On the Form, Growth, and Construction of Shells. 29 



of the Nautilus, since we have no positive assurance that it 

 does ever visit the surface except when driven up by storms. 

 From its form it is most likely an indifferent swimmer ; no 

 doubt it can swim backwards, like its relatives. The shell, 

 when placed in a bucket of water, turns over and floats with 

 its mouth downwards; if, however, a half ounce weight is 

 placed in the Opening, its centre of gravity is altered, and it 

 then floats with the spire turned more to the surface, so that 

 the weight is sustained in the shell without falling out. 



It has generally been taken for granted that the Pearly 

 Nautilus could rise and sink at will, and, as most authors have 

 attributed the fabulous properties of the Argonaut to its Oriental 

 relative, we might be misunderstood if we passed it over in 

 silence. All writers before Buckland, and some at the present 

 day, have explained the hydro dynamic parts by attributing 

 a second, no less fabulous power, namely, of pumping out the 

 water from its chambers in order to rise, and admitting it 

 again when it wished to sink. Nor was the difficulty of 

 explaining this feat the principal cause of its rejection. But 

 the author of the Geological Bridgewater Treatise knew that 

 in the beautiful Nautilus zic-zac of the Tertiaries, the siphuncle 

 consisted of a succession of shelly funnels, fitting continuously, 

 into one another, and totally excluding all communication with 

 the chambers, which were closed cells, forming a permanent 

 float. The Doctor was thus compelled to limit his hydrostatic 

 theory to the siphuncle itself, a most inadequate apparatus ! 

 But the specific gravity of a body is not altered by altering 

 its form ; and, if no other means exist, the Nautilus must 

 still depend on swimming to sustain itself in the water. 



The patentees of the Nautilus machine, have hit upon a 

 real method of varying the specific gravity of a submerged 

 body, which may some day come into use (whether the Nauti- 

 lus employed it or no) . 



Teleology of Form. — The first explanation which presents 

 itself to our mind, in accounting for the variety in the form 

 and construction of shells, is the universal law that " nature 

 never repeats," and that, when things are different, the 

 difference extends to every part of their organization. 



In the structure of shells there is a general adaptation to 

 the wants of the animal to which they belong. Thus we see 

 light shells for the floaters and swimmers, strength for the 

 Limpet and Periwinkle, space in the Cone and Nerite, conceal- 

 meat in Phorus, and roughness of surface in many others, 

 which invite parasitic growth, or colours assimilated to the 

 surface of attachment which the sedentary and fixed forms 

 affect. 



In considering the Origin of variations in the form of shells, 



