TEREDO. 223 



tudinal increase is not correspondent with the boring progress, 

 must, by being posteriorly fixed, either suspend excavation, 

 rupture the mantle, or have the power of advancing the muscle 

 of attachment. This advancement of the muscle is not a new 

 fact ; it has been observed in the Spondyli and Ostrete ; and 

 it cannot be doubted that nature has conferred on the present 

 species the power of detaching and advancing the muscle of 

 attachment, and that each hoop-shaped lamina, thrown out 

 for some point of the animal oeconomy, marks the periodic 

 removal of the muscle. The laminae are always more nume- 

 rous in the longer and older animals ; in very young specimens 

 there are only 1-3, and in the older ones 20-40. 



When authors have stated that this species has the tube 

 without concamerations, we presume they have only had 

 opportunities of examining very young or imperfect speci- 

 mens ; in all the specimens I have seen, many of which were 

 10 inches long, they were present, and I belive that no species 

 of Teredo is without them. 



The plugging up of the terminal volutions in Aporrha'is and 

 other Gasteropoda, and the consequent withdrawal of the 

 posterior parts of the animal, are analogous to this operation 

 in Teredo ; the same principle excites the action in both cases, 

 self-preservation . 



It will be observed that the alliance of Teredo with Pholas, 

 through the apophysary processes, is more decisive than be- 

 tween any two other bivalve families. I trust that I shall not 

 be considered fanciful if I venture to remark, that there are 

 points of analogy between Teredo and Dentalium se striking as 

 almost to give some weight to the idea that it forms the 

 passage to the Gasteropoda ; in support of these views I beg 

 malacologists to observe the similar vermiform character of 

 the animals, the attachment of their posterior parts to the 

 shells by sphincteroid muscles, the peculiar plan of the admis- 

 sion of the water by short siphons in conjunction with the 

 sphincter, the single branchial dorsal lamina on each side, 

 their separation from the body, and other minor analogies. 

 These concordances almost make me think my hypothetical 

 surmises have some foundation, and that the transfer of Pholas 



