SKELETONS, SHELTERS AND SPECIAL DEFENCES 



661 



hydrostatic pressure is maintained in the mantle fluid, which keeps the 

 boring end of the animal pressed against the head of the burrow. Teredo 

 lines the burrow with a calcareous secretion, and can close the external 



Dorsal 

 actuating' knob 



Blade-like 

 apophysis 



Ventral knob 



Fig. 15.15. Valves of Teredo norvegica 

 External view of right valve; internal view of left valve. (After Caiman (14).) 



opening with a pair of shelly pallets borne on the posterior end of the 

 body (12a). 



A tropical genus of shipworms Bankia attains large dimensions, exceed- 

 ing one metre. Two genera of pholads also bore in wood. Xylophaga 

 excavates shallow burrows in floating timber and bores in the same manner 

 as Teredo. Martesia, found in the tropics, has similar habits. 



Species of gribble Limnoria (Fig. 15.16) are found throughout the world 

 and are very destructive to marine timbers. This animal is about 4 mm 



(a) 



Fig. 15.16. Wood-boring Crustaceans 



(a) Limnoria lignorum; (b) Chelura terebrans; (c) Sphaeroma terebrans. Enlarged. 

 (Redrawn from Caiman (14).) 



long and excavates galleries some 25 mm in length immediately under the 

 surface of the wood. The galleries open to the surface at regular intervals 

 by small apertures, forming respiratory pits through which the animals 

 draw in water. A male and female are usually found together, but the 

 gallery is excavated solely by the female. Boring is accomplished by the 



