which separate the festoons. The whole of the plaits are twenty-eight, fourteen on each side, and they are margined by an equal number of great longitudinal vessels. The vessels which compose the tissue are excessively fine; the transverse, however, less delicate than the others, and not so closely set, accommodate themselves very well by their curvature to the outline of the festoons. This description, I feel, needs the aid of Savigny's figure, of which I gladly avail myself, and I am certain that in few other creatures will you find a structure more wonderfully fashioned. (Fig. 53.*)

Although the branchial tissue apparently covers the whole inner surface of the sac with a continuous network, yet it is really divided into two halves by a furrow in which the trunks of the blood-vessels lie; and this structure becomes obvious in some families where, as in Pyrosoma, the interspace is considerable; and is still more remarkably obvious in the Salpæ, in which, in fact, the branchial vessels are not disposed on the walls of the sac, but occupy the margin of two narrow linear leaflets of very unequal lengths that lie across the cavity. These are formed by a duplicature of

Fig. 53.

the inner tunic, and the superior margin is garnished with a close series of little vessels which run parallel to one another in a transverse direction; a form and disposition which, says Lamarck, has very little analogy with what is regarded as the respiratory organ in the Ascidiæ; but which, on the contrary, Carus seems to think is just the link that connects these with the bivalves, "appearing to constitute the transition from the Ascidia to the

Teredines, where there are two elongated branchial la

* Mém. sur les Animaux sans Vertèbres, 2de partie, passim.

+ Anim. s. Vert. iii. 114.

minæ above the intestine and within the tubular cloak, to which the water has access and egress by means of two tubes placed at the posterior extremity of the body."

Let us, before proceeding, reflect a moment on that wonderful diversity in the structure of the same organ here exhibited to us in one class of animated beings: it is a fine example, among many, of that variety in which the Creator of all has seen good to indulge in the production of his works, as if, to use the words of a favourite author, He "willed to shew those whose delight it is to investigate his works, by how many varying processes he can accomplish the same end." I see in it also a proof, that neither external and physical circumstances, nor self-born desires, have that great and almost creative influence in framing or modifying animal structures which many imagine they have; for here are before us a crowd of animals whose soft bodies, it will be allowed, are as susceptible of changes, or of being moulded to one type, as any animals can be, and the uniformity of whose nervous system seems to prove that their faculties and desires are much on a par; yet, if we select any large family from among them, we shall find them living in the same seas, and in the same depths, and in the same latitudes, and on the same food, and all breathing the same air; but, so far from showing a perfect agreement in their exterior organs, on which these causes are said to operate so efficiently, we find all is diverse, whether we look to the position, the form, or the structure of the organs. These are now, my friend, such as they were when the creatures

*Comp. Anat. Trans. ii. 147.

+ "Most of the Ascidia are coarse, unsightly, deformed-looking animals, utterly void of that external symmetry and beauty, rendering many of the tenants of the waters so interesting. Nor is it in this only, that they should fail to attract the spectator's notice. They testify neither instinct, action, nor motion, nor even the symptoms of life, farther than slight enlargement and reduction of size, together with contraction and expansion of the two tubular orifices of the body. No sensible alteration follows abstinence or repletion; the external form undergoes scarcely any modification from health or disease; even the lapse of time, that universal consumer, seems hardly to make any impression on the shapeless mass, which is rooted immoveably from the first moment, on the same spot to vegetate, live, or die. "Such is the case, with but few exceptions, during any ordinary or reasonable period, that may be occupied in observation.

"Yet, let us recall what is disclosed when this rude object is stripped of its external integuments, from which some of the germs may be withdrawn, almost as from a bag. How complex the structure thus displayed; what a wonderful arrangement of muscular, respiratory, circulating, secretive, and digestive organs, adapted for discharging the vital functions, all proving the handiwork of the Great Architect, as directed towards a common end."-SIR J. G. DALYELL.

came into existence from Infinite Wisdom, perfect, and complete, and immutable; and, notwithstanding all their variations, ever suited with special adaptation to the element and the place they were fore-ordained to inhabit. "Their forms are His special invention and construction, and their principle of life is also His special and communicated gift," is the just conclusion of an historian eminently distinguished for his learning, his good sense, and his piety.*

Now, the distinction which has been drawn between the Mollusca with lungs and gills, however anatomically correct, is not always physiologically true; for although I am not cognisant of any pulmoniferous species that can breathe water, or ever does so voluntarily, yet there are many branchiferous ones that can and do respire the uncombined air. A great number of bivalves are alternately submerged and exposed to the air, according to the fluctuations of the tide; but then the animals are covered up with a wet soil, or the concavity of the lower valve enables the animal always to retain some moisture around its gills, and I believe they do not open their shells freely unless when covered with water. Of the Cephalopods the Octopoda are said to come ashore frequently, and live among the rocks for days together; and the Pteropods and the naked Gasteropods in general love to swim at the surface in calm weather, particularly at the time of sunset, apparently to enjoy the respiration of a lighter and more oxygenated medium. There are other Gasteropods with gills which pass so large a portion of their term of life completely out of the water, that they seem to merit the appellation of amphibious. The most remarkable of these is an Australasian species of Neritina, which loves to forsake its native rivers or ponds for the green shade of the trees, amid whose foliage it is found abundantly. The Patellæ and the Littorinæ are also good examples. Our common species of the latter genus seem, indeed, to prefer spots where they can be covered only at high water, and I have seen myriads of them, when

Mr. Sharon Turner. His Sacred History of the World, from which the quotation is taken, I earnestly recommend to the attentive perusal of students of natural history.

+ Peron seems to have believed that a species of Onchidium (a pulmoniferous genus) found on the shores of New Holland, lived always in a state of submersion, but there is no doubt of some mistake here.-See Audouin and Milne-Edwards in their Litt. de la France, i. 118.

Kellia rubra spends the greater portion of its life out of the water, and is often for at least a fortnight uncovered with it.-See CLARK in Ann. and Mag. N. Hist. Ser. 2, iii. 455, and iv. 144.

young, clustered in hollows of rocks that were many feet above the highest tides. Still, their respiratory organs are, as they ever have been, branchial; nor does it seem easy, on the Lamarckian hypothesis, to account for their nonimprovability: why these shell-fish, so fond of air, have not acquired, by their residence in it, the lungs of the snail, and betaken themselves to the land; why their shells have not become lighter to enable them to move with more alacrity; and why their eyes have not risen to a higher elevation than the base of the tentacula, that they might scan the landscape and avoid its perils. The habits of the Chitonidæ are similar to those of the Littorinæ. "These animals," says the Rev. Mr. Guilding, "frequent the rocks and stones of the sea-coast, and are distributed nearly over the whole globe. Many of the species are constantly under water, while others ascend above low or even high-watermark, spending the day exposed to the hottest sun, or selecting a resting-place which is only occasionally moistened by the rude and restless surf. In Chitonellus and Cryptoconchus there are certain minute organs on the zone, which bear a strong resemblance to the spiracula of the annulose animals. From their habit of quitting the watery element, like many of the Turbinidæ, I once supposed that the organs for the aëration of the circulating fluid might be of a compound nature (pulmono-branchiati). It is, however, far more probable (as in the case of some crustaceous genera which I am now investigating) that this process is capable of a diurnal or a temporary interruption, or that the branchiæ, so long as they are kept moist, and shielded from atmospheric influence, may perform their functions, though much more slowly."

It is probably this capableness of protecting the branchia from the drying influence of the air that enables even such mollusks as habitually live submerged to survive their removal from the sea for a very considerable time, so that they may be thus carried to the greatest distances without injury, while any attempt to carry them, or to preserve them, in sea-water would have failed, without a care to renew the water that could seldom be bestowed. The death in experiments of this kind does not ensue from the abstraction of oxygen from the water, but from its corruption; and hence you will find that, if not mechanically prevented, the animal will creep from the poisoned bowl to expose itself to a slower death in an element which it cannot breathe. But what you would little anticipate to hear, fresh water is to these pelagic molluscans even a deadlier poison, and quicker

in its operation, than ardent spirits. Aristotle was aware of this fact, for the Purpuræ, he tells us, died in the course of one day in fresh water, although they could live fifteen days under the open atmosphere. But the poison works in general much more rapidly than is here stated, and we have rarely found a truly marine species resist its influence above a few minutes, some not as many seconds. You will perceive in this fact the ordained means of retaining these animals, in general carnivorous, within their natural parks, if I may so speak, and the surest of all guards against their inroads on the defenceless tribes which inhabit large rivers and lakes. But to occupy the ground that lies between these two tribes -the marine and lacustrine-we find another race created, whose delight is to dwell in brackish water,

"Or on the beached margent of the sea,"

and who can bear, with comparative impunity, either the river or the purer sea. Several Littorina and Rissoæ are in this condition; as well as the common mussel, the cockle, and some littoral Tellinides. The Potamides, a marine genus, is often found at the mouths of rivers; and Rang has found at the isle of Bourbon, in a fresh water tank, not far removed, however, from the shore, Pintadines and an Aplysia living in society, under stones, with Neritinæ and a Melania.* In South America, near Rio de Janeiro, Mr. C. Darwin found a Limneus in great numbers in a lake into which, the inhabitants assured him, the sea annually, and sometimes oftener entered, and made the water quite salt. M. Gay has stated that he found, in the neighbourhood of Rio, the marine genera Solen and Mytilus, and the fresh water Ampullariæ, living together in brackish water.+ Facts of this kind ought to be kept in remembrance by the geologist, but yet not with the fixedness of gaze that would rather blind than guide him.

And apropos to this hint:-In some strata of comparatively recent formation, marine and fluviatile shells have been found commingled, and some have made a noise about the discovery as if it were one which led to important deductions; nor need we dispute its value. Beudant was induced by the discovery to institute some experiments with the view of determining whether or not marine mollusca could not, by adopting proper precautions, be gradually habituated to fresh water and enabled to live in it, and if fluviatile mollusca could not, on their side, be accustomed to the sea; and his experiments incline him to think this might be done,— + Darwin's Journal, iii. 24.

* Manual, p. 47.