The “embranchement” or sub-kingdom Mollusca, as defined by Cuvier, included the following classes of shell-fish :— 1, the cutties or poulps, under the name Cephalopoda; 2, the snails, whelks, and slugs, both terrestrial and marine, under the name Gastropoda ; 3, the sea-butterflies or winged-snails, under the name Pteropoda ; 4, the clams, mussels, and oysters, under the name Acephala ; 5, the lamp-shells, under the name Brachiopoda; 6, the seasquirts or ascidians, under the name Nuda ; and 7, the barnacles and sea-acorns, under the name Cirrhopoda.

The main limitations of the sub-kingdom or phylum Mollusca, as laid down by Cuvier, and the chief divisions thus recognized within its limits by him, hold good to the present day. At the same time, three of the classes considered by him as Mollusca have been one by one removed from that association in consequence of improved knowledge, and one additional class, incorporated since his day with the Mollusca with general approval, has, after more than forty years, been again detached and assigned an independent position owing to newly-acquired knowledge.

The first of Cuvier’s classes to be removed from the Mollusca was that of the Cirrhopoda. Their affinities with the lower Crustacea were recognized by Cuvier and his contemporaries, but it was one of the brilliant discoveries of that remarkable and too-little-honoured naturalist, J. Vaughan Thompson of Cork, which decided their position as Crustacea. The metamorphoses of the Cirrhopoda were described and figured by him in 1830 in a very complete manner, and the legitimate conclusion as to their affinities was formulated by him (4). Thus it is to Thompson (1830), and not to Burmeister (1834), as erroneously stated by Kef erstem, that the merit of this discovery belongs. The next class to be removed from Cuvier’s Mollusca was that of the Nuda, better known as Tunicata. In 1866 the Russian embryologist Kowalewsky startled the zoological world with a minute account of the developmental changes of Ascidia, one of the Tunicata (5), and it became evident that the affinities of that class were with the Vertebrata, whilst their structural agreements with Mollusca were only superficial. The last class which has been removed from the Cuvierian Mollusca is that of the Lamp-shells or Brachiopoda. The history of its dissociation is connected with that of the class, viz., the Polyzoa or Bryozoa, which has been both added to and again removed from the Mollusca between Cuvier’s date and the present day. The name of J. Vaughan Thompson is again that which is primarily connected with the history of a Molluscan class. In 1830 he pointed out that among the numerous kinds of “polyps” at that time associated by naturalists with the Hydroids, there were many which had a peculiar and more elaborate type of organization, and for these he proposed the name Polyzoa (6). Subsequently (7) they were termed Bryozoa by Ehrenberg (1831).

Henri Milne-Edwards in 1844 demonstrated (8) the affinities of the Polyzoa with the Molluscan class Brachiopoda, and proposed to associate the three classes Brachiopoda, Polyzoa, and Tunicata in a large group “Molluscoidea,” coordinate with the remaining classes of Cuvier’s Mollusca, which formed a group retaining the name Mollusca. By subsequent writers the Polyzoa have in some cases been kept apart from the Mollusca and classed with the “Vermes;” whilst by others (including the present writer) they have, together with the Brachiopoda, been regarded as true Mollusca. The recent investigation by Mr. Caldwell (1882) of the developmental history of Phoronis (9), together with other increase of knowledge, has now, however, established the conclusion that the agreement of structure supposed to obtain between Polyzoa and true Mollusca is delusive; and accordingly they, together with the Brachiopoda, have to be removed from the Molluscan phylum. Further details in regard to this, the last revolution in Molluscan classification, will be found in the article Polyzoa.

As thus finally purified by successive advances of embryological research, the Mollusca are reduced to the Cuvierian classes of Cephalopoda, Pteropoda, Gastropoda, and Acephala. Certain modifications in the disposition of these classes are naturally enough rendered necessary by the vast accumulation of knowledge as to the anatomy and embryology of the forms comprised in them during fifty years. Foremost amongst those who have within that period laboured in this group are the French zoologists Henri Milne-Edwards (20) and Lacaze Duthiers (21), to the latter of whom we owe the most accurate dissections and beautiful illustrations of a number of different types. To Kölliker (22), Gegenbaur (23), and more recently Spengel (24), amongst German anatomists, we are indebted for epoch-making researches of the same kind. In England, Owen’s anatomy of the Pearly Nautilus (10), Huxley’s discussion of the general morphology of the Mollusca (11), and Lankester’s embryological investigations (12), have aided in advancing our knowledge of the group. Two remarkable works of a systematic character dealing with the Mollusca deserve mention here—the Manual of the Mollusca by the late Dr. S. P. Woodward, a model of clear systematic exposition, and the exhaustive treatise on the Malacozoa or Weichthiere by the late Professor Keferstein of Göttingen, published as part of Bronn’s Classen und Ordnungen des Thier-Reichs. The latter work is the most completely illustrated and most exhaustive survey of existing knowledge of a large division of the animal kingdom which has ever been produced, and, whilst forming a monument to its lamented author, places the student of Molluscan morphology in a peculiarly favourable position.

Classes of the Mollusca. — The classes of the Mollusca which we recognize are as follows :— [table]

General Characters of the Mollusca. — The forms comprised in the above groups, whilst exhibiting an extreme range of variety in shape, as may be seen on comparing an Oyster, a Cuttle-fish, and a Sea-slug such as Doris; whilst adapted, some to life on dry land, others to the depths of the sea, others to rushing streams; whilst capable, some of swimming, others of burrowing, crawling, or jumping, some, on the other hand, fixed and immobile; some amongst the most formidable of carnivores, others feeding on vegetable mud, or on the minutest of microscopic organisms—yet all agree in possessing in common a very considerable number of structural details which are not possessed in common by any other animals.

The structural features which the Mollusca do possess in common with other animals belonging to other great phyla of the animal kingdom are those characteristic of the Coelomata, one of the two great grades (the other and lower being that of the Ccelentera) into which the higher animals, or Enterozoa as distinguished from the Protozoa, are divided (13). The Enterozoa all commence their individual existence as a single cell or plastid, which multiplies itself by transverse division. Unlike the cells of the Protozoa, these embryonic cells of the Enterozoa do not remain each like its neighbour and capable of independent life, but proceed to arrange themselves in two layers, taking the form of a sac. The cavity of the two-cell-layered sac or Diblastula thus formed is the primitive gut or arch-enteron. In the Coelentera, whatever subsequent changes of shape the little sac may undergo as it grows up to be Polyp or Jelly-fish, the original arch-enteron remains as the one cavity pervading all regions of the body. In the Coelomata the arch-enteron becomes in the course of development divided into two totally distinct cavities shut off from one another—an axial cavity, the met-enteron, which retains the function of a digestive gut; and a peri-axial cavity, the coelom or body-cavity, which is essentially the bloodspace, and receives the nutritive products of digestion and the waste products of tissue-change by osmosis. The Mollusca agree in being Coelomate with the phyla Vertebrata, Platyhelmia (Flat-worms), Echinoderma, Appendiculata (Insects, Ringed-worms, <fcc.), and others,—in fact, with all the Enterozoa except the Sponges, Corals, Polyps, and Medusae.

In common with all other Coelomata, the Mollusca are at one period of life possessed of a prostomium or region in front of the mouth, which is the essential portion of the “head,” and is connected with the property of forward locomotion in a definite direction and the steady carriage of the body (as opposed to rotation of the body on its long axis). As a result, the Coelomata, and with them the Mollusca, present (in the first instance) the general condition of body known as bilateral symmetry; the dorsal is differentiated from the ventral surface, whilst a right and a left side similar to, or rather the complements of, one another are permanently established. In common with all other Coelomata, the Mollusca have the mouth and first part of the alimentary canal which leads into the met-enteron formed by a special invagination of the outer layer of the primitive body-wall, not to be confounded with that which often, but not always, accompanies the antecedent formation of the arch-enteron; this invagination is termed the stomodaeum. Similarly, an anal aperture is formed in connexion with a special invagination which meets the hinder part of the met-enteron, and is termed the PROCTODAeUM.

In common with many (if not all) Coelomata, the Mollusca are provided with at least one pair of tube-like organs, which open each by one end into the coelom or body cavity, and by the other end to the exterior, usually in the neighbourhood of the anus. These are the nephridia.

Like all other Coelomata, the Mollusca are also provided with special groups of cells forming usually paired or median growths upon the walls of the coelomic cavity, the cells being specially possessed of reproductive power, and differentiated as egg-cells and sperm-cells. These are the gonads. As in other Coelomata, the cells of the gonads may escape to the exterior in one of two ways—either through the nephridia, or, on the other hand, by special apertures.

As in all other Coelomata, the cells, which build up respectively the primary outer layer of the body, the lining layer of the met-enteron, and the lining layer of the coelom, are multiplied and differentiated in a variety of ways in the course of growth from the early embryonic condition. Tissues are formed by the adhesion of a number of similarly modified cells in definite tracts. As in all Coelomata, there is a considerable variety of tissues characterized by, and differentiated in relation to, particular physiological activities of the organism. Not only the Coelomata but also many Ccelentera show, in addition to the EPITHELIA (the name given to tissue which bounds a free surface, whether it be that of the outer body-wall, of the gut, or of a blood-space), also deeper lying tissues, of -which the first to appear is muscular tissue, and the second nervous tissue.

The epithelia are active in throwing off their constituent cells (blood-corpuscles from the wall of the coelom), or in producing secretions (glands of body-wall and of gut), or in forming horny or calcareous plates, spines, and processes, known as cuticular products (shells and bristles of the body-wall, teeth of the tongue, gizzard, &c.).

In the Mollusca, as in all other Coelomata, in correspondence with the primary bilateral symmetry and in relation to the special mechanical conditions of the prostomium, the nervous tissue which is in Ccelentera, and even in Flatworms, diffused over the whole body in networks, tends to concentrate in paired lateral tracts, having a special enlargement in the prostomium. The earlier plexiform arrangement is retained in the nervous tissue of the walls of the alimentary canal of many Coelomata, whilst a concentration to form large nerve-masses (ganglia), to which numerous afferent and efferent fibres are attached, affects the nervous tissue of the body-wall.

In all Coelomata, including Mollusca, muscular tissue is developed in two chief layers, one subjacent to the deric or outer epithelium (somatic musculature), and a second surrounding the alimentary canal (splanchnic musculature). Thus, primarily, in Coelomata the body has the character of two muscular sacs or tubes, placed one within the other and separated from one another by the coelomic space. The somatic musculature is the more copious and develops very generally an outer circular layer (t.e., a layer in which the muscular fibres run in a direction transverse to the long axis of the body) and a deeper longitudinal layer; to these oblique and radiating fibres may be added. The splanchnic musculature, though more delicate, exhibits a circular layer nearer the enteric epithelium, and a longitudinal layer nearer the coelomic surface.

In Coelomata and in many Coelentera there are found distributed between the tracts of muscular tissue, bounding them and giving strength and consistency also to the walls of the body, of the alimentary canal, of the coelom, and of the various organs and tissue-masses (such as nerve-centres, gonads, &c.) connected with these, tracts of tissue the function of which is skeletal. The skeletal tissue of Mollusca, in common with that of other Coelomata, exhibits a wide range of minute structure, and is of differing density in various parts; it may be fibrous, membranous, or cartilaginous. The Mollusca, in common with the other Coelomata, exhibit a remarkable kind of association between the various forms of skeletal tissue and the epithelium which lines the coelomic cavity. The coelomic cavity contains a liquid which is albuminous in chemical composition (bloodlymph or haemolymph), and into this liquid cells are shed from the coelomic epithelium. They float therein and are known as blood corpuscles or lymph corpuscles. The coelomic space with its contained haemolymph is not usually in Coelomata, and is not in Mollusca, a simple even-walled cavity, but is broken up into numerous passages and recesses by the outgrowths, both of the alimentary canal and of its own walls. By the adhesion of its opposite walls, and by an irregularity in the process of increase of its area during growth, the coelom becomes to a very large extent a spongy system of intercommunicating lacunae or irregular spaces, filled with the coelomic fluid. At the same time, the coelomic space has a tendency to push its way in the form of narrow canals and sinuses between the layers of skeletal tissue, and thus to permeate together with the skeletal tissue in the form of a spongy, or it may be a tubular, network all the apparently solid portions of the animal body. This association of the nutritive and skeletal functions is accompanied by a complete identity of the tissues concerned in these functions. Not only is there complete gradation from one variety of skeletal tissue to another (e.<∕., from membranous to fibrous, and from fibrous to cartilaginous) even in respect of the form of the cells and their intercellular substance, but the coelomic epithelium, and consequently the haemolymph with its floating corpuscles derived from that epithelium, is brought into the same continuity. The skeletal and blood-containing and -producing tissues in fact form one widely-varying but continuous whole, which may be called the skeleto-trophic system of tissues.

In many Coelomata not only do the skeletal tissues allow the coelomic space with its fluid and corpuscles to penetrate between their layers, but a special mode of extension of that space is found, which consists in the hollowing out of the solid substance of elongated cells having the form of fibres, which thus become tubular, and, admitting the nutritive fluid, serve as channels for its distribution. These are “capillary vessels,” and it has yet to be shown that such are formed in the Mollusca. Larger vessels, however, concerned in guiding the movement of the coelomic fluid in special directions are very usually developed in the Mollusca, as in other Coelomata, by the growth of skeletal tissue around what are at first ill-defined extensions of the coelomic space. In this way a portion of the coelomic space becomes converted into vessels, whilst a large part remains with irregular walls extending in every direction between the skeletal tissues and freely communicating with the system of vessels. As in many other Coelomata, muscular tissue grows around the largest vessel formed from the primitive coelom, which thus becomes a contractile organ for propelling the bloodlymph fluid. This “heart ” has in Mollusca, as in most other Coelomata in which it is developed, a dorsal position. A communication of the blood-lymph space with the exterior by means of a pore situated in the foot or elsewhere has been very generally asserted to be characteristic of Mollusca. It has been maintained that water is introduced by such a pore into the blood, or admitted into a special series of water-vessels. It has also been asserted that the blood-fluid is expelled by the Mollusca from these same pores. Recent investigation (14) has, however, made it probable that the pores are the pores of secreting glands, and do not lead into the vascular system. There is, it therefore appears, no admission or expulsion of water through such pores in connexion with the blood, although in some other Coelomata it is established that water is taken into the coelomic space through a pore (Echinoderms), whilst in some others there is no doubt that the coelomic haemolymph is occasionally discharged in quantity through pores of definite size and character (Earthworm, &c.).

We have thus seen that the Mollusca possess, in common with the other Coelomata—1, a body composed of a vast number of cells or plastids, arranged so as to form a sac-like body-wall, and within that a second sac, the met-enteron, the wall of which is separated from the first by a coelom or blood-lymph space ; 2, a stomodaeum and a proctodεeum; 3, a prostomium, together with a differentiated dorsal and ventral surface, and consequently right and left sides, i.e., bilateral symmetry; 4, a pair of nephridia ; 5, gonads developed on the wall of the coelom; 6, deric epithelium (producing horny and calcareous deposits on its surface), enteric epithelium, and coelomic epithelium; 7, laterally paired masses of nerve-tissue, especially large in the prostomial region (nerve-centres or ganglia); 8, muscular tissue, forming a somatic tunic and a splanchnic tunic; 9, skeleto-trophic tissues, consisting of membranous, fibrous, and cartilaginous supporting tissues, and of blood-vessels and the walls of bloo<Lspaces, the coelomic epithelium, and the liquid tissue known as haemolymph (commonly blood).

Schematic Mollusc.— Starting from this basis of structural features common to them and the rest of the Coelomata, we may now point out what are the peculiar developments of structure which characterize the Mollusca and lead to the inference that they are members of one peculiar branch or phylum of the animal pedigree. In attempting thus to set forth the dominating structural attributes of a great group of organisms it is not possible to make use of arbitrary definitions. Of Mollusca, as of other great phyla, it is not possible categorically to enunciate a series of structural peculiarities which will be found to be true in reference to every member of the group. We have to remember that the process of adaptation in the course of long ages of development has removed in some cases one, in other cases another, of the original features characteristic of the ancestors from which the whole group may be supposed to have taken origin, and that it is possible (and actually is realized in fact) that some organisms may have lost all the primary characteristics of Molluscan organization, and yet be beyond all doubt definitely stamped as Mollusca by the retention of some secondary characteristic which is so peculiar as to prove their relationship with other Mollusca. An example in point is found in the curious fish-like form Phyllirhoë (fig. 58), which has none of the primary characteristics of a Mollusc, and yet is indisputably proved to belong to the Molluscan phylum by...