238 THE MECHANICAL FUNCTIONS. 



tentacula as oars on either side, to direct, as well 

 as accelerate its motion. No sooner does the 

 breeze freshen, and the sea become ruffled, than 

 the animal hastens to take down its sail, and 

 quickly withdrawing its tentacuhi within its shell, 

 by which means it renders itself specifically heavier 

 than the water, sinks immediately into more tran- 

 quil regions beneath the surface.* The Pearly 

 Nautilus (N. Pompilius) is provided with a similar 

 sailing apparatus. 



The formation of external polythalamous shells 



* The habits of the Arg;onaut or Ocytho'e, as it is sometimes 

 called, are still very imperfectly known. Doubts have been enter- 

 tained whether the shell it occupies is formed by the animal itself, 

 or whether it is the production of some other, but unknown species 

 of Mollusca, and is merely taken possession of by the Argonaut as 

 a convenient habitation ; for it is now ascertained that the animal 

 does not adhere to it in any part. It appears, however, to be satis- 

 factorily determined by the recent inquiries of Professor Owen, that 

 the shell is really formed by the Argonaut. 



There exists an extensive series of very minute, and even 

 microscopic polythalamous shells, which were considered by M. 

 D'Orbigny as having been constructed by Mollusca belonging to 

 the family of Cephalopoda (Ann. Sc. Nat. VII., 96): but the 

 more recent inquiries of M. Dujardin have shown that the supposed 

 analogy is without foundation, and that the inhabitants of these 

 shells really belong, not to the class Mollusca, but to that of 

 Zoophyta. 



Professor Ehrenberg has lately discovered that limestone and 

 chalk contain a vast multitude of microscopic, and hitherto un- 

 known nautilites, or polythalamic shells, in size from the 3456th to 

 the 288th of an inch, so that each cubic inch of chalk contains 

 frequently a number of these shells, far exceeding a million. Many, 

 and probably all the chalk rocks of Europe are the products of 

 microscopic animalcules, some of which have formed calcareous, 

 and others siliceous coverings, which, at subsequent periods, have 

 been transformed into chalk and flint. See PoggendorfF's Annalen, 

 1839, No. 7 : and Edinburgh New Philosophical Journal, xxviii, 

 161. 



