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 8. 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 Helér) 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?. 
What has been stated may suffice for the explanation of the 
most common and necessary terms?. 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 vit. 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—1I. 
2 Comp. A. Murray Fundamenta Testaceologie. Upsalie, 1771, 4to, (also trans- 
ferred to Linn. Amenit. Acad. VIII. pp. 107—150); BLAINVILLE in Dict. des Sc. nat. 
X. pp. 168—225, article Cénchologie (and the same writer in his Manual cited above) ; 
DESHAYES, article Coquille, Dictionn. classique d’ Hist. nat. Tome Iv. 1823, pp. 431— 
449, Ke. 
