MOLLUSC si; THE MOLLUSCS 255 



tufts arranged in rows along the back. The body is often 

 strikingly and variedly colored. These soft, naked " sea- 

 slugs ' ' live near the shore, creeping about among the 

 rocks and seaweeds. About a thousand species of nudi- 

 branchs are known. 



Among the shell-forming marine Gastropods there is 

 great variety in the size and shape and coloring of the 

 shells. Many are beautifully colored and patterned ; 

 others are oddly and fantastically shaped. The cowries, 

 or porcelain shells, familiar in collections of ocean curiosi- 

 ties, have a large body whorl and a very short flat spire, 

 and the brightly colored shell looks as if enamelled. 

 Some of the coast tribes of Africa once used, and perhaps 

 still use to some extent, cowries as money. The limpets 

 (fig. 104) are among the most abundant of the seashore 

 molluscs, their low, broadly conical shells being plenti- 

 fully scattered over the rocks between tide-lines. The 

 * ' oyster-drills ' ' are Gastropods with odd spiny shells which 

 do much harm in oyster-beds by settling down on the 

 oysters, boring holes through the shells and eating the 

 soft parts within. The helmet-shells, from which shell 

 cameos are cut, are composed of layers of shell material 

 of different colors. Among the specially beautiful shells 

 are the cone-shells, the olive-shells, the ivory-shells, etc. 



Squids, cuttlefishes, and octopi (Cephalopoda). 



TECHNICAL NOTE. Small squids preserved in alcohol or formalin 

 can be had of all dealers in biological supplies (see p. 453), and 

 specimens should be examined. 



The squids (fig. no), cuttlefishes, octopi or "devil- 

 fishes," and the three living species of Nautilus constitut- 

 ing the class Cephalopoda are very different from the other 

 molluscs in appearance, and are in fact different in im- 

 portant structural characters. They can move swiftly, 

 have strangely modified organs of prehension, strong biting 

 mouth-parts, and eyes of very complex organization. 



