JANTHINA.— 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 ^^5^- 54.-iANTHiNA, 

 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 !he ocean. When 

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

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

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

 cone, projecting from one side. 



45, The Nerit^ are distinjjuished by 

 the columella being a straight line, which 

 makes the aperture of the shell either 

 semi-circular or semi-ellipticaf. They 

 are divided into Natica, in which the shell 

 is umbilicafe {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 o])erculum 

 stony, and into Neritina, in which the 

 shell is also without an umbilicus, very 



Fig. 55. 



NERITA PELORONTAo 



Bleeding Tooth. 



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



45. Row is the genus Neri'ta distinguished ? 



