Part 7 (1/2)

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 When these tentacles are withdrawn, they are gathered up in a bundle, like the stays of an umbrella.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.