MOLLUSCS 



417 



and in course of time excavates an oval " scar " by means of 

 the edge of the shell and the action of the foot. As these scars 

 are formed in hard siliceous rocks as well as in calcareous ones, 

 corrosion by an acid secretion plays no part in the matter, as 

 has been suggested by some observers. The conical shape of 

 the shell is calculated to resist the wash of the tide and waves, 

 to which it offers but little purchase. It must not be supposed 

 that all Gastropods with a limpet-shaped shell are of necessity 



Fig. 942. — Limpet [Patella valgata) leaving its Scar at Low Tide 



closely related. The same arrangements would appear to have 

 been independently evolved in several different groups, as the 

 result of adaptation to similar conditions. 



There is one brackish -water snail {Potaniides) that lives in 

 estuaries, and is reminiscent of those bivalves which attach them- 

 selves by means of a byssus (see p. 407). For it spins a number 

 of strong threads, which serve to hold it firmly to the roots of 

 mangrove-trees. 



Head-Footed Molluscs (Cephalopoda). — The eggs are 

 here comparatively large and contain a good deal of food- 

 yolk. Those of Squids {Loligo, fig. 943) are enclosed in gela- 

 tinous tubes, of which a considerable number are associated, 

 and radiate from a stone or other firm body to which they are 



