TEREDO. 145 



nally by a very thin^ pellucid, and film-like membrane 

 or sheath. These tube-sheaths are irregularly annular, 

 like the testaceous sheath or case which lines the exca- 

 vation in the wood ; and they bear a considerable re- 

 semblance in shape to the stem of Tubularia indivisa, 

 though differing from it in texture and colour. The 

 sheath of the alimentary tube is about an inch long, 

 and the other is half that length. Their annular struc- 

 ture evidently arises from successive accretions of growth. 

 The use of these membranous sheaths may be either to 

 prevent the delicate tubes, which they cover for about 

 half their length, being choked or obstructed by the 

 accumulation of the flocculent pulp lying outside, or 

 else to protect them from the attacks of minute preda- 

 cious animals. They are renewed from time to time ; 

 and in one of the specimens two separate pairs of these 

 membranous sheaths were attached to the outer opening 

 of the testaceous sheath in the wood, one pair having 

 been apparently disused and a new set formed. The 

 Teredines grow and multiply with astonishing ra- 

 pidity. Quatrefages has given us an instance. A ferry- 

 boat plying between two villages on the opposite sides 

 of the mouth of Guibuscoa harbour in the Bay of Pas- 

 sages, on the north coast of Spain, was accidentally 

 sunk in the beginning of spring. Four months after- 

 wards some fishermen raised the boat, hoping to make 

 use of the materials. But in this short space of time 

 the Teredo [T. pedicellata) had made such ravages, that 

 the planks and beams were quite worm-eaten and de- 

 stroyed. Sailors have given me some interesting ac- 

 counts of hair-breadth escapes which they had, while 

 engaged in boat duty for a few weeks at a time on 

 foreign stations, in consequence of the paint having been 

 rubbed off the sides of the boat below the water-line : 



VOL. III. H 



