| OCR Text |
Show 302 MOLLUSCA. ber and position, as in the ~ ertebrata,_ and the irregularity is still more striking in the viscera, particularly as respects the position of the heart and respiratory organs, and even as regards the structure of the latter ; for some of. them respire elastic air, and others salt or fresh water. Their external or· gans, however, and tho8e of locomotion, ~re generally arranged symmetrically on the two sides of an ax1s(l ). The circulation of the Mollusca is always double; that is, their pulmonary circulation describes a ~eparate and distinct circle. This function is at least always aided by a fleshy ven· tricle, situated between the veins of the lungs and the arteries of the body, and not as in Fishes between the veins of the body and the arteries of the lungs. It is then an aortic ventricle. The Cephalopoda alone are provided besides with a pulmo· nary ventricle, which is even divided into two. The aortic ventricle is also divided in some genera, as in .JJ.rca and Lin· gula; at others, as in other bivalves, its auricle only is divided. When there is more than one ventricle they are not joined in a single mass, as in the warm-blooded animals, but are fre· quently placed at a considerable distance from each other, and in this case the animal may be said to have several hearts. The blood of the Mollusca is white or bluish, and it appears (1) N.ll. Linna:us united all invertebrate animals without articulated limbs in a single class, under the name of VERMES, dividing them into five orders: the I:NTESTIN.A., embracing some of my .O.nnelides and Jntestina,· the MoLLUScA, co!ll· pre bending my Naked Mollusca, my Echinodermata, and part of my Intestina and ZoophytC3,· the TESTACEA, comprising my Mollusca and .O.nnelides with sheila; the LYTROPRYTA, or Stony Corals; and the ZooPHYTEs, embracing the r~mainder of the Polypi, some of the Intestina and the Infusoria. No regard whatever was paid to nature in this nrrangement, and Ilrugiere, Eneycl. Method., endeavoured to rectify it. He there established six orders of worms, viz. the IN:rumou; the INTESTINA, including the Annelides; the ?tloL~na· cA., uniting several of my Zoophytes to my true Mollusca; the EcmNODE!Uf.lT!, which only ~omprised Echinus and Asterias; the TEsTACEA, nearly the same as those of Linna:us; and the ZooPHYTF.s, under which name he included the Corals only. This arrangement was merely superior to that of Linnreus in the more complete approximation of the Annelides, and by the distinction it effected ofa part of the Echinodermata. I proposed a new arrangement of all the invertebrate animals, founded on the~ intet·nal structure, in a paper read before the Societe d'Histoire Naturelle 0.n the lOth of May 1795, of which rny subsequent labours on this part of natural history are the development MOLLUSCA. 303 to contain a smaller proportionate quantity of fibrine than that of the Vertebrata. There are reasons for believing that their veins fulfil the functions of absorbent vessels. Their muscles are attached to various points of their skin, forming tissues there which are more or less complex and dense. Their motions consist of various contractions which produce inflexions and prolongations of their different parts, or a relaxation of the same, by means of which they creep, swim, and seize upon various objects, just as the form of these parts may permit ; but as the limbs are not supported by articulated and solid levers, they cannot advance rapidly, or per sal tum. The irritability of most of them is extremely great, and remains for a long time after they are divided. Their skin is naked, ve,ry sensible, and usually covered with a humour that oozes from its pores; no particular organ of smell has ever been detected in them, although they enjoy that sense; it may possibly reside in the entire skin, for it greatly resembles a pituitary membrane. All the Acalepha, Brachiopoda, Cirrhopoda, and part of the Gasteropoda and Pteropoda, are deprived of eyes; the Cephalopoda on the contrary have them at least as complicated as those of the warm-blooded animals. They are the only ones in which the organ of hearing has been discovered, and whose brain is enclosed within a particular cartilaginous box. Nearly all the Mollusca have a development of the skin which covers their body, and which bears more or less resemblance to a mantle; it is often however narrowed into a simple disk, formed into a pipe, hollowed into a sac, or extended and divided in the form of fins. The .Naked Mollusca are those in which the mantle is simply membranous or fleshy; most frequently however one or severallaminre, of a substance more or less hard, is formed in its thickness, deposited in layers, and increasing in extent as well as in thickness, because the recent layers always overlap the old ones. When this substance remains concealed in the thickness of the mantle, it is still customary to style the animals Naked \ |
