19 



and it is difficult to see by what means it recognizes its 

 danger. 



The peculiar mode of life of Teredo has led to the 

 development of the palettes (firr. 20). These are protec- 

 tive strxictures peculiar to the "Ship-worms". They differ 

 somewhat in the various specios, but are essentiallj' cal- 

 careous paddles, attached one on either side of the poste- 

 rior end of the body. In Xylotrya the paddle part con- 

 sists of a series of funnel-shaped calcareous structures, 

 set one within the other ixpon a cylindrical handle, while 

 in Teredo it is composed of a single piece. The handle 

 of the paddle is imbedded in an invagination of the mantle 

 and the paddles project freely behind, whore by means of a 

 set of muscles, they may be protracted forcibly so as to 

 completely close the outer end of the burrow, aj^ainst the 

 intrusion of any enemy from the outside. AIgo, when the 

 burrow extends upwards and is more or less exposed at low 

 side, the palettes may so hermetically close the external 

 opening as to retain the water in the b' :';-ow, and to pre- 

 vent the collapse of the body of the "Ship-worms". 'i'-.e 

 action of the pfflettes is sliown in fi/^Ui'es 21 and .::.. V/hen 



