STRUCTURE AND FUNCTIONS OF VIBRACULA. 441 have enjoyed in watching the ludicrous movements of these anomalous productions. With regard to the development of the "Avicularia,” little has been made out. At first, they make their appearance as convex or hemispherical knobs, surmounting a simple cylindrical stem, and as growth advances progressively, the bird's-head form is gradually attained. In some species of Polyzoa, a moveable appendage of a totally different description, named the "vibraculum," affords an interesting subject for microscopical observation. This consists of a hollow filament, situated at the upper and outer angle of each cell, filled with a fibrous contractile substance, whereby it is enabled to effect movements of a very remarkable character. These movements occur at irregular, occasionally very short, intervals. First, the filament sweeps downwards over all the posterior surface of the polypidom within its reach, and then returning on its former track, descends in the opposite direction. The use of this singular apparatus is at present conjectural, for its movements are quite independent of the polyp, and continue for days after its death. It has been suggested, that it may be useful, by serving to rid the Polyzoon of intruding vagrants, and to cleanse away accidental defilement by sweeping across the orifice of the cell. Such are the principal and most instructive features of the Polyzoa met with upon our coasts; but, because we have selected only a few of the more ordinary species for the illustration of their general habits, the reader must by no means infer that the nenes of his ass are scaly éstributed, or of ZA HOTAMs in the exorgy of Nature; on the - vid der innumerable dent aspects isterse aing every coast, from des of the gas and the torrid zone. The Janis tenses LT 30 Dore abundant, neither MY DAY BÀ Dice listing proof of the universality f der ersteDES - l the lush hervei ni shymn of time." From the earliest appearance of life upon our globe,— from the Silliman rocks to the most modern deposits. - Tizaci medim miss so think as London fog," the skeletons of the Polyca present themselves in rith profision, testifying that, although "men were none" to see, much less appreciate such a spectacle, the cilia worked as vigorously upon the arms of extinct races, as on the tentacula of the Flustræ and Bowerbankie in our own aquaria. TUNICATA.-. 443 CHAPTER XLVIII. TUNICATA.-ASCIDIANS. "I find thee apt; And duller shouldst thou be than the fat weed THE scene is altered! Who that saw the sun go down last evening would have dreamed of such a change as this? "Shapes rose from the ocean to greet him; They curtain'd his bed: Such blue as the amethyst hides In the depths of her breast:— Then, all was calm and cheerful, and the peaceful sea, extended like a mirror, only served to multiply the glories of the parting day; but now, the hardy fisherman, with sail half-hoisted, hastens back to port to seek a shelter from the coming storm. The wind is up, and the dark heaving swell rolls heavily upon the beach, curling in sheets of foam, which, as they hurry onwards, seem to try to shun the maddening fury of the waves behind. Here let us stand awhile, and contemplate the vast uproar ;-look at the long array of foam-crowned billows stretching far to seaward, over which the shrieking gulls, blending their wild screams with the howling blast, wheel to and fro, as though in ecstasy, or ride triumphant o'er the yeasty waves! See! where is now the rocky reef o'er which we clambered yesterday?-far out from land, like a black streak among the wallowing seas; its hidden ridges only traceable by yonder line of breakers, rising like a wall amidst the eddying water, which, as in desperation, hurls itself upon the craggy obstacle that dares to interrupt its seemingly resistless course: "And it bubbles and seethes, and it hisses and roars, A bad day this for the naturalist! do you say? By no means; this furious storm will save us an infinity of trouble. Let us wait awhile till the retreating tide enables us to gather up the spoils these waves have reft from their deep hiding in the Laminarian "Und es wallet und siedet und brauset und zischt, FORMS OF SIMPLE ASCIDIANS. 445 zone; and, trust me! we shall reap a most abundant harvest : "Suave, mari magno turbantibus æquora ventis E terra magnum alterius spectare laborem!" that is, as we will take the liberty of translating it, -It is very agreeable to sit still here upon the shore, and watch the sea doing our work for us, especially while we enjoy a little luncheon. And now the tide is ebbing, and the beach strewn with all sorts of objects left by the retreating waves: -let us to our task, and see what next presents itself for observation; for we may rest assured, the products of deep water will be met with here. And first, these large ASCIDIANS claim our notice. Rarely is the dredge drawn up from any sea-bed at all prolific in submarine creatures, without containing few or many irregularly-shaped leathery bags affixed to sea-weed, rock, or shell, by one extremity, or by one side, free at the other, and presenting two more or less prominent orifices, from which, on the slightest pressure, sea-water is ejected with great force. On the sea-shore, when the tide is out, we find similar organisms attached to the under surface of stones, or lodged in crevices of rocks. They are variously, often splendidly coloured, but otherwise are unattractive, or even repulsive in their aspect. These creatures are ASCIDIE, properly so called. Numbers of them are often found clustering amongst tangles, like bunches of some strange semitransparent fruit; others are enclosed in a gelatinous case*, which is often en The word Ascidian is derived from dokòs (askos), a leather bag, or wine-skin. |