MARINE BIOLOGICAL STATION AT POUT ERIN, 



99 



MOLLUSCA. 



(Figs. XIX. and XX.) 



The Molluscs, such, as cockles and whelks, are the 

 true " shell-fish." They have no joints or segments, and 

 no legs, and the soft body is covered by a hard limy shell 

 which is generally in only one (univalve, such as whelk) or 



Fig. XIX. 



in two (bivalve, as in oyster and cockle) pieces, the valves. 

 A few Molluscs, the sea-slugs (fig. XX.), have no 

 shell when adult ; while the Cuttlefishes have either no 

 shell, or a shell of a very special kind unlike that of 

 common univalves and bivalves. Figure XIX. shows a 



