第46章

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 fromZoophytology to a place in the system of nature more nearlyconnected with Molluscan types.Some of them come so near to thecompound ascidians that they have been termed, as an order,"Zoophyta ascidioida." The simplest form of polype is that of a fleshy bag open at oneend, surmounted by a circle of contractile threads or fingerscalled tentacles. The plate shows, on a very minute scale, atfigs. 1, 3, and 6, several of these little polypiform bodiesprotruding from their cells.But the Hydra or Fresh-water Polypehas no cell, and is quite unconnected with any root thread, or withother individuals of the same species.It is perfectly free, andso simple in its structure, that when the sac which forms its bodyis turned inside out it will continue to perform the functions oflife as before.The greater part, however, of these HydraformPolypes,althoughequallysimpleasindividuals,are connected in acompound life by means of their variously formed POLYPIDOM, as thebranched system of cells is termed.The HydroidZoophytes arerepresented 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, asin 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