MOLLUSCA 



THE MoUusca form one of the great " phyla," or sub- 

 kingdoms of the Animal Pedigree or Kingdom. 



Literary History of the Group: — The shell-bearing forms 

 belonging to this group which were known to Linnseus were 

 placed by him (in 1748) in the third order of his class 

 Vermes under the name "Testacea," whilst the Echino- 

 derms, Hydroids, and Annelids, with the naked Molluscs, 

 formed his second order, termed " Zoophyta." Ten years 

 later he replaced the name "Zoophyta" by "MoUusca," 

 which was thus in the first instance applied, not to the 

 MoUusca at present so termed, but to a group consisting 

 chiefly of other organisms. Gradually, however, the term 

 MoUusca became used to include those MoUusca formerly 

 placed among the "Testacea," as well as the naked MoUusca. 



It is important to observe that the term (JtaXaKia, of which 

 MoUusca is merely a Latinized form, was used by Aristotle 

 to indicate a group consisting of the Cuttle-fishes only. 



The definite erection of the MoUusca 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 t3rpe (1).^ 

 An independent anatomical investigation of the MoUusca 

 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 " embranchement " or sub-kingdom MoUusca, as de- 

 fined by Cuvier, included the following classes of shell-fish : — 

 1, the cuttles orpordps, 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 Ptbeopoda ; 4, the clams, 

 mussels, and oysters, under the name Acbphala; 5, the 

 lamp-shells, under the name Beachiopoda ; 6, the sea- 

 squirts or ascidians, under the name Nuda; and 7, the 

 barnacles and sea-acorns, under the name Cieehopoda. 



The main limitations of the sub-kingdom or phylum 

 MoUusca, 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 MoUusca 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 MoUusca 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 brilUant discoveries of that 

 remarkable and too-Uttle-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 MoUusca 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, 

 one of the Tunicata (5), and it became evident that the 



'■ 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 MoUusca were only superficial. 

 The last class which has been removed from the Cuvierian 

 MoUusca 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 MoUusca 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 MoUuscan 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 affi- 

 nities of the Polyzoa with the MoUuscan class Brachiopoda, 

 and proposed to associate the three classes Brachiopoda, 

 Polyzoa, and Tunicata in a large group " MoUuscoidea," 

 coordinate with the remaining classes of Cuvier's MoUusca,- 

 which formed a group retaining the name MoUusca. By 

 subsequent writers the Polyzoa have in some cases been kept 

 apart from the MoUusca and classed with the " Vermes ; " 

 whilst by others (including the present writer) they have, 

 together with the Brachiopoda, been regarded as true Mol- 

 lusca. The recent investigation by Mr. CaldweU (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 MoUusca is 

 delusive ; and accordingly they, together with the Brachi- 

 opoda, have to be removed from the MoUuscan 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 MoUusca 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 KoUiker (22), Gegenbaur (23), and more recently Spengel 

 (24), amongst German anatoniists, 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 MoUusca (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 MoUusca deserve mention here — the Manual of the 

 MoUusca 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-Eeichs. 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. 



