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. Subsequently⁶ they
were termed Bryozoa by Ehrenberg (1831).
Henri Milne-Edwards in 1844 demonstrated 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,” co-ordinate 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 they have, together with the Brachiopoda, been regarded as true Mollusca. 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, were removed from the Molluscan phylum by Lankester in his article in the 9th edition of this work (on the which present article is based). Further details in regard to this, the last revolution in Molluscan classification, will be found in the article Polyzoa.
As thus purified by successive advances of embryological research, the Mollusca were 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. Foremost among those who between 1840 and 1880 laboured in this field are the French zoologists Henri Milne-Edwards⁹ and Lacaze Duthiers,¹⁰ to the latter of whom we owe the most accurate dissections and beautiful illustrations of a number of different types. To Kölliker,¹¹ Gegenbaur,¹² and more recently Spenger,¹³ amongst German anatomists, we are indebted for epoch-making researches of the same kind. In England, Owen’s anatomy of the pearly nautilus,¹⁴ Huxley’s discussion of the general morphology of the Mollusca,¹⁷ and Lankester’s embryological investigations,¹⁹ 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 Dr S. P. Woodward, a model of clear systematic exposition, and the exhaustive treatise on the Malacozoa or Weichthiere by Professor Keferstein of Göttingen, published as part of Bronn’s Klassen und Ordnungen des Thier-Reichs.
The arrangement adopted by Ray Lankester in the 9th edition of the Ency. Brit. (art. “Mollusca”; 1883) was as follows: Of the four Cuvierian classes mentioned above, the Pteropoda were united with the Cephalopoda, on account of the apparent similarity of the cephalic tentacles in some of the former to the arms of the latter. An additional class was instituted for the reception of Dentalium and its few allies, and for this class Bronn’s name Scaphopoda was used. The Chitons and their allies were placed under the Gastropoda, as a distinct branch called Isopleura, and for the Acephala de Blainville’s name Lamellibranchia was substituted. The latter were regarded as forming a distinct branch, equivalent in rank to the other three classes together, the latter all possessing the radula which is wanting in Lamellibranchs.
Since the 9th edition of the Ency. Brit. was published important advances have been made in our knowledge of the Mollusca, as the result of researches largely due to the interest excited in the subject by Lankester’s article. Attention has been especially directed to the investigation of the most primitive forms in each group, and accordingly we can now form much more definite conceptions of the phylogeny and evolution of the various classes. The most important and extensive contributions to this progress have been made by the Belgian zoologist, Dr Paul Pelseneer, who has made the Mollusca his special study.
The Chitonidae and the Aplacophora are now separated from the Gastropoda and raised to the rank of a distinct class, under the name of Amphineura. On the other hand, Boas and Pelseneer have shown that the Pteropoda have nothing to do with the Cephalopoda, but are Gastropoda modified for a pelagic life; they are therefore now united with the Gastropoda. The Lamellibranchia are no longer regarded as a distinct branch in contrast to the remaining Mollusca; according to Pelseneer they are allied to the Gastropoda and Scaphopoda, all three classes being derived from a common hypothetical ancestor, called Prorhipidoglossum. These three classes have therefore been united by Grobben into one branch or grade, the Prorhipidoglossomorpha.
General Characters of the Mollusca.—The forms comprised in the various 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 Coelentera) into which the higher animals, or Metazoa as distinguished from the Protozoa, are divided. The Metazoa all commence their individual existence as a single cell or plastid, which multiplies itself by transverse division. Unlike the cells of Protozoa, these embryonic cells of the Metazoa do not remain each like its neighbour and capable of independent life, but proceed to arrange themselves into 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, on the other hand, there is another cavity, dividing the body-wall into two layers: an internal layer surrounding the gut, and an external layer. This cavity is excavated in a third mass of cells distinct from the cells lining the gut, forming the endoderm, and the cells covering the surface of the body, the ectoderm. This third mass of cells is the mesoderm. The Mollusca agree in being coelomate with the phyla Vertebrata, Platyhelmia (flat-worms), Echinoderma, Appendiculata (insects, ringed-worms, &c.), and others—in fact, with all the Metazoa 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 archenteron; 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.
The coelom is primarily and essentially the generative cavity: the reproductive cells arise from its walls, i.e. from the coelomic epithelium. True nephridia do not primarily open into the coelom, as was formerly taught, but are intra-cellular ducts in the mesoderm. Such organs are absent in Mollusca in the adult state, but a pair of nephridia usually occurs in the larva. The coelom opens to the exterior by ducts which are primarily genital ducts by which the ova or sperms are discharged. These ducts, however, as well as the coelomic epithelium, may assume excretory functions. In Mollusca the coelom is reduced and consists of two parts, the pericardial cavity which surrounds the heart, and the cavity of the gonads or generative organs. There is usually one pair of coelomic ducts leading from the
