JANTHINA. NERITA. 



- 54. -JANTHINA. 



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 



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 ia 



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



cone, projecting from one side. 



45. The NERITA 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 (Jig. 55), and the operculum 

 stony, and into Neritina, in which the 

 shell is also without an umbilicus, very 



Fig. 55. 



NERITA PELORONTA. 



Bleeding Tooth. 



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



45. 



\s *h* % enus Neri'ta distinguished ? 



