336 



ANIMAL DEFENCES 



the genus Murex (fig. 495), also Scorpion -Shells [Pferoceras), 

 while Thorny Oysters {Spondylus) and the Spiny Cytherea 

 (Cytherea dione) are good illustrations of spiny bivalves. It is 

 particularly interesting to notice in the last case that the spines are 

 massed at the posterior end of the shell, the part most exposed 

 to attack, for, although the animal may bury itself in the loose 

 covering of the sea-flioor, the siphons projecting from its hinder- 

 end must be placed so that currents of water may enter and 

 leave (see p. 249) by the apertures there present, respectively 



Fig. 495. — A spiny Sea-Snail \I\ftircx) 



Fig. 496. — Opercula of various Gastropods. 



a. River Snail [Paliidhia] ; b. Whelk {B:tccinunt) ; r, ]}htrex; 

 d, Cyclostovia; e, TrocJiHS;/^ Neriia. 



carrying food and oxygen mwards, and waste products of all sorts 

 outwards. 



In many univalve Molluscs (Gastropoda) the shell is so roomy 

 that the animal can withdraw entirely into it. Such a creature 

 often possesses a special plate (operculum) by means of which it 

 can, as it were, close the door behind it (fig. 496). This plate 

 may be simply horny, as in a Whelk or Periwinkle, though not 

 infrequently it is composed of dense shelly material, as, for in- 

 stance, in Nerita and Neritina. 



Some of the 'tween-tide Gastropods which live on rocks, such 

 as the Limpet {Patella) (p. 197), are descended from ancestral 

 forms which possessed a large spiral shell into which the body 

 could be withdrawn. The conical structure by which this has 

 been superseded is, however, a very efficient defence, for when 

 alarmed a Limpet holds on with great force by means of its 

 powerful foot, and pulls down the shell so that its edges touch 

 the surrounding rock. To dislodge the animal from its hold 



