GASTROrODS. 



39 1 



starfish with Thyca ; a, Shell of latter enlarged. 



like most other gastropods. In the family of the naticas (Naticida ), which is 



numerous in both recent and fossil species, the shells are more or less globular 



or ear-shaped, and the 



animals remarkable for 



the enormous develop- 

 ment of the foot, which 



is used as a plough to 



drive through the sand 



in search of other mol- 

 luscs — mostly bivalves 



— upon which they 



feed. The foot is so 



large as almost to 



conceal the shell, and 



the front part of it 



(the propodiuni) hides 



the head, which has no 



visible eyes. The 



shells are quite close 1 



in some of the groups 



by a paucispiral operculum, which is either horny or shelly. Naticas are found in 



all parts of the globe. The largest living species (Lunatia keros) occurs on the 



eastern coast of North America, but some 

 of the more highly -coloured forms are 

 found in tropical seas. About half a 

 dozen species occur in Britain. The 

 spawn-cases are curious objects, resem- 

 bling a broad sandy strap coiled round 

 like a horseshoe. 



SECTION PTENOGLOSSA. 



In this unimportant group of Pectini- 

 branehs — which includes only the families 

 Ta nth I a idd' and Scale ft idit — the radula 

 consists of numerous rows of pointed 

 teeth, arranged in cross series forming an 

 angle in the middle. There is no central 

 or rhachidian tooth. The violet sea-snails 

 (Ianthinidce) have thin trochiform shells, 

 adapted for a pelagic life. These are 

 mostly of a violet colour, the base or 

 underside, which is turned upwards and 

 exposed to the light when the animal is 

 swimming at the surface of the sea, being 

 more deeply tinted than the rest of the 



6. 





VIOLET SEA-SNAIL (Id llfh hid) WITH ITS FLOAT. 



b, A bubble, drawn, somewhat too large, aboul to In 



joined to the anterior end of the float ; c, Shell 

 I, Float ; p, Foot ; t, Head. 



