18 



LECTURES ON MOLLUSCA. 



Ammonites did the work of destruction in ancient times, which is now 

 performed by murices, strombs, whelks, and their allies. 



The chambered shell is always pearly within, but with an external 

 p^rcellanous layer. The Chinese are fond of leaving patterns carved 

 on the Nautilus while the body of the shell is uncoated, to show the 

 nacre. In fossils sometimes the outer coat has perished, sometimes the 

 inner, and sometimes both. The chambers are always connected by a 

 siphuncle, through which the animal maintains a connection with the 

 deserted chambers. These are lined with a very thin living membrane 

 in the Nautilus; in the Orthoceratites they show the marks of blood- 

 vessels, &c., which prove that they played some unknown part in the 

 economy of the animals. That these air-chambers serve as a float, to 

 balance the weight of the shell and enable the creature to swim if 

 needful, cannot be doubted ; but the stories of their filling the cells 

 with air or water at pleasure, and so sailing at the top or descending 

 to the bottom, appear to be fables, like the classical legends of the 

 Argonaut. The living Nautilus only comes to the surface occasionally, 

 when the sea bottom has been agitated by storms ; and it is believed 

 that the fossil species inhabited depths not greater than thirty fathoms. 

 The chambers are filled with nitrogen gas, without oxygen or carbonic 

 acid. The animal is attached to the shell by powerful adductor mus- 

 cles. As these grow onwards, the animal gradually deserts the last 

 chamber; and, at periodic periods of rest, a fresh septum is formed.* 



If a diving bell had explored what is now called New York and 



*The following lines have the rare merit of not losing truth at the same time that they are 

 highly poetical. They are copied from the "Atlantic Monthly." Let the reader take in his 

 hand a pearly Nautilus cut through the middle, and say 



This is the Ship of Pearl, which, poets feign, 



Sails the unshadowed main; 



The venturous bark, that flings 

 On the sweet summer wind its purpled wings, 

 In gulfs enchanted, where the siren sings, 



And coral reefs lie bare; 

 Where cold sea-maids rise, to sun their streaming hair. 



Its web of living gauze no more unfurl ; 



Wrecked is the ship of pearl! 



And every chambered cell, 



Where its dim, dreaming life was wont to dwell, 

 As the frail tenant shaped his growing shell, 



Before thee lies revealed; 

 Its irised ceiling rent, its sunless crypt unsealed! 



Year after year behold the silent toil 



That spread his lustrous coil; 



Still, as the spiral grew, 

 He left his past year's dwelling for the new; 

 Stole, with soft step, its shining archway through; 



Built up its idle door, 

 Stretched in his last-found home, and knew the old no more. 



