248 ZOOLOGY 



colored olive-brown or red, exactly like the seaweeds on 

 which they live ; others (Chromodoris) are extremely 



External gills 



End of foot 

 Mantle 



Drawing by R. Weber (after MacFarland) 



FIG. 65. Sea slug or nudibranch (Chromodoris porter a) from the coast of California ; 

 showing warning coloration. It is bright ultramarine blue, with the band along 

 each side of the mantle bright orange (enlarged). 



conspicuous, with purple or blue and orange colors. 

 These latter secrete substances which make them dis- 

 tasteful, and it is supposed that they possess "warning 

 coloration," enabling fishes to recognize them and let 

 them alone. Many of the gill-breathing Gastropoda, 

 especially those living in the sea, possess a circular 

 shelly or horny plate, the operculum, with which they 

 close the mouth of the shell when alarmed. When the 

 animal is in motion the operculum is seen attached to 

 the outer surface of the body, held somewhat as the 

 shield of a marching Roman soldier. 



4. The Amphintura, formerly classed with the Gas- 

 tropoda, include the Chitons, a marine group which has no 

 spiral shell and looks as much like a crustacean as like a 

 mollusk. The body is flat and usually broad, and on the 

 upper surface are eight transverse shelly plates, giving a 

 false appearance of segmentation. The creature is bi- 

 laterally symmetrical, without any of the torsion so 

 characteristic of the snails. In many cases the shell 

 valves bear minute eyes, which may number many 

 thousands in a single individual. Related to the chitons 



