strata; although we will concede that the gregarious species are fewer, and perhaps less accumulative, than they were in the primeval seas, which some geologists have imagined were overcharged with carbonate of lime, affording a more abundant supply of matter to the shell-fish, and a consequent greater multiplication of them. But the introduction of Man on the scene has proved a more certain, and a very powerful check to the accumulative results of the present races of molluscans; for the species which are most conducive to the process, he dredges up in millions, to be used for food, for bait, for ornament, and for the purpose of conversion into lime and manure. Reflect for a moment, and you will readily admit how widely and surely this check operates. How countless are the loads of oysters alone which are annually dredged up on our coast, and on every habitable shore! multiply these by some two or three thousands of calculate their increase from one generation to another (which would have gone on, in a more than geometrical progression, had they been left undisturbed. in their native haunts), and then you may estimate the depth and width of their banks which would have covered the sea's bottom. And this is to take one species only into the calculation; but, in fact, no mollusk, which occurs in sufficient numbers to alter it materially, is left to its natural spread and increase, for what he cannot use as food or bait, man burns into lime, or strews on his land as manure,— a purpose which he has ascertained they answer admirably well, their superiority to common lime depending doubtless on the animal matter which enters into their composition.

years;

Nor are you to underrate the numbers used in this way, for they are incalculable. In our own land, indeed, where limestone is abundant, and the supply of recent shells precarious and limited, we reckon not much upon them*; but the greater part of the lime used in America, for agricultural and architectural purposes, is made of calcined shells. The

* In 1662, Mr. Ray saw the people on the Welsh coast "burning cockleshells, thereof to make lime. The manner thus. They make an hole in the ground, therein they put furze, upon that wood, upon the wood small stone coal, and then a layer of cockle-shells, and so shells and coals, s. s. S., and then put fire to them, these burnt, make excellent lime.” Sel. Rem. 245.-Lister asserts, that in his time, living mussels were extensively used in manuring the fields in Lancashire.-Hist. An. Ang. 183. Numerous examples of their application in this manner might be specified, but the most remarkable, and the only one I shall instance here, is contained in the Phil. Trans. for 1708, and communicated to the Royal Society by the Archbishop of Dublin." Marl is not used in the north parts of Ireland; but about the seaside the great manure is shells: towards the eastern part

H

inhabitants of the maritime parts of Africa, of India, China,* and of New Holland, are equally dependent on the same source for their supply of this necessary article,—hence the consumption of shells over this the greater half of the globe must be enormous, and must retard, if not entirely nullify, their accumulation into banks and beaches. To restore the molluscans to their pristine influence in the mutations of the earth's surface, it would be necessary to render it first unfit for the residence of man, or that he should grant them the petition of the dead-" leave me, leave me to repose!"

I here conclude what I had to tell you in relation to the economy and uses of the mollusca. In my next, my endeavour shall be to give you a view of the system which Cuvier has invented for their classification, when we shall be prepared to enter on what relates to their structure and physiology.

of the bay of Londonderry, commonly called Loughfoyle, lie several eminences, that hardly appear at low-water; these consist of shells of sea-fish of all sorts, more particularly of periwinkles, cockles, limpets, &c. The countrymen come with boats at low-water, and carry loads of these shells away; they leave them in heaps on the shore, and there let them lie till they drain and dry, to render them lighter for carriage; they then carry them by boats as far as the river will permit, and then in sacks on horses, perhaps six or seven miles into the country. They allow sometimes forty, but mostly eighty barrels to an acre. These shells agree with boggy, heathy, clayey, wet, or stiff land, but not with sandy." "The manure continues so long, that none can determine the time of its duration. The reason of which seems to be, that these shells dissolve every year a little, till they be all spent, which requires a considerable time; whereas, lime, &c. operates all in a manner at once; but it is to be observed, that in six or seven years the ground grows so mellow, that the corn on it grows rank and runs out in straw to such a length that it cannot support itself, and then the land must be suffered to lie a year or two, that the fermentation may abate a little and the clods harden, and then it will bear as long again, and continue to do so, with the like intermissions for twenty or thirty years. In the years in which the land is not ploughed, it bears a fine grass, mixed with daisies in abundance, and it is pleasant to see a steep high mountain, that a few years before was all black with heath, on a sudden look white with daisies and flowers. It fines the grass, but makes it short, though thick." "Some thousands of acres have been improved by the shells, and that which formerly was not worth a groat per acre, is now worth four shillings; they have in many places thus improved the very mountains, that before were mere turf bogs.' Phil. Trans. abridg. v. 404-5. See also Phil. Trans. abridg. ix. 82. On the use of shells in agriculture in America, see Gould's Report Invert. Massachus. 361.

The Chinese "not only burn lime from the oyster-shells, but likewise make use of the largest in their buildings, instead of bricks."- OSBECK'S Voy. to China, ii. 317. See also STAUNTON'S Embassy, iii. 432.

99

AN

LETTER VII.

EXPOSITION OF CUVIER'S ARRANGEMENT OF MOLLUS-
COUS ANIMALS.

I HAVE already mentioned that Cuvier was the first who grouped together the animals to which the name of Molluscous is now properly restricted. Previous to the publication of his memoir on the classification of avertebrate animals in the year 1795, the Mollusca were intermingled with worms and with zoophytes, while a great number of them stood detached from their allies under the ordinal designation Testacea, merely because they were enclosed in hard calcareous shells, the knowledge of the inferior tribes being then too little advanced to admit of the application of any characters but those that were derived from exterior form and consistence. By his numerous careful dissections, Cuvier was early enabled to detect and appreciate the unnaturalness of the prevalent systems; and when his labours had convinced him that their overthrow was necessary to the progress of science, they had at the same time furnished him with the materials out of which he sought to erect a new system, which has been of incalculable advantage to scientific conchology, and which remains untouched in all its grand lineaments, though his successors have certainly improved and worked out many of the minor details. These we shall have a future opportunity of discussing. I mean at present to give you an outline of the original as it came from its author's last revision, without note or comment; and when you are made aware that this great naturalist continued to regard it as one of the principal works on which his fame might safely rest, and watched it with a degree of parental jealousy, unwilling that the parentage should be either doubted or divided, you can have no need of being urged to make yourself its master. "It is well known," Cuvier himself tells us, * "how much care and time I have devoted to the anatomy of the mollusca in general, and in particular to the knowledge of the naked mollusca. The determination of the class, its principal divisions and subdivisions, all repose upon my proper observations; for the

* Règne Animal, tom. i. pref. xxvi. Paris, 1829.

magnificent work of M. Poli aided me no further than by some descriptions and some anatomies useful to my end, and these were confined to the multivalves and bivalves. I have verified all the facts which that able anatomist has furnished me; and, as I think, have determined with more accuracy the functions of some of the organs. I have also sought to characterize the animals to which the principal forms of shells belong, and to classify these in accordance with the organization of their inhabitants, leaving the ulterior divisions of them into genera and subgenera to those who devote themselves in particular to this work."

According to Cuvier there appear to be four general plans or models, if we may so speak, after which all animals seem to have been created. The variations superinduced on these plans may at a first glance seem considerable, and have, indeed, properly given rise to certain divisions in each of them; but by whatever name we designate these-classes, or orders, or legions—a close analysis of their organism demonstrates that the variation has been the result not of any change in the essence of the type, but of a slight or graduated modification of some one of the organic systems,—of the motive organs, of the nervous or circulating apparatus,—to which some parts, not inconsistent with the model, have been added or subtracted. Now, under the second of these four plans (sub-kingdoms) are reduced all the creatures to which the name of mollusca is assigned. The essential character lies in the peculiar arrangement of their nervous system, which consists not of a medullary ganglionated chord, but of some ganglions scattered, as it were, irregularly through the body, and from which nerves issue to its various parts. Hence the mollusca are less symmetrical in general than any other class of beings; hence also the poverty of their instinctive faculties, and the deficiency of their muscular activity, for in these respects they are decidedly inferior to insects and annulose worms, and scarcely superior to the radiated animals. They have no skeleton nor vertebral column, so that the muscles are inserted to the skin, which forms a soft envelope contractile in every direction, and in or upon which are generated, in very many species, hard calcareous plates called shells-analogous, in the opinion of Cuvier, to the

"Le corps tend toujours à se composer de parties binaires placées symétriquement des deux côtés d'un plan médian; mais cette symétrie n'est jamais complete, et ce plan, au lieu de se développer suivant une ligne droite, tend à décrire une courbe. Il en résulte que d'ordinaire la bouche et l'anus sont plus ou moins rapprochés, et que le corps, considéré dans son ensemble, présente en général un aspect plus ou moins difforme."-MILNE EDWARDS, Elem. de Zoologie, iv. 237.

corpus mucosum of vertebrates. This muscular envelope contains the viscera, among which the nervous system lies unseparated, the principal ganglion or brain, as some call it, being placed under the gullet and encircling it with a filament as with a collar. Of the proper senses the mollusca possess those of taste and sight only, and the latter often fails. A single class has organs of hearing. There is in all a complete system for the circulation of a white serous blood; and respiration is performed by special organs. The organs of digestion, and of the secretions, are nearly as complicated as they are in the vertebrated animals, and not less remarkable for their variety and curious adaptations. Such are the leading characters of the mollusca; and although this type of organization is not distinctly revealed by a very uniform correspondency between it and exterior configuration, yet there is such a degree of harmony and mutual dependency between the inner anatomy and the outward show, that only a little experience in their investigation is wanting to make us perceive and acknowledge it.

The mollusca defined in this general manner form a subkingdom, which is next divided into six classes, † their distinctions being founded on modifications of the organs of progression. For reasons already assigned one of these classes (Cirrhopods) is now considered as properly pertaining to the annulose sub-kingdom, and the five remaining may be shortly characterized as follows:

I. CEPHALOPODS.-Body enclosed in a muscular sac containing the branchiæ, and open superiorly, where the head projects. This is well developed, and is surmounted with a circle of eight or ten subulate cotyligerous appendages (feet) subservient to progression and the capture of their prey.

II. PTEROPODS.-Body not sacciform; the head has no appendages, or only very small ones; the principal organs of locomotion are two wings, or compressed fins, situated at the sides of the neck, and which, in many species, perform the function of branchiæ.

III. GASTEROPODS.-These are also cephalous, but less distinctly so than the preceding: they crawl on the belly or on a fleshy flattened disk, which is sometimes, though very rarely, compressed into the form of a fin, different, however, from those of the Pteropods, and distinguished by its ventral position.

IV. ACEPHALES.-Headless mollusca in which the mouth lies concealed in the base of the mantle, which also envelopes

It will be remembered that I am almost translating from Cuvier. The sense of hearing is not so limited in the class.

+ Règ. Animal, iii. 6, 7.