194 



THE OPAL SEA 



Cephalo- 

 pods. 



The pearly 

 nautilus. 



thystine-purple ; and have lines that gracefully 

 mimic sea waves in their rise and fall. 



The cephalopods with their coiled shell have 

 many extinct species, and many livinp; that are 

 as remarkable for their ability to change their 

 color at will as for the color itself ; but only one 

 of the living group has interest for those who 

 are not scientists. This is the pearly nautilus, 

 the sole modern representative of the Tetra- 

 branchiates. It has become familiar in almost 

 every household through the poem of Dr. 

 Holmes, for it 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." 



Pliny, centuries ago, wrote of it as stretching 

 out a " membrane of marvelous thinness which 

 acts as a sail spread out to the wind." With 

 this sail it "makes its way along the deep, 

 mimicking the appearance of a light Liburnian 

 bark, while if anything chances to cause it 

 alarm, in an instant it sinks to the bottom." 

 Whether it sails the sea or not is still a mooted 

 question ; but there is no doubt about its spiral 

 shell with its different compartments in which 

 the animal has successively lived, its " irised 



