204 



the root of the two small tubes, which appear out of the wood: It ter- 

 minates in a small double fold, forming a cap, on the inside of which 

 are fixed the long small stems of the two opercula, which become broad 

 and flat towards their other extremity. These, when brought together, 

 shut up the shell, and enclose the two contracted tubes within it : not 

 one operculum corresponding to each tube, but in a transverse direction. 

 In the Teredo gigantea, the opercula are similarly situated, each shutting 

 up one half of the bifurcation. 



The Teredo gigantea is found imbedded in a different substance from 

 that in which the Teredo navalis is found, and may have many other cha- 

 racteristic differences; although it appears, from comparing the shells 

 in which they are incased, that they are formed of exactly the same 

 materials. 



The Teredo gigantea, when arrived at its full growth, closes up the end 

 of the shell. This, the Teredo navalis does also. In some of the speci- 

 mens of Teredo gigantea the shell is just covered in, and that part close 

 to the termination is extremely thin, but in others it is increased in thick- 

 ness twentyfold : in others, again, the shell has not only become thick, 

 but the animal has receded from its first enclosure, and has formed a 

 second three inches up the tube, and afterwards a third two inches on ; 

 and has made the sides thicker and thicker, to diminish the canal in pro- 

 portion to the diminution of its own size. 



These facts prove, that the Teredo gigantea t when arrived at its full 

 growth, or whenever prevented from increasing its length, closes up the 

 end of its shell, and lives a long time afterwards, furnished with food 

 from the sea-water it receives, like the actinia. The Teredo navalis closes 

 up its shell in the same manner : it must, therefore, after that period, be 

 supplied with food entirely through the medium of sea-water *. 



Whilst treating of serpulse we found, that in those shells a similar pro- 



* Observations on the Shell of the Sea-worm, &c. by Everard Home, Esq. Philosophical 

 Transactions, 1806. 



