are of two sorts;* one opaque and granular, the other clear and possessed of a nucleus. The corpuscles may be obtained for examination by pricking the sinus of the heart and placing a drop of the fluid, which exudes, upon a glass slide. If now we remove this and examine it with a power of four hundred diameters, we shall see the two varieties. It is possible that at first we may see only the opaque forms, but by treating them with water we render them transparent, and their nuclear and other contents quite perceptible, as shown in the plate, fig. 3. It is strange that although colourless blood is the rule among the gastropoda, the planorbis has a circulating fluid of a reddish hue.

The peculiar glandular apparatus which surrounds the heart, and which I have called the pericardial gland, has been called the kidney by anatomists, and writers upon this subject. It is exceedingly wrong to style an organ a kidney merely upon a vague supposition that its function may be that of an urine gland, when its structure and position do not seem to support such a view. Yet this is what has been done by Cuvier, Jacobson, Müller, Siebold, and a host of others. Let us see what grounds there are to support this opinion.

In the urine of most animals is found a peculiar insoluble substance termed uric acid. Now, chemists have discovered that where this is acted on by aquafortis (nitric acid) and hartshorn (ammonia) a characteristic red colour is produced, and since there is not any other substance (?) capable of being affected in a similar manner, it follows, logically enough, that if in this organ of the slug we find a material whose chemical reaction with aquafortis and hartshorn is similar to that of uric acid, that material must be uric acid. Jacobsont asserts most positively that he has found a deposit in the heart gland of the slug which gave rise to a reddish stain when acted on by aquafortis and subjected to the fumes of hartshorn-hence the deduction.

I have made a series of experiments upon the same subject, and have invariably found

1st. That the stain was not of the ordinary red hue, but of a dirty reddish brown.

2nd. That it was produced by the aquafortis alone without the employment of the hartshorn, and

3rd. That the same stain was produced when portions of the liver had been placed under similar conditions.

This was first distinctly indicated by Mr. Wharton Jones in his admirable memoir in the Philosophical Transactions for 1846; for though Leydig described two forms of corpuscles in his admirable memoir, Ueber Paludina Vivipara, yet he was ignorant of their developmental relations to each other.

+ Meckel's Archives, VI., p. 370. 1820.

Which three results are quite sufficient in themselves to show that there is not much reliance to be placed in the statements of those who assert that the gland is a kidney.

Again, if this structure really fulfilled the office of an urine gland it is evident that it should have attached to it some channel through which the excrementitious fluid might be carried out of the body. Even the opposite side admits this. Here, however, we differ. Siebold, Cuvier, Müller, and others maintain that there is a tube connected with the gland, and opening near the lung aperture. I, on the other hand, contend that no such canal is present. I have dissected many specimens with the hope of discovering something of the kind, but I have invariably failed to observe it; and I can only account for the mistaken observations of such distinguished naturalists by assuming that in emaciated German and French slugs the end of the intestine (which pursues exactly the route of the supposed duct,) has been looked on as the canal of the gland.

The pericardial gland is of a dark reddish brown colour, and measures from side to side (including the heart and sinus) more than half an inch. It is made up of a great number of lamellæ or plates lying against each other, like the leaflets of a fish's gill. Each of these examined under the microscope appears to be composed of numerous irregular vacuoles, containing within them solid, round, opaque, incompressible nuclei. Passing between the plates may be observed hundreds of blood vessels journeying from the lung to the sinus, and on their passage giving off several branches, which wind about the vacuoles and anastomose frequently. I cannot see that there is any necessity for supposing that a slug has a kidney at all. But even did I suppose the creature possessed of such a commodity, I cannot conceive why I should pitch upon the pericardial gland as the organ most likely to subserve the function, simply because one of the compounds found in human urine was discovered (or said to be discovered) here also. As well might a person ignorant of anatomy, contend that the skin of man was the human kidney, because there is undoubted proof of the statement that urea (a constituent of the renal secretion,) is thrown out from the body in the perspiratory fluid. So long as scientific men frame generalizations upon such vague and careless observations as those I have alluded to, so long will comparative anatomy be uncertain and unreliable. If the reader will take the trouble to investigate the matter for himself he will then ascertain how loosely many of the conclusions of comparative anatomists have been formed, and how important it is to accept no statements as truths until they have been verified by personal research.

EXPLANATION OF PLATE.-Fig. 1.-Plan of the circulation; the arrows indicating the direction of the currents; ll, the lungs; pp, the pericardial gland; s, the sinus; h, the heart; a, the aorta; aaaa, the various arterial branches; i, the digestive canal; lulu, the great lateral veins; vvvv, their branches. Fig. 2.-Iuner surface of the membrane of the lung, showing the network of blood-vessels. Fig. 3.-Blood corpuscles; a, opaque and granular; b, with projecting processes; c, the nucleus faintly seen; d, the nucleus distinctly seen. Fig. 4.— The heart and sinus, surrounded by the pericardial gland. Fig. 5.-Imaginary longitudinal section in the vertical plane of the animal; h, the head; a, the abdomen; c, the cavity which contains the heart, sinus, gland, and shell. Fig. 6.Imaginary vertical section in the transverse plane; , the lungs; sb, the shell-bag; p, the pericardium; h, the heart; pg, the pericardial gland. Fig. 7.-Plan of the pulmonary organ; ll, the lung sac of each side; pp, the passages of intercommunication; i, the isolated patch, containing the heart, sinus, shell-bag, etc., etc.

AILSA CRAIG AND ITS BIRDS.

BY ROBERT GRAY,

Secretary to the Natural History Society of Glasgow.

Ir is hardly necessary to dwell upon the general features of this remarkable rock, for, like other isolated rocks of the same character, it presents scenes which have often been describedsteep precipices towering on all sides; high ranges of fractured pillars, sometimes overhanging and shadowing the deep; cliff seeming to rise above cliff, their massive walls streaked with endless rows of soldierly sea-fowl; the whole island, from its solitary position, rising upon the view in "unattended majesty," and impressing the beholder with a feeling of awe.

When nearing it in calm weather no one can fail to be struck by the grandeur of its precipitous cliffs, reaching 1200 feet above the sea; the eye at once comprehends its vastness, owing to what Dr. M'Culloch has called its" sudden and unexpected magnitude," especially when the summit is invested with floating mist; and while speculations naturally force themselves upon the mind of the spectator, whether geologist or bird-hunter, regarding the time when in awful convulsion the great mass first upheaved its "broad bare back into the clouds," it must be confessed its present aspect is uncommonly peaceful, forming a scene, indeed, in which are combined many

elements of beauty. The air-worn precipices, as seen about sunrise, when the long yellow rays come streaming across the water, lighting up the perches of the sea-fowl, are gleaming with innumerable birds; and, as these pure white figures contrast with the rusty background, every object tells in the picture. From base to peak these cliffs are occupied by long lines of guillemots and razorbills, and huddled groups of gannets and kittiwakes; myriads of puffins stud the fallen fragments reaching to the water's edge, a curious effect being added by the tree mallows, throwing a purple tinge on the face of the rock; while over all, the green slopes which carry the eye to the cloud-capped summit are covered with the greater sea-gulls-these splendid birds appearing, from a distance, like large white flowers amongst the grass.

Seen at break of day, therefore, few objects could be more impressive to a naturalist than this prodigious bird-hive, where many thousands of its feathered natives are in sight at one view, before the masses break up and wing their flight to the surrounding sea: it can only be exceeded at sunset, when these multitudes return for the night, the craig being then invested with a solemn splendour which words can never do justice to.

Yet ninety years ago Pennant, who landed upon Ailsa in the month of June, called it a scene of horror, and wondered why thrushes could exert their melodies in such a place as cheerfully as they did in the groves of Herefordshire; overlooking the fact that these were Ailsa-born throstles, shouting with glee, and making the columns and halls of their rocky home resound with a mavis' welcome!

On the south-west side of the rock the tidal waves have rolled and polished the stones so as to form a rough kind of beach, on which a landing can be safely effected. The visitor is here welcomed by the keeper and his dogs-a group not the less picturesque by a continued residence, summer and winter, in their lonely kennel. One cannot help thinking of this modern Crusoe being troubled occasionally in his sea-girt abode by the waves in stormy seasons roaring to one another in the nighttime, and flinging themselves with a crashing sound at his very door; but in answer to our inquiry, he declared that once only did he feel solitary, and that was when something went wrong with his eight-day clock; the familiar ticking having ceased, he became conscious, he said, of the "noise outside."

Proceeding by a rugged path along the base of the cliffs, it is possible to make a circuit of the island, a task, however, by no means easy to accomplish, the distance being computed at three miles, over a mass of angular rocks, which have fallen from the upper ridges, and now lie piled in terrible confusion to a height, in some places, of 300 feet. As the road ceases

altogether about half way, any one desirous of completing the journey is compelled to surmount the difficulties of the adventure by indulging in a series of most undignified attitudes. The most prudent course the visitor can adopt is to pause for a time at the safest point of observation, where he will obtain a view of the basaltic pillars, one of the grandest ranges in Britain. These stupendous columns attain an elevation of 400 feet a height six times greater than the famous pillars of Staffa. Many of these gigantic blocks have become partially disconnected, and look as if ready to fall at the slightest touch. The extraordinary accumulation of fallen rocks already spoken of is the result of such blocks giving way; and as, throughout its whole extent, this mass has now become the chosen abode of countless puffins, we may imagine the singular spectacle afforded by a legion of these birds pouring out of their holes when a few tons of trap happen to fall upon their territories.

A steep and somewhat dangerous footpath forms the ascent to an old castle in ruins, the surface beyond that elevation being irregularly covered with immense pieces of rock, which lie scattered in the wildest confusion. The hollows are filled with plants of a monstrous growth, tall, fierce nettles, and umbrellalike blades, amongst which the tourist, as he brushes his way to the summit, stumbles at intervals, and not unfrequently sinks out of sight, getting wedged in the most awkward postures in the cold embrace of these boulders. The only satisfaction, while so situated, is to listen to the tinkling sound of a miniature river which has its source at a marshy hollow some distance above, and which now runs fresh and clear, singing its quiet tune in strange contrast to the noise of the great reservoir below; but the best reward of all awaits those who have patience to toil up to the Cairn, as the magnificent view of the Frith of Clyde is one of memorable interest.

Of the various birds to be found upon this "lone isle " (there are upwards of forty observed as regular visitors), by far the most numerous is the puffin (Mormon fratercula). On nearing the craig in a boat, this comical species is the first to make himself known by rising on all sides out of the water; he looks at you for a second or two, then with a diverting turn of his little fat body, he throws up his painted heels and plunges out of sight. It is only on the rock itself, however, that a near acquaintance can be made with the puffins, for the ornithologist has but to sit down anywhere near their burrows, and keep the peace, when the Mormons will show their confidence by alighting in hundreds within a few feet of the spot he occupies. There is, it must be confessed, a risk of censure incurred while speaking of bird flights darkening the air, and making a noise like the rushing of wind, yet the scene I wit