FRESH-WATER MUSSEL. 131 



individual filaments, originally tubular, become nearly solid at the same time by the 

 development of rods of a condensed gelatinous tissue in their interior. The 

 vascular channels then run chiefly in the junctions. These changes are carried to 

 a great extent in Anodonta. The surfaces of the filaments are clothed with cilia. 

 The ciliated cells are distinguishable into three sets : frontal, with medium-sized 

 cilia ; lateral-frontal, forming a single row of large cells with very long cilia ; and 

 lateral with much the shortest cilia. The vascular channels in the gills have been 

 stated by Kollmann to posses an endothelium. This is certainly the case in many 

 forms, e. g. Area ; but Bonnet has failed to prove the fact in Anodonta. The 

 vascular channels have a very complex arrangement in Anodonta, and have been 

 fully described with figures by the last named author. The tissue of the branchiae 

 in the Unionacea (Unio, Anodonta, &c.), contains a very large amount of lime 

 carbonate. Hence the brittleness of these structures. 



From the account given above it is clear that the two gills so-called of each 

 side in the Lamellibranch are really parts of a single gill, a highly complex and 

 modified ctenidium. 



The cilia covering the surfaces of the gills cause the currents of water to flow 

 from the outer free surfaces into the interlamellar spaces and thence outwards 

 through the superior siphonal notch. The cilia at the margins of the lamellae 

 are said however to cause currents towards the mouth and thus subserve ali- 

 mentation. 



Lamellibranchiata, Bronn, Klass. und Ordn. des Thierreichs, iii. i. 1862 ; Do. 

 with Anodonta as a type, Ray Lankester, ' Mollusca,' Encyclopaedia Britannica 

 (ed. ix), xvi. 1883; cf. Haren Noman, Niederland. Archiv fur Zool. i. Suppl. 

 1881-82. 



Anodonta, Howes, Atlas of Practical Elementary Anatomy, London, 1885. 



Keber^s organ. Grobben, Arb. Zool. Inst. Wien. v. 1883, p. 40 ; cf. Griesbach, 

 A. N. 43, 1877. 



Absorption of ivater, &c. Schiemenz, Mitth. Zool. Stat., Naples, v. 1884 (with 

 lit.) ; cf. Schiller, A. M. A. xxv. 1885. Movements of foot in Lamellibranchiata, 

 Fleischmann, Z. W. Z. xlii. 1885 ; for valve of vein from foot, see p. 419, and Fig. 5, 

 p. 420. 



Byssus gland. Carriere, Arb. Zool. Zoot. Inst., Wurzburg. v. 1882. Mode of 

 attachment. Cattie, Tijdschr. Nederl. Dierk. Vereen, vi. 1882-85. 



Gills. Posner, A. M. A. xi. 1875 ; Peck, QJ.M. xvii. 1877 ; Mitsukuri, Q. J. M. 

 xxi. 1 88 1 ; circulation and endothelium of, Kollmann, Z. W. Z. xxvi. 1876 ; A. M. A. 

 xiii. 1877, and Festschrift zur Feier des 3oo-jahrigen Bestehens der Universitat 

 zu Wurzburg, 1882, p. 42 (with lit.); Bonnet, M. J. iii. 1877. Rods of gills. Bonnet, 

 op. cit. p. 321 ; cf. Kollmann, SB. Bayer. Akad. 1876, p. 163. Development of gills, 

 De Lacaze-Duthiers, A. Sc. N. (4) v. 1856. 



Ciliary-currents, Sharpey, Encyclopaedia of Anat. and Physiol. i. 1835-36, 

 p. 621. 



K 2 



