CONCHIFEKS. 717 



As auditory organ, SIEEOLD regards a part met with by him in 

 Cyclas, Anodonta, Unio, My a, Cardium, Tellina, &c., an organ 

 situated in front of and near to the nerve-mass of the foot. Here 

 namely on both sides he found a vesicle in which a flat, round, 

 transparent nucleus is in free motion. The nucleus is a concrement 

 comparable to the lapilli in the auditory sac of the bony fishes 1 . 

 Neither of this enigmatical organ nor of eyes has any vestige been 

 met with in the Brachiopoda. 



The motions of these animals are very simple. Many Lamelli- 

 branchiata, and all the Brachiopods, are fixed to their places, and 

 are not able to move from them. Other conchifers have a springing 

 motion by means of the foot, a name given to a production of the 

 abdomen which is muscular and very firm, possesses great con- 

 tractility and irritability, and may assume very different forms 2 . 

 By means of it bivalves move at the bottom of the water in which 

 they live. 



The geographic distribution of conchifers deserves a more 

 special investigation than has hitherto been bestowed upon it. 

 All conchifers live in water, part of them in Afresh water, but 

 the greatest part in the sea. Amongst the genera that live in 

 the sea are some of which species are met with in all parts of 

 the world, as genera Solen, Mi/a, Anatina, Mactra, Tellina, Lu~ 

 cina, Donax, Venus, Cardium, Area, Pectunculus, Mytilus, Pecten, 

 Ostrea. It is however far from the fact that all these genera 

 are found in like manner in different seas ; of the genera Venus, 

 Cardium, Area, Ostrea, the species are much more numerous in 

 the Indian Sea and the South Pacific, than in seas of the northern 

 hemisphere. Glyciimris appears to be a northern form, of which 



(ib. p. 153); they are particularly large and conspicuous in the species last named 

 (Tab. 27, figs. 5, 14); POLI was not able to investigate their internal structure; and 

 the later writers on the molluscs neglected this peculiarity altogether. Only within 

 the last few years has it been adequately illustrated by GRUBE (MUELLER'S Archiv , 

 1840, s. -24, Taf. in. figs, i, 3), KROHN (ib. s. 381 386, Taf. xi. fig. 16) and WILL 

 (FRORIEP'S Neue Notizen, xxix. Bd. January, 1844, No. 622, 623). 



1 C. TH. VON SIEBOLD Ueber ein rathselhaftiges Organ einiger Bivalvcn, MUELLER'S 

 Archiv, 1838, s. 4954 (transferred to Ann. des Sc. nat., sec. Sdrie, x. Zool. p. 312), 

 and WIEGMANN'S Archiv, 1841, Ueb. d. Gehororgan der Mollusken, s. 148 and foil., 

 Ann. d. Sc. nat., sec. S^rie, xix. p. 193, PI. 2 B, fig. i. [Also LEYDIG Ueber Cyclas 

 cornea, MUELLER'S Archiv, 1855, pp. 51, 52. PI. vi. fig. 18.] 



2 See POLI, I. Introd. p. 37. 



