on its outside, and a narrow and shallow lagoon between it and the coast. An Atoll is generally found where the water is deep, the outer side of the coral bank shelving down at an angle of 45 degrees to the depth of 200 or 300 fathoms, whilst the inner side gradually slopes so as to form a cavity somewhat like a saucer ; but this also is in great measure filled up by fragments of coral detached by the surf from the outer part of the reef, so that even in the centre of a lagoon the water is rarely more than 50 feet deep. The outer surface is continually washed by a strong surf which renders the island difficult of approach, unless it should happen, as it not unfrequently does, that the force of the surf has been sufficient to make a breach in the coral; then, however rough may be the passage to the breach, the navigator will be amply repaid by the smooth water of the lagoon, and if he be a Zoologist, no spot that he could choose is more fertile in animal life. Here fishes swarm, here is found the gigantic Clam, with an endless variety of smaller shell-fish; here the coral animals and other Zoophytes revel in enjoyment, here sea-weeds grow in abundance, and turtles are not wanting to feed upon them.

Many of these Atolls are of large size, and the coral barrier is covered with a luxuriant vegetation. As the coral animals cannot live in more than 20 or 30 fathoms below the surface, it may be asked, how can the reef be formed at a depth of 200 or 300 fathoms ? Mr. Darwin holds the opinion that the Atolls are based

upon land which was once dry, but has since subsided, carrying with it the dead corals. He has found also that the surface of the coral is generally covered with Nullipores, sea-weeds and shells, and that the detritus of these, the dead coral, shells and sand together, form a soil fitted for the growth of stray seeds and fruits, such as the cocoa-nut, carried to them by the ocean currents. Some idea may be formed of the beauty of one of these Atolls by the diagram before you, which is a representation of Whitsunday Island in the Pacific; the vegetation is most luxuriant, and capable, as has been proved on more than one occasion, of supporting for a considerable period the shipwrecked mariner; the cocoa-nut, the turtles, and shell-fish of the lagoon furnishing him with abundance of food, whilst fresh water was readily obtained by digging small wells in the sand, into which water from the sea gradually found its way, and having a large stratum to pass through, was deprived of nearly all its saline constituents. The largest barrier-reef, upwards of 1000 miles in length, occurs on the north-east coast of Australia, rendering the navigation of this part of the globe exceedingly dangerous. It has been described by Mr. Joseph B. Jukes,* who was employed during the years 1842-1846 as naturalist to the expedition under Captain Blackwood, of H.M.S. 'Fly,' for the purpose of surveying Torres Straits, New Guinea, and

* "Narrative of the Surveying Voyage of H.M.S. 'Fly,'" London, 1847.

other islands of the Eastern Archipelago. The upper and outer parts of the reef are composed of living species of the genera Porites and Millepora, the former being described by Darwin as forming masses from four to eight feet broad, and of similar thickness, and by the naturalists of the United States' Exploring Expedition as of a more or less nodulated figure, while the Millepore is composed of thick vertical plates, intersecting each other at various angles, forming an exceedingly strong honey-comb mass of a circular figure, the external plates alone retaining their vitality.

These corals are found near the surface, but lower down there are other large stony species. Small fragments of Millepora alcicornis have been brought up from a depth of twelve fathoms; but so firmly are the corals bound together, even at this depth on the margins of the reef, that chains and anchors have been lost in the attempt to detach them. Shells of different species are also occasionally found forming a stratum of two or three feet thick, within reach of the tide or spray of the water.

The remains of Madrepores and other corals abound and were mainly concerned, in the formation of many limestone rocks. The specimens constantly exposed for sale at Clifton are principally Madrepores and Oolites, and some of the former still clearly exhibit the large cells for the polypes and the radiated disposition of the septa, the cells being closely approximated, and the interseptal spaces filled with transparent calcareous matter.

In other specimens, the cells are more widely separated, and the intervening spaces occupied by dark calcareous material, fractured in such a manner as to produce a cellular appearance, so that even in the fossil state the indications of coral structure are very evident. As the entire mass of the coral is composed of cells filled with carbonate of lime, it follows that the vulgar opinion of its being built up by polypes is erroneous; indeed, the study of the development of these animals clearly proves that the cells are formed before the polypes, and that their function is that of collecting and digesting food-in fact, that they constitute the digestive system for the nutrition of the flesh of the entire animal. This connection of the polypes with the flesh, and the conveyance of the nutritious matter to the latter, is strikingly evident in the tubular keratophytes. The polypes then, perform the same function towards the coral-mass as the oyster to its shell; for if the oyster die the shell also dies, but so long as the oyster retains its vitality, every portion of the shell, however distant from the animal, is endowed with life; and in the same manner, so long as the polypes remain alive, the vitality of the entire coral mass will be sustained. Collectors of shells can immediately distinguish those that have been taken out of the water alive, from dead shells; the former always retain their polish and colour; the latter are dull, opaque, and of little value.

Corals, like shells, possess the power of repairing injuries; this I had long suspected, although I had no

direct proof of the fact until, having occasion to examine an old collection belonging to the Museum of this College, I found some specimens in which fractures had occurred. In one of these, as shown at A in Fig. 82, several branches had been broken off and cemented to others in the position in which they had fallen; in another species-Millepora alcicornis-there are several examples of a similar repair of fractures; whilst a third, of an extremely interesting character, shows that the broken extremities of the branches have been subsequently rounded off and covered by the flesh, in the same manner as a bone after amputation of a limb. These specimens all belonged to John Hunter, and were probably selected by him for the illustration of the process of repair after injury.

When speaking of lime as forming the skeleton of plants in my first Lecture, I made brief allusion to a class of organized beings termed Lithophytes; these

FIG. 83.

Corallina officinalis.

were considered by Lamarck and other

high authorities to be

animals; modern investigation has proved that the Corallines, and even the harder Nullipores, are essentially plants more or less coated with cal

careous material.