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 

 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 [Litkodomus, 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. 



Fig. 934. — Date-Shells [Lithodomus] 



