MOLLUSC A 



123 



secretion of which a very highly valued purple dye used 

 to be obtained in ancient times. 



Fio. 78. — The Common Cowry [Cypraea {Trivia) europoea). 

 a. The reflected mantle. 



Patella. 



Patella differs from those thus far considered, 

 Tl^^fmnet ^" having a simple conical shell, and also in having 

 — instead of the typical gill of the others — a ring 

 of delicate, vascular, plate-like outgrowths from the under sur- 

 face of the mantle, which can be 

 projected just beyond the foot. 

 It is in these plates that the 

 blood is aerated. The Common 

 Limpet {Patella vulgaris) lives in 

 great numbers all round our 

 coasts, clinging so closely to the 

 rocks when these are left un- 

 covered at low tide, that water 

 is shut into the shell, and so 

 the gills are kept moist till high tide again. When sub- 

 merged, the animals loosen their hold and move about, feeding 

 on seaweed, but each returns to the same rock after feeding 

 and often wears away a little basin by constantly clinging to 

 the same spot — its own little foothold of land. 



Fig. 79. — The Common Limpet 

 {Patella vulgaris). 



Practical Work on Water Snails 



How to ^^ order to keep water snails under close observa- 



start a tion, a, fresh-water tank is necessary. A rectangular tank 



Fresh-water is best, either one made entirely of glass, or one of 

 Tank. -vvrood and glass, such as that the construction of which 



is fully described in Fresh Water Aquaria, by G. 0. Bateman. 



The aquarium should stand, if possible, in a north window, 



and should be covered with a sheet of glass or perforated zinc. 



If in a sunny window, the side of the tank next the window 



