150 TEREDINlDiE. 



that it abandons by slow degrees the narrower end of 

 the canal. The pallets of course increase in size rela- 

 tively to the growth of the body ; and as the sheath 

 enlarges inwards_, new plates are formed in that direc- 

 tion to accommodate the increased size of the pallets. 

 Although the body is contractile to a certain extent (as 

 we see in dead specimens), it is fixed to the sheath by 

 the muscular ring which contains the pallets, and there- 

 fore cannot be withdrawn into the canal beyond that 

 line ; the other extremity is employed in excavation, 

 until the canal has been completed. When a Teredo 

 has ceased to excavate before attaining its full growth, 

 and has interposed a barrier in front, its valves become 

 stunted and somewhat altered in shape, although their 

 sculpture is similar to that of ordinary specimens. The 

 same fact is observable in many other bivalves that in- 

 habit cavities or confined spaces, whether they are of a 

 boring or of a free nature. The cap-shaped plug, often 

 formed in front of the valves by individuals of every 

 age, serves as a partition wall between adjoining canals, 

 as well as indicates that the animal has ceased working; it 

 is formed like the sheath, but its substance is thinner. 

 Sometimes two or more of these plugs may be seen, one 

 after another, at various distances apart, as if the animal 

 had withdrawn and thus strengthened its inner line of 

 fortifications. Fischer was disposed to regard this secre- 

 tion as analogous to the epiphragm of land shells. That, 

 however, is only constructed for a temporary or occa- 

 sional purpose, and can be dissolved by the snail at 

 pleasure. It does not appear that the Teredo can do 

 this and resume its work of perforation. Laurent be- 

 lieved that the plugs or caps of the Teredo are made for 

 hybernation, an idea that is open to the same objection 

 as that of Fischer. The tubes or siphons, when in 



