202 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The Shell. (Fig. 6.) 



Redi describes the shell of Aplysia as " most delicate and 

 quite smooth, so that it seems to be polished and looks like 

 transparent talc." The shell in Aplysia is in process of 

 disappearing. It is completely hidden from view by the 

 upward growth of the mantle, and in allied forms such as 

 Phyllaplysia has disappeared altogether. It has lost the 

 three-layered condition of the shell of most Molluscs, what is 

 called the shell being only its outer horny layer. Beneath 

 this, but separated from it and often broken in pieces, lies all 

 that remains of the calcareous layers.* The shell has become 

 flattened like the visceral hump which it covers, and has lost 

 all trace of the spiral coiling which occurs in the larval stage. 

 The columellar muscle has disappeared though remnants of it 

 still remain, and the shell is held in place by the upgrowing 

 mantle folds. 



In shape the shell is depressed, with a rounded growing 

 margin situated anteriorly and a pointed umbo with recurved 

 edge. The umbo and recurved edge form the rostrum (Fig. 6, 

 ros.) of some authors. The umbo is posterior, and is directed 

 downwards. The shallow arch of the shell is greatest near 

 the umbo. The shell is slightly asymmetrical owing to the 

 presence of a concavity (an. in.) on the right border into 

 which the anal spout fits. This concavity is much deeper 

 in Dolabella. Rings of growth are visible on the shell. 



[Lift up and remove the shell.] 



Beneath the shell lies the thin transparent mantle, through 

 which can be seen the organs of the visceral hump. The latter 

 are moulded to the shape of the shell, a recurved projection 

 of hepatic tissue filling the concavity of the shell near the umbo. 



* The calcareous matter readily dissolves in acid, and will therefore 

 not be found in a specimen preserved in an acid fixative. 



