260 



MOLLUSCA 



modifications in the development of terrestrial and 

 fresh-water molluscs. 



CUutifcation. Branch A. Those with a well- 

 developed head and a rasping tongue : Class I. 

 Crphaloi>o<ia(a.\.). See also CUTTLK KISH, NAUTI- 

 LUS. 'Class II. Gasteropoda (a) More or leas 

 unsyrnmetrieal forms (see GASTEROPODA, also LIM- 

 PET, SNAIL, WHKI.K, &c.); (6) Symmetrical ami 

 primitive forms e.g. Chiton (q.v.) ; (c) A distinct 

 sub-class of jx?lagic forms, the Pterot>oda (q.v., 

 sometimes called a class, or unite<l with Class I.). 

 Class III. Scitphopoda A few very distinct forms, 

 of which Dentaliuiu (q.v.) is the hest-kno\vn repre- 

 sentative. Branch B. Those in which the head- 

 region U very slightly developed, ami without a 

 rasping tongue : Class IV. Lamellibruncliiata or 

 Bivalves (q.v.). See also MUSSEL, OYSTER, iVe. 



History. Provided as most molluscs are with 

 -uh-taiitial calcareous shells, their fossil remains 

 have been to that extent very perfectly preserved, 

 and long series have been traced out with remark- 

 able success. Itcnmins of the great types occur 

 even in the Cambrian rocks, and more abundantly 

 from the Lower Silurian onwards. The earliest 

 Nautilus type of cephalopod has waned away 

 through the ages, and only one genus (see NAU- 

 TILI'S) now persists, while the cuttle-fish type 

 which appeared much later is still represented by 

 numerous species. As a whole, however, this class 

 seems to have dwindled, for there are towards 4000 

 fossil species distributed of course over many ages 

 as against less than '2<K) living forms. With the 

 gasteropoda, however, the cose has been very differ- 

 ent, for the number of genera steadily increases 

 from the Cambrian and Silurian onwards, and of 

 the total numlier of species, computed some years 

 ago at 23,000, the extinct forms are estimated at 

 7000, the living forms at 10,000, which proves 

 lieyond doubt a notable increase in numbers. Nor 

 can we deplore any modern diminution of lamelli- 

 branchs, for, although the extinct species are 

 reckoned at about 9000 and the living at about 

 5000, the former were distrilmtcil over a much 

 vaster period, while in sonic families the numlter 

 of living species greatly exceeds that of the extinct. 

 As one would expect, the lung-breathing gastero- 

 poils and the fresh-water lamellibrancha are both 

 relatively late in appearing. 



AH to i\w pedigree, some of the characters of the 

 chiton-like gasteropoda and the general occurrence 

 of a trochosphere larva suggest the origin of mol- 

 lusos from some worm-like type, but this is not 

 saying much. Very helpful has been Professor 

 Kay Lankestcr's attempted reconstruction of the 

 ancestral or archi-moltusc, from which all the 



ff.pl. g.pe. pz.l.g.v. 



Fig. 1. Section of Schematic Mollusc (from Lankerter). 

 , mouth ; g c. cereDral g.fl. pleura), g.pe. pedal, g.n. vltcoral 



ganglia; p, foot; 1 1., Kit lnf* of dlgiiitlre gland : fc, genital 



I'pemnu ; 11, tinatrrinr end of the frxrt ; r, edge <if the mantle ; 



, kldnejr ; / lic- "f primary iliell-iac ; , ventricle of the 



heart ; , pericanlial cavity. 



known forms are supposed to have sprung. A 

 will l>e seen from the figure, this schematic con- 

 ception combines the general characters which we 

 have noted above, Its descendants must have 



early divided into two series, the sluggish bivalves, 

 with many structures (e.g. the atrophied head) 

 modified in relation to a sedentary life, and the 

 active aggressive gasteroixxls and ceplialomids. in 

 which the general features of the archi-mollusc are 

 more evidently retained. 



As the economic importance of molluscs is recog- 

 nise,! in special articles (such as those on Bivalves, 

 Cuttlefish, Ciasten>]>da, Mussel, (hster, Snail, 

 Whelk, fce. ), we shall simply illustrate in con- 

 clusion their general zoological interest. For a lung 

 time the study of molluscs hardly penetrated U-yond 

 their shells ; but, though this Conchology (q.v.) 

 was often a collecting craze, and sometimes led 

 to positively misleading conclusions, no one will 

 dispute the charm of beauty and variety which 

 these externalities possess, nor deny that their in- 

 vestigation rightly pursued (in relation to the 

 tenanting animals and the surrounding conditions 

 of life) may yield most important scientific results. 

 It is very interesting also to consider how the 

 limits of Mollusca have been gradually changed. 

 As the original title, Malakia, means soft, it is 

 necessary to explain that Aristotle applied it 

 solely tn the practically shell less cut tic li- 

 wliile Linn;eiis used the Latinised term Mollusca 

 also in primary reference to naked forms, along 

 with which be included, however, hydroids, anne- 

 lids, and echinoderms. The shelled molluscs which 

 form the majority of the modern classes he separated 



Fig. 2. Earlier ( A ) and later ( B) ' Vcligcr ' of a 

 Gastcropod (after Gegcnbaur). 

 a, velum ; 6, foot ; c, visceral dome. 



off as Testacea. But strangely enough this last 

 title was gradually replaced by Mollusca, under 

 which Cnvier included not only ccphalopods, gas- 

 teropods, pteropods, and the 'acephalous' or head- 

 less bivalves, but the Uunp*belli or Bmchiopods, 

 the Nuda or Ascidians, and the barnacles or Cirrlio- 

 po'la. In modern times, as a reference to the 

 articles on Brachiopoda, Ascidians, and Cirri- 

 iM-dia will show, the icsca relics of Vaughun 

 Thompson. Kovalevsky. Milne Kdwai'ls. and otln i~ 

 have entirely freed the class of molluscs from 

 Cuvier's last three divisions. The incomparably 

 careful anatomical work of Lnca/.e -Hut biers, along 

 \\ith tin- re-e:iieln-> of Kiilliker and (legenbaiii, 

 Huxley and Spengel, the embryologieal discovei ics 

 of liay Lankester and others, and the gcnemlisa- 

 ticins which we owe to Ilu\lr\ and Lankcsicr are 

 notable illustrations of recent progress in the study 

 of molluscs. Moreover, on tlie problems of oolu- 

 lion no little light has been cast e.g. by Wiirlen- 

 l.rigei s comparison of the racial and individual 

 development of the Ammonites, bv Hyatt's monu- 

 mental monograph on tin- evolution of fossil 

 eej.halopods, or even by the single chapter in which 

 Hilgendorf worke<l oui the variations of Planorbis 

 in the t'p|ier Miocene beds of Steinheim. In other 

 directions, too. the inquiries of Dull and others into 



the deep sen molluscs, tl \iieriiiients by which 



Semper was able to impress changes 11)11111 various 

 forms by altering their surroundings. SdlasV re- 

 -eareh on the ongin of fresh water fauna, Lang's 

 study of the influence of sedentary life, Itateson's 

 account of the variations of the cockle, and many 

 other evolutionist investigations have yielded result* 

 not less valuable than those of the anatomists. 



