284 



CIVIC BIOLOGY 



shells. If kept in a balanced aquarium. Physa will serve to demonstrate 

 most of the interesting reactions locomotion, spinning mucous threads, 

 feeding, breathing, egg-laying of this group of mollusks. The eggs 

 will be laid in transparent masses of jelly on the glass, and will thus 

 afford opportunity to observe the embryological development of a 



gastropod. 



Tyrian purple, the dye, was obtained 

 from marine gastropods, whicli have 

 been known as purpuras since remote 

 antiquity. 



Cephalopods. No more interesting- 

 specimens for the marine aquarium can 

 be had than the young of our common 

 squids, with their flashing changes of 

 color, their hiding, ink-cloud maneuvers 

 (equaled only by the most astute politi- 

 cians), and their lightning-like efficiency in catching fish nearly as 

 large as themselves. It is almost impossible to believe that these keen, 

 active, intelligent creatures are really mollusks. 



The cephalopods furnish bait for our cod fisheries, sepia for artists, 

 and cuttle bone for canaries, and are used extensively for food along the 

 Mediterranean and among oriental peoples. Some of the deep-sea forms 

 reach enormous size ; we hear thrilling stories of their encounters with 

 whales, and they probably furnish whatever basis there may be for 

 sailors' yarns of sea serpents. 



FIG. 132. Atlantic squid 



