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 Ostree; 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 ceconomy, marks the periodic 
removal of the muscle. The laminz 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 Aporrhais 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 familes. I trust that I shall not 
be considered fanciful if I venture to remark, that there are 
points of analogy between Teredo and Dentalium so 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 lamima 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 
