Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/660

Rh 632 MOLLUSCA rjlHE Mollusca form one of the great &quot;phyla,&quot; or sub- J_ kingdoms of the Animal Pedigree or Kingdom. Literary History of the Group. The shell-bearing forms belonging to this group which were known to Linnaeus were placed by him (in 1748) in the third order of his class Vermes under the name &quot;Testacea,&quot; whilst the Echino- derms, Hydroids, and Annelids, with the naked Molluscs, formed his second order, termed &quot; Zoophyta.&quot; Ten years later he replaced the name &quot;Zoophyta&quot; by &quot;Mollusca,&quot; which was thus in the first instance applied, not to the Mollusca at present so termed, but to a group consisting chiefly of other organisms. Gradually, however, the term Mollusca became used to include those Mollusca formerly placed among the &quot;Testacea,&quot; as well as the naked Mollusca. It is important to observe that the term pxAaKia, of which Mollusca is merely a Latinized form, was used by Aristotle to indicate a group consisting of the Cuttle-fishes only. The definite erection of the Mollusca into the position of one of the great primary groups of the animal kingdom is due to George Cuvier (1788-1800), who largely occupied himself with the dissection of representatives of this type (I). 1 An independent anatomical investigation of the Mollusca had been carried on by the remarkable Neapolitan natur alist Poli (1791), whose researches (2) were not published until after his death (1817), and were followed by the beautiful works of another Neapolitan zoologist, the illus trious Delle Chiaje (3). The &quot; embranchement &quot; or sub-kingdom Mollusca, as de fined by Cuvier, included the following classes of shell-fish : 1, the cuttles 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 sea- squirts 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 con sidered by him as Mollusca have been one by one removed from that association in consequence of improved know ledge, 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 Mol lusca was that of the Cirrhopoda. Their affinities with the lower Crustacea were recognized by Cuvier and his contem poraries, 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 Crus tacea. 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 for mulated by him (4). Thus it is to Thompson (1830), and not to Burmeister (1834), as erroneously stated by Kefer- stein, 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 Eussian embryologist Kowalewsky startled the zoological world with a minute account of the developmental changes of Ascidia, o^eofj^ Tunicata (5), and it became evident that the 1 These figures refer to the bibliography at the end of the article^ p. 695. 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 con nected with the history of a Molluscan class. In 1830 he pointed out that among the numerous kinds of &quot; polyps&quot; 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 affi nities of the Polyzoa with the Molluscan class Brachiopoda, and proposed to associate the three classes Brachiopoda, Polyzoa, and Tunicata in a large group &quot;Molluscoidea,&quot; 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 &quot; Vermes ; &quot; whilst by others (including the present writer) they have, together with the Brachiopoda, been regarded as true Mol lusca. The recent investigation by Mr. Caldwell (1882) of the developmental history of Phoronis (9), together with other increase of knowledge, has now, however, estab lished the conclusion that the agreement of structure supposed to obtain between Polyzoa and true Mollusca is delusive ; and accordingly they, together with the Brachi opoda, have to be removed from the Molluscan phylum. Further details in regard to this, the last revolution in Mol luscan classification, will be found in the article POLYZOA. As thus finally purified by successive advances of em- bryological 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 Kolliker (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 dis cussion 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 Gottingen, published as part of Bronn s Classen und Ordnungen des Thier-Reiclis. The latter work is the most completely illustrated and most exhaustive survey of exist ing knowledge of a large division of the animal kingdom which has ever been produced, and, whilst forming a monu ment to its lamented author, places the student of Mol luscan morphology in a peculiarly favourable position.