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animals take their food by a mouth at one extremity of the body, plants by roots diverging from the other. The reproductive organs in the latter occupy the place and ornature of the nutritive ones in the former. The gay and varied colours of the blossoms, the infinite diversity of their forms, the delicious scent so many of them exhale, all are calculated to draw the attention and excite the admiration of the beholder, while the organs of nutrition are usually hid in the earth. Not so in the animal kingdom ; the nutritive organs, or rather those that prepare the nutriment, are placed in the most eminent and conspicuous part of the body, in the vicinity of all the noblest avenues of the senses, while those of reproduction are placed in the most ignoble station, and are usually found closely united with those passages by which the excretions of the body pass off. In the Tunicaries indeed the mouth and the anal passage are usually very near to each other, and in the polypes the same mouth that receives the food rejects the feces, and it even sometimes appears to happen than an animal has been swallowed, and after performing the ordinary revolution in the stomach, has been ejected again in a living state.
PLATE IV. Fig. 1.
Functions and Instincts. Tunicaries.
The animals we have hitherto been considering were all regarded by Cuvier as belonging to his first class, the Zoophytes, and are continued therein by Carus; the latter, however, allows that the Echinoderms are somewhat removed from the class by the commencement of a nervous system. Lamarck's next Class, the Tunicaries, which we are now to enter upon, form part of the headless Molluscans” of Cuvier, and belong to that section of them that have no shells. My learned friend, Savigny, in his elaborate and admirable work on The Invertebrate Animals, who also considers them as a separate class, denominates them Ascidians, dividing them into two Orders, Tethydans and Thalidans. Many alcyons of Linné and others, are now referred to the Class we are treating of.
The characters of the class may be thus stated : ANIMAL, either gelatinous or leathery, covered by a double tunic, or envelope. The external one, analogous to the shell of Molluscans, distinctly
| Tunicata. 3 Ascidice.
Mollusca Acephala. 4 Tethydes, Thalides.
organized, provided with two apertures, the one oral, for respiration and nutrition, the other anal ; the interior envelope, analogous to their mantle, provided also with two apertures adhering to those of the outer one. Body oblong, irregular, divided interiorly into many cavities, without a head; gills occupying, entirely or in part, the surface of a cavity within the mantle ; mouth placed towards the bottom of the respiratory cavity between the gills ; alimentary tube, open at both ends; a ganglion, sending nerves to the mouth and anus.
These animals are either simple or aggregate; fixed or floating: the simple ones are sometimes sessile,' and sometimes sit upon a footstalk. The aggregate ones possess many characters in common with the polypes, inhabiting, as it were, a common body, somewhat analogous to the polypary, except that it is more intimately connected with the animal that inhabits it: the mouth of all is surrounded with rays or tentacles, as is also, in many, the anal orifice; but in their organization they differ very widely, exhibiting traces of a nervous system, and even, in some, of circulation. The fixed ones are commonly attached to rocks or other inorganized substances, but sometimes they are parasitic; thus a species of botrylle envelopes, like a cloak, certain ascidians, and another of the Tunicaries' envelopes the madrepores, more or less, with a milk-white crust.
3 Botryllus polycyclus.
The Creator, when he filled the waters of the great deep with that infinite variety of animals of which every day brings genera and species, before unknown, to light, willed that many of them should, as it were, form a body politic, consisting of many individuals, separate and distinct as inhabiting different cells, but still possessing a body in common, and many of them receiving benefit from the systole and diastole of a common organ: thus, by a material union, is symbolized, what in terrestrial animal communities results from numerous wills uniting to effect a common object. The land, as far as I can recollect, exhibits no instance of an aggregate animal; nor the ocean of one, which, like the beaver, lemming, bee, wasp, ant, white ant, and many others, forms associations to build and inhabit a common house, and rear a common family. ---Probably the nature of the different mediums these several animals inhabit is the cause of this diversity; and Providence, when it willed the peopling of the waters, as well as of the earth and air, into which the effluxes of light and heat from the central orb could not so penetrate and be diffused as to act with the same power and
! Didemnum candidum, Sav.
energy as upon the earth's surface, and in its atmosphere, so formed them as to suit the circumstances in which they were to be placed. Instead of sending the social aquatic animals forth by myriads to collect food and materials for their several buildings, he took the vegetable creation for the type of their general structure, in many cases fixed them to the rock or stone, united them all into one body, which, under a common envelope, contained often innumerable cells from which were sent forth by the occupant of each a circle of organs to collect food, from which, by some chemical operation, they could elaborate materials for the enlargement of their common house ; and often cause that influx and reflux, to compare small things with great, resembling the oceanic tides, and by which the sea-water is alternately absorbed and rejected by these animals : but this function, in the case of some of the Tunicaries, the animals with which we are now concerned, seems to be affected by a central organ or pump common to the whole fraternity.
But although none of the marine associated animals are employed, like the terrestrial ones, in labours that require locomotion and the collection, from different and often distant parts, of materials for the erection of their several fabrics, and of food to store up for the maintenance of the various members of their community, yet