684 ON MOLLUSCS 



The internal cavity of the shell may extend uninterruptedly 

 through all the turns (cochlea monothalamia s. unilocularis] , or may 

 be divided by several partitions (cochlea polythalamia] . A cylindri- 

 cal tube traversing these partitions is named siphon (sipho}. Such 

 a many-chambered shell is that of the genus Nautilus. 



Cover (operculum) is the name given to a round calcareous or 

 sometimes horny lamina, which in many molluscs is attached to 

 the uppermost and hindmost part of the so-named foot, and which, 

 when the animal retracts itself within the shell, closes the aperture. 

 Sometimes a spiral line is seen on this part, as for instance in 

 the genus Turbo. Some writers name the shell, in this case, testa 

 subbivalvis. 



Finally, according to the direction of the wreaths, univalve 

 shells are distinguished as right- and left-turned. In most of them, 

 when placed on the aperture with the point backwards, the external 

 margin of this is on the right side, and the turns run from the point 

 to the aperture from the left side to the right (cochlea dextra}. If 

 the converse takes place and the external margin of the aperture 

 lies on the left, then the turns run from the right to the left side 

 (cochlea sinistra s. contraria), and with this a displacement of the 

 internal organs is connected. Some species of molluscs have such a 

 left-handed shell as the rule, and in others (as in Helix] shells of 

 this sort occur as varieties or deviations, just as a displacement of 

 the internal organs, of the heart at the right-side, &c. has been also 

 observed occasionally in man 1 . 



What has been stated may suffice for the explanation of the 

 most common and necessary terms 2 . But for the general anatomy 

 and physiology it is desirable to contemplate shells in another 

 point of view, and to investigate the mode of their origin and 

 composition. 



The formation of shells is effected by the external integument 



1 On such left-turned snail-shells CHEMNITZ has published various remarks and 

 observations in the Journal Der Naturforscher VIII. s. 163 178, XII. s. 76 84. 

 That from the left-handed variety of Helix Pomatia right-handed young are again 

 produced, was observed by him with perfect certainty ; ibid. xvii. s. i n. 



2 Comp. A. MURRAY Fundamenta Testaceologioe. Upsaliae, 1771, 4to, (also trans- 

 ferred to LINN. Amoenit. Acad. vin. pp. 107 150); BLAINVILLE in Diet, des Sc. not. 

 X. pp. 168 225, article Cdncholoyie (and the same writer in his Manual cited above) ; 

 DESHATES, article Coquitte, Dictionn. classique d'Hist. nat. Tome iv. 1823, pp. 431 

 449, &c. 



