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VARIETY OF LAPRALIE.

of rocks and algae. These, when carefully examined, exhibit the beauty and regularity of structure inseparable from the works of creation, but are commonly passed over by the collectors of pretty things, as merely white, scaly crusts, altogether devoid of interest. What the parasitic fungi are to larger vegetables, these little animals are to their more showy neighbours. But even in the humblest kinds, it is astonishing what a variety of beautiful structures are met with. The common observer may pass over the species of Lepralia without discrimination, as being merely rude scurfs, deforming the sea-weeds or shells over which they spread;

VARIOUS SPECIES OF LEPRALIE, MAGNIFIED.

but, if he carefully examine them, nearly forty kinds, distinguished by very curious and elegant varieties of form, will reward his labour. The polypidom in this genus consists of a single layer of cells, adhering by their under surface to rocks, shells, or sea-weeds, and disposed in regular order in a more or less perfectly circular manner, formed row beyond row, in concentric layers. Some of the more curious forms of the cells are represented magnified in our figure.

Several of the Polyzoa, especially those of the family called Escharide, have appendages to their cells of a

BIRD'S-HEAD APPENDAGES.

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very singular nature, the use of which has not yet been determined. These odd-looking organs are attached to the outer side of the cell, and resemble in form the head of a bird, furnished with a bill which

can open and shut like a pair of pincers. Each head is fixed on a flexible stalk, and while the creature lives, maintains a constant and regular motion up and down, opening and shutting the bill at intervals. All specimens of the same species do not produce them, nor are they found on all the cells of a single specimen ; and they exist indiscriminately on certain species of different genera, while often species, otherwise closely allied, are not furnished with them.

APPENDAGE OF
CELLULARIA.

From this group of the class Zoophytes we pass, by a very easy transition, to the more simple members of the MOLLUSCA, those forming the subdivision tunicata. They are so called, because their soft parts are enclosed, not in a shell, like the majority of the class, but in a tough, leathery, coat or tunic. The commonest example of a tunicated mollusc is found in the various kinds of Ascidiæ, or sea-squirts, some of which are found attached to sea-weeds and stones, in the littoral zone; others are frequently thrown up from deeper water on the beach, and may be dredged in abundance in almost any locality. Some are of a large size, several inches in length. Their outer form is that of a bag, with a smooth or variously roughened semi-transparent skin, furnished with two small openings, through which, on the slight

H

98

ASCIDIE, OR SEA-SQUIRTS.

est pressure, a jet of water is sent to a considerable distance. These creatures lead a very inactive life. Attached by their base to plants, they trust for nourishment to whatever small fry are brought to their mouths by currents in the water. They have not the elegance of form of the sea-anemones, but many are painted with the most gaudy colours. Their internal structure is very simple, and connects them closely with the division of mollusca which form bivalve shells, the tunic in the Ascidia being strictly analogous to the shell of the conchifer. Their metamorphoses have been watched by several distinguished naturalists, and offer highly curious points in their history. In the young or tadpole state, they are extremely active, swimming about by rapid motions of their tail, till the young creature finds a spot where he can take root. Then the tail disappears, and grasping fibres, or roots, spring from the body, which gradually assumes the form, and adopts the quiet life of the parent from which it sprung. It is thus, by giving to the young animal powers which she denies to the fully grown, that nature, in these and many other of the stationary lower animals, provides for the proper dispersion of the species. Among more perfect animals, it is the old take care of the young, and provide for them : here we find the young possess instincts which they lose at an advanced period of their life.

The Ascidia, which, because they are common and of large size, I have instanced as examples of the tunicated mollusca, are simple animals, each creature living by itself; but I should give an imperfect idea of the class if I did not allude to the compound ascidians, animals

of similar structure, which yet live associated, or connected together into a compound body, such as we have already seen among the zoophytes. These are very numerous on our shores; but the most varied forms are taken only by the dredge. Still, on the stems of seaweeds, within tide-marks, especially on the various kinds of Cystoseira, and on the Laminaria, numerous kinds, some of them extremely beautiful, may be found. The stems of the olive-coloured sea-weeds are often literally concealed, by clasping masses of firm jelly, whose surface is marked with radiating stars, blue, crimson, or orange, or various in colour, resembling a tessellated pavement, or the polished section of a Torquay madrepore-stone. These belong to the ani

BOTRYLLUS.

mals in question, and to the tribe Botryllida. The gelatinous crust is a matrix common to the whole community; while each star that glitters on its surface consists of numerous separate individuals, similar in most points of their structure to the baglike ascidia. It is impossible, without colour, to do justice to such delicate creatures by a figure, and the wood-cut in the margin is merely intended to guide the eye. A brief outline of what is known of the British species of this highly curious family, is given in the first chapters of Forbes' and Hanley's "British Mollusca," to which I must refer for further information: but a more complete history, accompanied by coloured figures, is required before the study of these curious creatures can be rendered popular.

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PROGRESSIVE STRUCTURE

These ascidians are among the humblest members of the great class of MOLLUSCA. This class includes the whole group of animals which produce what are properly called shells, the favourite study of the conchologist; also a multitude which are shell-less at all periods of their life; and others, whose shells are reduced to membranous plates concealed under the fleshy folds of their bodies. Commencing with the shapeless bag of the Ascidia; proceeding thence, through the bivalve shells into sea-slugs; and so, through the various tribes of univalve shells, we pass under review a great variety of animals, rising in complication of structure one above another, until we arrive at the Nautilus and the Cuttle-fish, which close the great group, by a type of structure in which the peculiar organization of the vertebrate is dimly sketched. The lowest mollusca are scarcely more organized than the zoophytes: the highest closely border on the most perfect animals. The study of this class therefore is, in all respects, highly important. To the mere student of comparative anatomy it offers a rich field of research; for here, within circumscribed bounds, he can trace the gradual development of organs from the first idea, as it were, to their full perfection. To the collector of beautiful objects, the countless varieties of shells, so easily preserved and so varied in contour and colour, afford continual sources of interest; and their proper classification, a pleasant problem for the exercise of ingenuity-although it must be admitted that the proper classification of shells cannot be arrived at, if the nature of the animal which forms them be not carefully studied. A striking proof of this is shewn in the