It has, I that had nearly slipt from my remembrance. think, been very generally received as an axiom in physiology that the power of locomotion bears a ratio to the perfection of the respiratory organs: the more perfect the latter, and the greater their capability of submitting the blood to the action of the oxygenating medium, the more vivacious and agile the animal in its movements. Thus, the cuttle-fish, with their laminated and highly-developed gills, move in the bosom of the ocean with quickness and vigour; and the pulmoniferous and pectinibranchial mollusks have been advantageously contrasted with the sedentary bivalves and the ascidians. But the latter comparison is surely an unfortunate one, for I know no mollusks in which the gills are so large in proportion to the body as in the fixed oyster and anchored mussel. Indeed, in regard to the mollusca the axiom will not hold good. Some of the bivalves, such as the Cyclas, move with little less rapidity than the Limneus and pulmoniferous Gasteropods; and the water-breathing Rissoæ are more quick of foot by far than the slugs and snails which breathe uncombined air. Even in the same tribe and family there are such differences in respect of speed that it seems impossible to ascribe much influence on it to any formation of the gills. Thus, the Buccinum undatum is preeminently tardigrade, but its near ally the Nassa maculata is quick and active; and similar examples may be easily pointed out. Nor do I believe that there is any connection between any structural peculiarities of the respiratory organs and the varying depths in which the mollusca do live. In his masterly anatomy of the Brachiopoda, Professor Owen would seem, perhaps, to intimate the contrary. He says, that to the Ligula a respiratory apparatus more complex and obvious than that of the Terebratula, was indispensable, because the former lives more commonly near the surface, and where it must meet with a greater variety and abundance of animal nutriment than can be found in those abysses in which Terebratula is destined to reside. He continues: "The respiration, indeed, as well as the nutrition of animals living beneath a pressure of from sixty to ninety fathoms of sea-water, are subjects of peculiar interest, and prepare the mind to contemplate with less surprise the wonderful complexity exhibited in the minutest parts of the frame of these diminutive creatures. In the stillness pervading these abysses they can only maintain existence by exciting a perpetual current around them, in order to dissipate the water already loaded with their effete particles, and bring within the reach of their prehensile organs the animalcula adapted for their support. The actions of Terebratula and Orbicula, from the firm attachment of their shells to foreign substances, are thus confined to the movements of their brachial and branchial filaments, and to a slight divarication or sliding motion of their protecting valves; and the simplicity of their digestive apparatus, the corresponding simplicity of their branchiæ, and the diminished proportion of their soft to their hard parts, are in harmony with such limited powers. The soft parts in both genera are, however, remarkable for the strong and unyielding manner in which they are connected together: the muscular parts are in great proportion, and of singular complexity as compared with ordinary bivalves; and the tendinous and aponeurotic parts are remarkable for the similarity of their texture and appearance to those of the highest classes. By means of all this strength they are enabled to perform the requisite motions of the valves at the depths in which they are met with. Terebratula, which is more remarkable for its habitat, has an internal skeleton superadded to its outward defence, by means of which additional support is afforded to the shell, a stronger defence to the viscera, and a more fixed point of attachment to the brachial cirri."* This interesting passage cannot fail to please you, and its scope is, generally speaking, correct and just; but if thence you infer that the Terebratulæ reside only in the unfathomed depths of the ocean, you will be misled, for specimens of the recent species have been taken from under rocks that were left uncovered by a springtide. Many bivalves too, which differ in nothing material from littoral species, have their abode in very deep water; and Mr. Broderip furnishes me with a striking instance of the various depths which species even of the same genus affect. In describing some species of Clavagella, he says: "Clav. australis was so near the surface at low water, that it was detected by its ejection of the fluid; Clav. elongata, from the nature of the coral in which it is chambered, could not have been living far beneath the surface; whereas Clav. lata was dredged up from a depth of sixty-six feet. Any inferences, therefore, as to the state of submersion of a rock during the life of the fossil species of Clavagellas which there occur, should be made with caution by the geologist."† 269 LETTER XVI. ON THEIR ORGANS OF RESPIRATION. THE respiratory organs of the mollusca have peculiar claims to the attention of the conchologist, not solely because of their function, which, indeed, is one of chief importance, but because they have furnished the principal characters on which modern systematists have proceeded to subdivide the class into orders and families. Cuvier, of whom, among recent naturalists, it may most truly be said that he was "Ordain'd to light with intellectual day The mazy wheels of Nature as they play," was the first to perceive their utility in this respect; and when it is considered that their position mainly determines the arrangement of the other viscera, and must consequently exert a powerful influence over the habits of the animals, you will feel disposed to admit that a happier choice could not have been made, the more particularly as the organs in question are in general easy to detect, and exhibit sufficient variety in location and form for every systematic purpose. Molluscous animals are either Pulmoniferous,* and breathe atmospheric air only, or they are Branchiferous, and respire it through the medium of water. In the former the respiratory organ is a simple cavity, commonly situated on the anterior part of the back; but sometimes, as in Testacella, near the tail. The air is admitted by a small circular aperture that opens outwards on the neck under the margin of the cloak, and which the animal opens and shuts at pleasure. Externally the cavity is protected either by a thick fold of the cloak, often strengthened with a horny or calcareous * Lamarck objects to the use of this term, as applied to the mollusca, Anim. s. Vert. vi. ii. 44; and hence some have called the order Pneumonobranchous. But there is no good reason in the objection. The distinction between a lung and a gill, rests on anatomical structure. When the air is operated on through the medium simply of an internal cavity or cavities, the respiration is pulmonary; when, on the contrary, processes or gills project from the cavity or from the surface, and expose the blood to the air by the vessels which are distributed on these processes and folds, the respiration is branchial.-MILNE-EDWARDS, Hist. Nat. des Crustacés, i. 91. plate, or by the body-whorl of the shell; while its interior walls, and more especially its roof, are covered with a fine vascular network, formed by the minute ramifications of the pulmonary vessels, which thus expose the blood freely to the influence of the air, alternately introduced and expelled by the alternate dilatation and contraction of the cavity itself. All the terrestrial mollusca, such as slugs and snails, and the great bulk of the Gasteropoda that inhabit fresh water, possess a respiratory apparatus of this kind; and, since these aquatic Pulmonifera (Limneus, Planorbis, and Ancyllus may be quoted as examples) are necessitated, from time to time, to inhale the fresh and uncombined air, so they will be found uniformly to be the denizens of shallow waters, and to spend a large portion of their lives at the surface. The Branchiferous Mollusca have the aerating organs greatly more diversified in every respect; and to countervail the disadvantages of breathing a medium little impregnated with air, the organs are likewise of greater extent and complexity. When placed within the body, the branchiæ, if distinct, are divided into multiplied lobes and leaflets; or, if a mere cavity, the surfaces are folded into innumerable plaits, all calculated to afford ampler space for the display and meanderings of their blood-vessels, and to expose a wider surface to the contact of the water: but, if the branchiæ are external and exposed, they are, it may be, less complicated, only because complexity seems unnecessary where fresh doses of unbreathed fluid are continually brought into momentary contact with them, and without any effort on the creature's part. The mollusca which have their branchiæ entirely exposed belong to two orders, the Pteropoda and Gasteropoda. A very few, as the Acteon (Aplysia viridis, Mont.) and the nearly allied genus Limapontia, appear to have no other respiratory organ than the common integuments, which have suffered no modification; but in the genus Cribella of Audouin and Milne-Edwards the skin of the mantle is, for this purpose, wrinkled on each side, and perforated with an infinite number of pores.* In a greater number the branchiæ are actually blended with the locomotive organs, as in Clio, a member of the former order, whose fin-like expansions are supposed to perform the office, not of progression only, but also of ventilating the blood as it circulates through the fine regular network with which their surfaces are covered. The Glaucus (Fig. 44) affords another example of the same union * Edinb. Journ. Nat. and Geogr. Sc. iii. 244. of functions. This is one of the most remarkable and most beautiful of the Gasteropods. The body glows with a fine cerulean blue colour, which deepens in hue towards the ends of the fringes of its ptero-branchiæ; the centre of the back is of pearly whiteness, bordered with a line of deep blue; and the sides are adorned with an interrupted series of fanlike laciniated gills, by aid of which, as I have said, it swims reversed at the surface of the Mediterranean Sea, in numerous swarms. * Fig. 44. Glaucus hexapterygius, copied from Fig. 45. But, generally, the external branchiæ are distinct and independent organs. Of the Pteropoda, almost each genus presents them under some new modification in form, or structure, or position, "as Nature in them strove to show variety." Thus, in the Pneumodermon (Fig. 45), they are placed nearly on the posterior extremity of the body, which is naked, and resemble two Cs placed back to back in this manner, oc, united by a little transverse bar across the middle, or at each end, the lines being garnished with a number of regular prominent leaflets of minute size. In the Hyales again, the branchiæ are pectinated, and lie concealed in a space between the lobes of the cloak, to which the water gains admission by copied from Cuvier. certain fissures on the sides of the shell; while, to make, as it were, the dissimilarity perfect, they appear, in the genus Cuvieria of Rang (Fig. 46), in the form Pneumodermon, An early, but anonymous, notice of this mollusk is worth quoting: "The mid-line of the back part appeared through a common magnifier like a single leaf, and was in continual undulating motion, either from the muscles or circulation of juices. Two side lines, extending the whole creature's length, and ending in one in the tail, of a deep blue. The fingers, or tentacles, end in a deep blue; a silvery cast intermingled with the blue over the whole back, or upper parts, where the blue is lighter."-Phil. Trans. liii. 58.-Mr. George Bennett has given a very interesting account of the habits of the Glaucus in the Proc. of the Zool. Society, pt. iv. 1836, p. 113, 119. |