CEPHALOPODS. PTEROPODS. GASTEROPODS. All the Decapods, The entire class. Heteropods ; e. g. Sepia, Loligines. Octopods; Nautilus. Ptero- soma; Testacellus; Natica; The majority of Bivalves ; most remarkable in Solen, Mya, Mactra, Pholas, Ostrea: Anomiæ; Pedum; The whole order. Ascidia and its al Spondylus, and footless Pectens: Aviculæ ; some lies, Botryllida. 168 LETTER XI. THEIR SYSTEM OF AQUEDUCTS. I HAVE often been struck, when examining the testaceous mollusca, with the great difference between the size of the animal when fully extruded and when contracted within the chambers of its shell. It may not be compared to that power of expansion and contraction which Milton assigns to the fallen spirits when they thronged the council-hall of Pandemonium,-who "To smallest forms "Reduced their shapes immense ;" but it is so remarkable that, when once observed, it can scarcely fail to raise a question of its cause and end. You may observe the difference in almost every marine mollusk, though we have no species on our shores that exhibits it in that excess of which we have an example in the "Yet" of Adanson, (Cymba neptuni, Sow.,) where the protruded foot far exceeds the entire bulk of the shell (Fig. 29). The Cowries and the Tun-shells (Dolium) are examples of the same excess, and in the former the breadth of the foot, and the extent of the mantle-lobes, contrast strongly with the narrowness of the shell's aperture. Nor is the fact less visible in the land tribes, for just recall to memory the size and figure of the common snail as it crawls along, and you will then admit it to be curious how such a broad elongated foot, and all the tentacula, can be so nicely compacted together as to be contained in the shell with ease, and with room enough to spare. I have already called your attention to * *Of Buccinum lævigatum, and B. achatenum, Mr. Swainson says,— "Both these have the foot of an immense size, so that it spreads over a circumference near three times as large as the shell, and is sufficient to envelope it entirely."-Malacology, 74. The genera of Gasteropods, which have the foot disproportionably large, are Dolium, Oliva, Ancillaria, Bulliana, Harpa, Voluta, Cymbium, Óvula, Cypræa, and Natica and Bulla. An orifice for the introduction of water within the foot is seen also in Conus and Nassa, where the foot is comparatively small. See "Figures of Molluscous Animals," etched by Maria-Emma Gray, vol. i. Lond. 1842: one of the most valuable works which the conchologist can place in his library. Fig. 29. the same phenomena as exhibited in the bivalved mollusca, in whom the foot can frequently be made to surpass the shell; nor is this capacity of temporary and varying increase in size limited to conchiferous mollusks, but is possessed also by the naked species. The slugs are more tumid and transparent in moist weather than in a drought; and the sea-slugs, whether Gasteropod or Pteropod, retain only their amplest dimensions when mersed in water; for when removed thence, they shrink and shrivel down to perhaps a half of their former bulk. This increase in any species is always accompanied with a greater transparency of the body; and only in this state can the tenta im cula and the other exterior organs of the animal be properly displayed, or progression accomplished. The mechanism by which the animals effect these relative changes in their bulk is curious and simple. Were you to fill a cup exactly with a dried sponge, you could make it rise above the rim, and expand over it on all sides by pouring some water into the cup. The water does not float the sponge, but is imbibed into its interior by percolating through its pores and canals. It is somewhat similar with some of the mollusca. These are framed with a system of canals and cavities, excavated principally in the foot and penetrating it thoroughly, which has a communication with the exterior surface on the one hand, and leads into the visceral cavity on the other. The circumfluent water entering by the external orifices flows into and through these canals, fills them and the cavities with which they communicate, and, as a necessary sequence, the organs are distended, enlarged, and rendered firm and more capable of muscular exertions. When the animal wills to come out from its shell, or to remove to another site, the water is made to flow inwards through the aqueducts; and when it wishes to reenter or conceal itself, the muscular compres sion of the parts forces the water again from the body by the same channels. The discovery of this remarkable system of aqueducts was made by the Neapolitan naturalist, S. Delle Chiaie, not more than thirty years ago. He detected it in many testaceous and naked Gasteropods; and ascertained that there were no traces of it in some freshwater genera, as in Limnæus and Planorbis,-a singular exception, for which probably you may find a reason in the comparative lightness of their shells, requiring for their support no mechanical additions to their inherent muscular powers. Delle Chiaie found similar aqueducts in the arms of various Cephalopods, in whom they serve to elongate their motive organs, and to distend their acetabula previous to their being fixed upon a surface; and the muscular fins of the Pteropods are permeated by analogous canals. The discovery has been since confirmed; and the same apparatus has been shown to exist in the Bivalves by Professor Baer, of Königsberg;† in many Ascidians by Delle Chiaie; and perhaps also in the pulmonated terrestrious Gasteropods by M. Kleaberg, for this seems to be the function of what he calls their " mucous ducts." It may, indeed, appear absurd to ascribe the office * Anim. s. Vert. Nap. ii. 259, &c. The foot of Lucina " is frequently twice as long as the diameter of the animal. When not contracted, it is much longer. It is remarkable that it is hollow throughout its entire length, and that this tube opens directly and widely into the spaces of the visceral cavity."-FORBES and HANLEY's Brit. Moll. ii. 42. The curiously ciliated arms of the Brachiopoda are extended by the same means. When treating of Terebratula psittacea, Professor Owen says: "The mechanism by which the arms are extended, is simple and beautiful. The stems are hollow from one end to the other, and are filled with fluid, which, being acted upon by the spirally disposed muscles composing the parietes of the canal, is forcibly injected towards the extremity of the arm, which is thus unfolded and protruded outwards."-Trans. Zool. Soc. i. 150. See also, p. 155, where the Professor states that in Orbicula these canals have no connection with the vascular system. "In the gasteropodous mollusca of the genera Limax, Arion, Helix, and Bulimus, we find under the mouth, between the two inferior lips, and the protuberance of the disk of the foot, the orifice of a canal, hitherto unobserved, which runs along the whole of the foot. This anatomical arrangement is not very distinct in the genus Succinea, which approaches nearer to the Lymnææ in internal structure. In the Arion empiricorum, which is entirely black, we perceive a trace of this canal, which appears in the form of a whitish band. The canal is not simple; it receives many little ducts, which come from the muscular sac in which the viscera are contained. In the Bulimus ovatus, Brug., a little gland, which has not been described, opens into this canal; it is of the size of a bean, trilobate, granulated, and situated under the oesophagus and the inferior ganglion of the cerebral ring, so that it is surrounded by nervous filaments passing from this ganglion. The distribution of aqueducts to any vessels of a terrestrious animal; but before you reject the conclusion, you will consider that this tribe is active only in a moist atmosphere, when too they swell out, and by their greater lubricity and clearness show that the body is saturated and distended with a liquid. I believe that they have absorbed this liquid from the air that surrounds them,-the moisture, however, very probably having entered through invisible pores in the skin into the loose textures and very bloodvessels of the animal.* After a long continuance of dry weather the land mollusca become extenuated; and retiring far within the shell, or under cover, they lie exhausted and incapable of any active exertion. This apparatus for absorbing and containing water is distinguished from the circulating, and from every other system of vessels by having always an outward communication with the circumfluent fluid; but the position of the external orifices is less uniform than might have been anticipated. In some Gasteropods (Cypræa), there is a long slit in the sole of the foot near its middle; in orders (Haliotis) there are two or three pores at each extremity; in others (Doris, Aplysia, Bulla, &c.) there are a series of orifices placed round its edges. Delle Chiaie says, that where these pores exist they are passages of admission to the water; but in a large proportion of the class the surface of the foot is imperforate, and in many of these (Turbo, Trochus, Murex, + Purpuriferæ, &c.) the water enters by a peculiar orifice placed in of all the ducts may be easily observed when filled with mercury: M. Kleaberg names them mucous ducts, but he has not been able to determine their use and importance."- Edinb. Journ. Nat. and Geogr. Science, ii. 63. "Spallanzani found that snails absorb an abundance of water, for their weight increases rapidly when they are placed in it. Jacobson has lately made experiments on the absorbing power of the Vine-snail (Helix pomatia). A solution of prussiate of potass, which was poured on the surface of animals belonging to this species, was absorbed with rapidity and passed into the mass of the blood. The blood can take up such a quantity as afterwards to acquire a deep blue colour, when sulphate of iron is added.”— TIEDEMAN'S Comp. Physiology, 90. In the anterior part of the foot of the Muricidæ, there are to be seen certain holes or antra, which are the apertures to as many little cavities lying underneath, and which permeate the interior substance of the foot. There are, besides, between these cavities, certain slender canals trending to the same holes or antra, by means of which the whole are connected and inosculated together. The water then entering by the syphuncle at the will of the animal, is sent upon the inferior surface of the foot, into its substance and into the antra; and flowing thence into the cavities, the foot is rendered turgid and firm; but when necessary the water, by a strong pressure, is made to transude from the substance of the foot, or is spontaneously ejected when life becomes feeble: the foot becomes then flaccid and extenuate.-DELLE CHIAIE, Anim. s. Vert. Nap. ii. 204. |