size of the piece of branch represented; but it must be remembered that this is only a small portion of the bushy shrub.

Campanularia syringa. Pl. I. fig. 8.

This Zoophyte twines itself parasitically upon a species of Sertularia. The cells in this species are thrown out at irregular intervals upon flexible stems which are wrinkled in rings. They consist of lengthened, cylindrical, transparent vases.

CAMPANULARIA VOLUBILIS. Pl. I. fig. 9.

A still more beautiful species, with lengthened foot-stalks ringed at each end. The polype is remarkable for the protrusion and contractile power of its lips. It has about twenty knobbed tentacula.

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 passes upwards through an intestinal canal 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 glass, 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.

FLUSTRA LINEATA. Pl. I. fig. 1.

In Flustræ, 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.

FLUSTRA 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 analogous to the birds’ heads in the Cellularæ.

PLATE V. CORALS AND SEA ANEMONES.

CARYOPHYLLÆA SMITHII. Pl. V. fig. 2. Pl. VI. fig. 3.