TEREDO. 59 



century, however, have made known to us its true orga- 

 nisation, and have proved that it is a true molhisk, and a 

 bivalve, closely allied to the PJiolas. The supposed head is 

 now known to be the main part of the body, including all 

 the viscera except a portion of the respiratory organs ; the 

 fancied tongue, or sucker, is the foot ; the reputed "jaws " 

 are the valves of the shell ; the " body" is the main part of 

 the siphonal tubes; the "tail" their bifurcated extremity; 

 whilst the tube, upon which so much stress was laid, turns 

 out to be the least important part of the whole creature, 

 and, in fact, a mere appendage. Most " chimseras dire," 

 when thus closely pressed, and forced to dwindle into their 

 true proportions, lose all their ancient terrors ; not so with 

 the Teredo. It still pursues its destructive course, unmind- 

 ful of the scalpel of the naturalist, sinking many a goodly 

 ship, and shaking many a stately pier. With the evil, 

 however, comes good, for it acts as a clearer of the seas, 

 breaking down into small fragments the useless masses of 

 floating timber and fragments of wreck, which might other- 

 wise prove serious and dangerous impediments to navigation. 

 The abnormal character of the animal of Teredo does not 

 lie merely in its vermiform shape, of which we had a paral- 

 lel instance in the Ascidian genus Pelonaia, but also in 

 peculiarities of its internal organisation. Thus, whilst in 

 other lamellibranchiate bivalves the gills inclose the intes- 

 tines and other viscera, in this they are in a great measure 

 placed apart, so that the intestinal sac comes directly into 

 contact with the mantle, whilst the respiratory organs are 

 continued posteriorily, or into the tubes. In this relative 

 position of the branchise and the mass of the viscera, we 

 see an affinity with the Tunicata indicated, borne out 

 by the minute structure of the external covering or mantle, 

 which, according to the observations of Frey and Leuck- 



