422 MOLLUSCA phylum vi 



sphere and a Veliger larval stage ; by possessing bilaterally symmetrical, 

 unsegmented bodies ; a larval shell gland, from which a Härder exoskeleton 

 or shell is secreted, though not always permanently retained ; a mouth, 

 intestinal canal and anus ; a closed, but partly lacunary circulation, assisted 

 by a heart with one or more auricles, and containing a usually colourless 

 body fluid or haemolymph ; a nervous System with at least three pairs of 

 ganglia connected by commissures ; sexual reproduction by ova and sper- 

 matozoa ; audition and equilibration provided for by otocysts ; respiration 

 by ctenidial or secondary gills, or by the tegumentary surface, which may be 

 invaginated to form a pulmonary sac ; locomotion by a muscular organ called 

 the foot, or by special parapodial structures, or by swimming; the organs 

 typically paired, and protected by a sac-like integument called the mantle ; 

 and the visceral sac having a tendency toward torsion, so as to become usually 

 asymmetrical. Sexually Mollusks are usually dioecious ; or, if monoecious, 

 are incapable of self-fertilisation. 



Owing to the homogeneity of the group, its division into classes has been 

 attended with some differences of opinion, depending upon the point of view, 

 the anatomist laying more stress upon certain groups of characters, and the 

 morphologist upon others. From a general Standpoint, the Mollusca are 

 readily divisible into five classes, as follows : Pelecypods, Scaphopods, 

 Amphineura, Gastropods and Cephalopods. The first of these is well marked 

 ofF from the rest by the presence of a bivalved shell and the absence of a 

 distinct head and of a radula, and the two groups have been contrasted as 

 Aglossa (or Lipocephala) and Glossophora (or Cephalophora). 



' Olassl. PELECYPODA Goldfuss.^ 



(Lamellibranchiata Blainville ; Conchifera Lamarck ; Bimlvia (Bonanni) Linn6 ; 



Lipocephala Lankester.) 



Aquatic, bilaterally symmetrical, acephalous Mollusks, protected hy a pair of 

 shelly valves, which are secreted by the lateral portions of the mantle, connected by an 



1 Literature : Neumayr, M., Zur Morphologie des Bivalvenschlosses. Sitzungsber. Akad. 

 Wiss. Wien, 1883, vol. Ixxxiii,, and Denkschr. 1891, vol. Iviii,— üeber die Herkunft der Unioniden. 

 Sitzungsber. Akad. Wiss. 1889, vol. xcviii. — Beiträge zu einer morphologischen Eintheihmg der 

 Bivalven ; mit Vorwort von E. Suess. Denkschr. Akad. Wiss. Wien, 1891, vol. Iviii. — White, 

 CA., Review of the fossil Ostreidae of North America, 4th Ann. Report U.S. Geol. Survey 

 (1883), 1S8A.— Jackson, R. T., Phylogeny of the Pelecypoda. The Aviculidae and their Allies. 

 Mem. Boston Soc. Nat. Hist., 1890, vol. iv. no. 8. — Minegaux, A., Recherches sur la circulation 

 des Lamellibranches marines, 1890. — Hyatt, A., Remarks on the Pinnidae, Proc. Boston Soc. 

 Nat. Hist., 1892, vol. xxv. — Moynier de Vülepoix, R., Recherches sur la formation et l'accroisse- 

 ment de la coquille des mollusques, 1893.— ^erwar^, F., Serie de notes sur le developpement et la 

 'morphologie de la coquille chez les Lamellibranches. Bull. Soc. Geol. France [3], 1895-97, vols. 

 xxiii. -XXV.— Dali, W. ff., Anew Classification of the Pelecypoda. Trans. Wagner Inst. Sei. Phila- 

 delphia, 1895, vol. iii. pt. 3. Also Proc. U.S. Nat. Mus., 1895, vol. xvii. no. 1Q^2.— Hyatt, A. 

 Terminology proposed for the Description of the Shell in Pelecypoda. Proc. Am. Assoc. Adv. 

 Sei., 1895, vol. xliv.— Fischer, ff., Resume des travaux de M. F. .Bernard sur le developpement 

 de la coquille des Pelecypodes. Journ. de Conch., 1897, vol. xlv., no. A.—March, M. C, General 

 Classification of the Pelecypoda. Ann. Mag. Nat. Hist., [8] 1912, vol. x. 



A. On Paleozoic Forms : Barrande, J., Systeme Silurien du centre de la Boheme. Acephales, 

 i.-iv., 1882.— ßee«ie, /. W., Carboniferous Invertebrates. Kansas Univ. Geol. Surv., 1900, vol. vi. 

 —Clarke J. M., Naples Fauna in western New York. Mem. N.Y. State Mus., vi., 1900.— //aZ^, J., 

 Geol. Surv. New York. Palaeont., vol. v., 1884-85.— /rZew, Whitfield, R. P., Meek, F. B., and 

 Ulrich, E. 0., Description of Palaeozoic Fossils. Palaeont. Ohio, 1873-75, vols. i., ii., and Geol. 

 Ohio, 1893, vol. \\\.~ffind, W., Monograph of the British Carboniferous Lamellibranchiata. 



