JANTHlftA. NERITA. 51 



in the form of the animal. It has no oper- 

 culum, but carries on its foot a substantial 

 vesicular organ, which resembles froth bub- 

 bles, which hinders it from crawling, but en- 

 ables it to float on the surface of the water. 

 Its head is in the form of a trunk, and has a 

 forked tentacle on each side. The common Fi Z' ^.-J 

 species Janthinafragilis with a very thin violet-coloured shell, 

 is very common in the Mediterranean, and the seas of all warm 

 regions. Great numbers of them are frequently met with in per- 

 fectly calm weather floating on the surface of the ocean. When 

 the animal is touched, it ejects a deep violet-coloured liquor, which 

 dyes the water all round it. The vesicular or froth-like buoy is 

 about three times the size of the animal ; it is in the form of a 

 cone, projecting from one side. 



45. The NERIT^E are distinguished by 

 the columella being a straight line, which 

 makes the aperture of the shell either 

 semi-circular or semi-elliptical. They 

 are divided into Natica^ in which the shell 

 is umbilicate (fig. 20, page 34) and the 

 operculum horny ; into Nerita, properly 

 so called, in which the shell is not umbili- 

 cate, thick ( fig. 55), and the operculum Fi %' 55 ' 

 stony, and into Neritina, in which the N **' PELORONTA ' 

 shell is also without an umbilicus, very *'"* j 

 thin, and the operculum is horny. The last live in fresh waters. 



45. How is the genus Neri'ta distinguished ? 



