Malacology ((Jr. •, soft, and , discourse), that department of zoology which treats of the mollusca, some of which were termed even by Aristotle malakia (soft animal-), including the examination both of the external shells and the internal organs. In the article Conouology the outer shells of mollusks have been sufficiently described, and their internal organization and habits will be noticed under Mollusca; it only remains here to enumerate briefly some of the principal systems of classification. Linmeus (1766) placed mollusks in his 6th and lowest class of vermes, with worms and zoophytes. As early as 1812 Cuvier had given to the world his views on the classi-fication of animals, founded principally upon his researches in comparative anatomy; he makes the mollusca his second branch, with the classes: 1, cephalopoda (like cuttle fishes); 2, pteropoda (like clio or whale bait); 3, gasteropoda, with orders pulmonata (slugs and snails), nudibranchia (naked marine genera without shells, like doris), infer'obranchia (phyllitlia), tectibranchia (bulla and aplysia), heteropoda (rarinaria), pectinibranchia (most of the marine univalves, turbo, trochus, &c), tubulibranchia (like siliquaria), scutibranchia (haliotis, &c), and cyclobranchia (patella and chiton ); 4, acephala, with orders testacea (oys-ter, clam, and most bivalve shells) and tunicata (ascidians); 5, Irachiopoda, like terebratula, crania, and lingula; and 6, cirrhopoda (like barnacles), now placed among art tat lata in the class Crustacea. - Lamarck (1815-'22) arranged the mollusks in two classes: one his 1lth, conchifera or bivalves, with the orders dunyaria (having two separated muscular im-pressions on the inside of the shells) and morwmyaria (with a nearly central single impression); the other his 12th class, mollusca, with the orders pteropoda, gasteropoda, trache-hpodsi (helix, &c), cephalopoda, and heteropoda (carinana); be placed the ascidians in his 4th class, tunicata, among his apathetic animalshe made of the cirripeds his 10th class, with the orders sessilia and pedunculata, ranking them and the next two classes among sensitive animals. - Ehrenberg (1836), in his division of ganglioneura (with ganglionic nervous system), and subdivision sphygmozoa (with a heart and pulsating vessels), makes his 4th section of mollusca, characterized by absence of articulations to the body and by the irregular dispersion of the nervous ganglia; he gives the classes cephalopoda, pteropoda, gasteropoda, acephala, brachiopoda, tunicata (simple ascidians), and aggregata (compound ascidians); the cirripeds he places among crustaceans. - Owen (1843-'58), in his "Lectures on Comparative Anatomy," and article "Mollusca " in the " Encyclopaedia Britannica" (8th edition), divides the province mollusca or heterogangliata into two sections, acephala and encepliala, according to the absence or presence of a head and its accompanying parts.
Acephala, with the classes: 1, tunicata; 2, brachiopoda; 3, lamellibranchiata, with the groups monomy-aria and dimyaria, with one or two adductor muscles.
Encepliala, with the classes: 4, pteropoda; 5, gasteropoda, with the divisions moncecia and dicecia; and 6, cephalopoda, with orders tetrabranchiata and dibranchiata. The cirripeds he places among articulates, though in a class distinct from crustaceans, and he, with his predecessors, retains the bryozoa among radiates. - Siebold (1848) makes three classes as follows: 1, acephala, with orders tunicata, brachiopoda, and lamellibranchia (with suborders monomya, dimya, and in-clusd); 2, cephalophora, with orders pteropoda, heteropoda, and gasteropoda (with suborders apneusta, heterobranchia, tubicolod, pectinibranchia, and pulmonata)', and 3, cephalopoda, without orders, but with families nautili-na, octopoda, and loligina. (See Burnett's translation, Boston, 1854.) - Leuckart (1848) divides mollusca into four classes: 1, tunicata, with orders ascidim and salpo3 (he is inclined to make these not simply a class, but a type intermediate between echinoderms and worms); 2, acephala, with orders lamellibranchiata and brachiopoda; 3, gasteropoda, with orders heterobranchia, dermatobranchia, heteropoda, ctenobranchia, pulmonata, and cyclobranchia; and 4, cephalopoda. - Before giving the classifications of Milne-Edwards and Agassiz, which seem to be the truest to nature, it will be instructive to glance at a few physio-philosophical and embryological systems as compared with the preceding founded upon anatomical structure.
Oken (1809-'43) places the mollusca in his province of dermatozoa (sensitive or tegumentary animals) or splanchnozoa (visceral or fieshless animals), and in the circle of vascular, sexual animals, equivalent to mala-cozoa and conchozoa (glandular or shell animals); according to the anatomical system, the vascular animals are either venous (like mussels), arterial (like snails), or cardiac (like kraken or cuttle fishes); according to the development of the feeling sense, the sexual animals (the same as the vascular) are either ova-rial, orchitic, or renal. In his system (see his "Physiophilosophy," Ray society ed., 1847) the first class of mollusks (venous, ovarial animals or mussels) has the following orders: I. Protozooid mussels. II. Conchozooid mussels; this corresponds to the acephala, and is characterized by a membraneous heart with two auricles. The second class (arterial, orchitic animals or snails) has the following orders: III. Protozooid snails or androgyni (bisexual). IV. Conchozooid snails or dicecii (with separate sexes); this class corresponds to gasteropods, having a membraneous heart with one auricle. The third class (cardiac, nephritic animals or kraken) has the following orders: V. Protozooid kraken. VI. Conchozooid kraken.
It will be seen from this system that the principles of Cuvier respecting the different plans of the four great divisions of the animal kingdom are entirely set at nought; orders, according to Oken, representing in their respective classes the characteristic features of the lower types. - Among the em-bryological systems may be mentioned those of Von Baer, Kolliker, Van Beneden, and Vogt. Von Baer (1827-'8) calls the mollusks the massive type, as the body and its parts are formed chiefly in round masses, the shape un-symmetrical, the nervous ganglia diffused and appearing late, and the movements slow and feeble; in the course of development identical parts are produced, curving around a conical or other space. According to Kolliker (1844), in the mollusks the embryo arises from a primitive part, grows uniformly in every direction, and either entirely encloses the embryonal vesicle, early in gasteropods and acephala, or late (forming a temporary vitelline sac) as in Umax, or else contracts above the embryonal vesicle, forming a genuine vitelline sac, as in cephalopods.
Van Beneden (1845-55) places mollusks with worms and radiates under his group of allocotyledones or allovitellians, in which the vitellus or yolk enters the body neither from the ventral nor from the dorsal side; his class mollusca, at the first date, included cephalopods, gasteropods, pelecypods, and brachiopods; in his later work he added acephala, tunicata, and bryozoa, removing the last two from the polyps; the cephalopods, however, are not allovitellians, and any classification which unites in one group mollusks, worms, and radiates cannot be founded on correct principles. Vogt (1851) adopts the distinction of Kolliker, of animals in which the embryo is developed from the whole yolk, and those in which it arises from a definite part of it, in the former of which he places mollusks, with worms and radiates; he makes a primary division of the cephalopoda, in which the yolk is cephalic, with a class of the same, with the orders tetrabranchiata and dibranchiata. In the division mollusca, with an irregular disposition of the organs, he makes the followrog classes: cephalophora, acephala, tunicata ctenophora, and bryozoa. The last three classes constitute his mollvscoidea.
The separation of the cephalopods is unjustifiable, and the transfer of the ctenophora from acalephan radiates to mollusks cannot be maintained.