MOLLUSC A 



123 



secretion of which a very highly valued purple dye used 

 to be obtained in ancient times. 



FIG. 79. The Common Limpet 

 (Patella vulgar is}. 



FIG. 78. The Common Cowry (Gypraea (Trivia] europoea). 

 a, The reflected mantle. 



Patella differs from those thus far considered, 

 navm o 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. 



PRACTICAL WORK ON WATER SNAILS 



jj ow {.Q In 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. WO od and glass, such as that the construction of which 



is fully described in Fresh Water Aquaria, by G. C. 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 



