466 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



325). As the animal grows, the extending mantle adds 

 to the length and size of the outer chamber until finally 

 the animal withdraws from contact with the last cham- 

 ber wall and secretes another partition, and thus adds 

 another to the series of empty chambers. Cephalopods 

 are classified into two groups — viz., the naked, like 

 squids and cuttlefish, and the shelled, like the nautilus; 

 the former have two gills (dibranchs), the latter four 

 gills (tetrabranchs). 



The exquisite beauty and variegated iridescent luster 

 of molluscous shells when polished is the result of the 

 superposition of extremely thin, translucent plates and 

 the interference of light thus produced. The play of 

 color becomes more splendid if the plates are corru- 

 gated, as in the abalone {Haliotus). 



ECHINODERMS. 



The shell of a sea urchin is a wonderful structure 

 when examined under a microscope. Imagine a calca- 

 reous shell composed of several layers, each layer a 

 reticulation of calcareous fibers, the several layers sepa- 

 rated from one another by calcareous pillars, but the 

 openings or mesh of the several layers not coinciding — 

 and we have a structure which is a marvel of lightness 

 and strength combined with perfect permeability. This 

 structure is easily seen under the microscope, and the 

 skin structures of echinoderms generally furnish some 

 of the most beautiful of microscopic preparations, and 

 are very characteristic of the several orders. 



CORALS. 



The deposits of coralline limestone in corals are some- 

 what peculiar, yet, as they are formed by epidermal cells, 

 like epithelial cells, and therefore are of similar origin, 



