contrary, which were forced into combinations as artificial as by any the most arbitrary methods. What, for example, but adherence to an arbitrary rule could bring the Mactra and the Nucula into close affinity, and fix in the same order the Pectunculus and the Chama with the Solenes, the Tellinæ, and the Venusida; and the Arcæ with the mussels and the Aviculæ ? It is time, says Deshayes, to renounce these artificial arrangements founded on a single character, arbitrarily assumed and made to dominate over all others. To be natural, a classification ought to take into consideration the whole essentials of the organism in the definition and circumscription of the principal groups. It is by acting on this principle that the best zoologists have arrived nighest to the purposed end of their ingenious labours. A review of these various systems will have satisfied you, I think, that none of them are superior, or even equal, to Cuvier's, the source and parent of them all. The first outline of Cuvier's was published on the 10th of May, 1795; the last finish was given to it in March, 1830, and the strong impress of it on the progress of Conchology can never be erased. He has lifted the subject above raillery and ridicule, and placed, in its due rank, a large class of animals, than which none other more deservedly claim the attentions of the naturalist, the physiologist, and the geologist. Note to page 532. To understand the original points in this and subsequent classifications of the Tunicata, it is necessary to keep in view that of Jules-César Savigny, founded on the most delicate and elaborate dissections. Hence I shall here give a synopsis of it, with a few preliminary remarks. Dr. John Albert Schlosser was the first to describe a beautiful and common production on our shores, which Linnæus named Alcyonium schlosseri. It is a firm gelatinous crust growing on sea-weeds and, less frequently, on stones; the surface marked with numerous small pear-shaped bodies arranged round a central dot, and thus simulating a star as this is figured in geographical maps. Schlosser believed that all these bodies which entered into the composition of each star were intimately united at their smaller end, and constituted a true animal "much more beautiful than any polype, but quite of a different structure." In each pear-shaped body he detected, near the outer extremity, a small circular hole, which contracts and opens frequently; and in their common centre another "opening of a circular, oval, or oblong figure, forming a kind of rising rim like a cup, which, when the animal is alive and at rest, contracts and expands itself to many different degrees, with great alertness and velocity, though sometimes it remains a great while expanded or contracted." At neither of the apertures could he perceive any tentacula, "but by looking into them very narrowly, he saw something like very tender little fibres moving at the bottom of their insides." He believed the smaller aperture to be the mouth. Phil. Trans. an. 1755, abridg. x. 670. Ellis also examined this production. He found that all the interstices between the stars were filled with eggs of different sizes, "each adhering by one end to a very fine capillary filament. The smallest eggs are globular, and as they advance in size, they change to an oval figure; whence they assume the shape of one of the radii of the stars." Ellis ascertained" that each radius is a distinct animal by itself." Lib. cit. x. 671.-In his Essay on Corallines, Ellis described another species, but his account of its structure is very inaccurate. Pallas and Linnæus referred these species to the genus Alcyonium, under a false view, evidently, of their organisation. Gartner confirmed Ellis's opinion that each ray of the star was a separate animal; and, after a long interval, MM. Peron and Lesueur and M. Desmarest described one or more other species, indicating their structure to be of higher complexity than other polypes, without, however, attempting to give them a proper position or arrangement in the Animal Kingdom. For this great step in classification we are entirely indebted to M. Savigny, who published his "Mémoires sur les Animaux sans Vertèbres" in 1816. His systematic table of the Tunicata is as follows:- Mollusques hermaphrodites et acéphales. Class-ASCIDIÆ. Shell soft, formed by an exterior distinctly organised envelope, furnished with two apertures, a branchial and an anal one. Cloak forming an interior tunic, also furnished with two apertures corresponding with those of the shell and adherent to them. Branchia occupying wholly or in part the surface of a membranous cavity attached to the inner surface of the cloak. Mouth without labial laminæ, and situated at the bottom of the respiratory cavity between the two branchiæ. Order I. ASCIDIA TETHYDES. Cloak adhering to the envelope or shell only by the two apertures. Branchia equal, large, forming the two side-walls of the respiratory cavity. Branchial orifice furnished interiorly with a membranous and denticulated ring, or with a circle of filaments. 1. Family-TETHYÆ. Body fixed. Apertures not opposite nor communicating by the cavity of the branchiæ. Branchial cavity open at the superior extremity only, and its aperture furnished with tentacular filaments. Branchia united or coalescent on one side. I. SIMPLE TETHYES. 1. Section. Apertures with four rays. 1. BOLTENIA. Body pedunculated. 2. CYNTHIA. Body sessile. 2. Section. Apertures with more than four rays, or without distinct rays. 3. PHALLUSIA. Body sessile. 4. CLAVELINA. Body pedunculated. II. COMPOUND TETHYES. 3. Section. Both apertures with six regular rays. 5. DIAZONA. Body sessile, orbicular; a single system. single system. 4. Section. Branchial aperture only with six regular rays. 8. SYNOICUM. Body pedunculated, cylindrical, vertical ; a single system. 9. APLIDIUM. Body sessile, multiform; systems without central cavities. 10. POLYCLINUM. Body sessile, multiform; systems with central cavities. 11. DIDEMNUM. Body sessile, fungous, crustaceous; systems without central cavities. 5. Section. Apertures without rays. 12. EUCŒLIUM. Body crustaceous; systems without central cavities. 13. BOTRYLLUS. Body crustaceous; systems with central cavities. 2. Family-LUCIE. Body floating. Apertures exactly opposite, and communicating by the cavity of the branchiæ. Branchial cavity open at both ends, the superior aperture without tentacular filaments, but margined with a denticulated ring. Branchia disjunct. I. Simple Luciæ. II. Compound Lucia. 14. PYROSOMA. Body in form of a tube closed at one extremity; a single system. Order II. AsCIDIE THALIDes. Cloak adherent everywhere to the envelop. Branchie unequal, narrow, consisting of two leaflets attached to the anterior wall, and to the posterior wall of the respiratory cavity. Branchial aperture furnished with a valve. Obs. Savigny did not classify this Order. 572 LETTER XXVIII. RECENT IMPROVEMENTS IN THE CLASSIFICATION OF THE MOLLUSCA. THE system of Cuvier prevailed over the naturalist world, and the happy influence was immediately felt in the higher aims of its followers. Conchology was now understood to be the study of a series of beings which occupied a large space in the eye and plan of Him who created all things, and pronounced them very good. The structure He had given the Mollusca-the functions with which He had endowed them -the relations in which He had placed them towards each other, and to the animal kingdom in general,-and their use and office in the world's economy, were worthy subjects of study to the only intelligence that walked visible amidst this scene of life; created in part for the exercise and improvement of his intellect; and wherein he might, perhaps, discover something of the wisdom and design that planned and coordinated the whole. And so it was seen in the higher talent which occupied the field; than which no other, in natural science, has of late been more cultivated and made more productive of good results to the physiology of life, and to the unveiling of the physical transitions of the globe itself. pro The discoveries in every direction which resulted, proved also the soundness of the principles on which Cuvier had ceeded; for it has not been found necessary to alter, in any essential manner, the main sections of his system. These were marshalled in a linear series, and in a descending scale, commencing with the most complexly organized species, and ending with those which were least so.* Lamarck, you may * As I have previously mentioned, Cuvier was, however, careless of the finical niceties affected by others in their orderly localisation of families, well aware of the impossibility of arranging these in a book according to their affinities in nature. M. Emile Blanchard has well said:" Tout en ayant l'intention de ne pas négliger absolument l'ordre des rapports naturels dans la description des espèces, je dois le dire, j'attache peu d'importance ici à mettre un genre, une famille même, après ou avant l'autre. Dans le classement des représentants d'un ordre quelconque du règne animal, on ne parvient jamais à ranger les genres et les espèces sur une seule ligne sans rompre les affinités les plus évidentes."-Ann. des Sc. Nat. xi. (1849), 74. remember, proceeding in the contrary direction, considered the Tunicata to be a very distant tribe, from which the Mollusca could trace back only a doubtful descent; and Blainville made of the Tunicata and one or two other small tribes, a sort of border clans, with which he tenanted a space, otherwise waste, that lay between the Mollusca and the worms. Mr. Macleay's decision as to the rightful position of the orders of the Mollusca, and their catenation, was more ingenious; and in accordance with his general theory, that the series of organic creations form, in their progress, a series of circles, "rolling wheel within wheel, ad infinitum." Every circle was a separate class, with its members forming an intimately associated community, yet intimately connected with all the adjacent and even the furthest remote circles, either by ties of organic affinity or of analogy; the ties of affinity proving the immediate connection of the circles and their proper position in the one that embraced the whole; the analogy declaring itself in certain common peculiarities of form and habits which proved how the corresponding groups of separate and even distant circles, were made to symbolise each other. Every circle, moreover, was presumed to be formed of five other circles, each of which was resolvable into five lesser ones, and still even then revolving with a narrower and narrower range as they successively descended to embrace in the circling vortex, the lower and the lowest entities of creation. Thus the animal kingdom was supposed to form a circle that included five great circles, one containing the Acrita or Polypes, and infusory animalcules; the second the radiated; the third the annulose; the fourth the vertebrated; and the fifth the molluscous animals, which, in their most degraded forms, return to and merge into the circles of Acrita. The circle of the Mollusca should, of course, have five classes; but Mr. Macleay's attention not having been particularly directed to this tribe, he could only determine certainly the Acephala and the Pteropoda as occupants of it, and the Brachiopoda doubtfully; while the Cephalopoda and the Tunicata were deemed to be osculant groups, the former allying the Mollusca with the Vertebrata, and the latter making a similar alliance with the Acrita.* Mr. Swainson, an able and zealous disciple of Macleay's, first attempted the application of this theory, but in a modified form, to the classification of the Mollusca in 1835,† and more fully in 1840. Of the groups which constitute every Kirby and Spence's Introd. to Entomology, iii. 12; and iv. 359. + Elements of Modern Conchology. Lond. 1835. Duod. Malacology; or, Shells and Shell-fish. Lond. 1840. Duod. |