MOLLUSCS 



411 



moving back to the proper position. Such slipping, however, is 

 impossible, for there is a tongue-shaped projection from the back 

 of the right valve which fits into the opening of the burrow and 

 serves as a holdfast. 



The notorious Ship- Worm (Teredo navalis, fig. 933), which 

 in the days of entirely wooden vessels played such havoc with 

 their timbers, belongs to a family which is closely related to 

 that including the 

 Piddocks and their 

 allies. The body is 

 not rounded like that 

 of the last -men- 

 tioned bivalve, but 

 resembles a long 

 slender cylinder. 

 The shell is of rela- 

 tively very small size 

 and placed at the 

 front end of the 

 body, while the two 

 siphons project at 

 the other. The 

 burrow has a smooth 

 calcareous lining, 

 formed by the hard- 

 ening of a fluid 



which is exuded Fig. 934. Date-Shells (LWuntonttts) 



from the skin. If 



in the course of its boring operations this mollusc reaches the 



outside of the wood it closes the opening with a shelly plate. 



The Date- Shells (Lithodomus, fig. 934) are relatives of the 

 Edible Mussel that burrow only in limestone rocks. In this 

 case the work is supposed to be chiefly done by an acid fluid, 

 which acts as a solvent. The shell is covered by a tough horny 

 layer, and is thus protected from the corrosive action to which 

 its calcareous substance would otherwise be exposed. 



TUSK- SHELLS (SCAPHOPODA). These are burrowing forms 

 with a curved conical shell open at both ends. As to their 

 life-history, it need only be said that the eggs are discharged 

 freely into the water, where they hatch out into trochospheres, 



