ON MOLLUSCA. 187 
Perhaps we should consider in the same way the ligament of 
the arca and some kindred genera. ‘The external ligament is 
always more thick, more gibbous, and more elastic. It 
always occupies the back of the shell, just behind the sum- 
mits. Finally, the internal ligament, simple or multiple, is 
that which is more inside than the line of articulation. Its 
fibres are usually short and straight. ‘The mactra, crassatella, 
pectines, &c, present us with examples of this kind of liga- 
ment. 
One of the most remarkable peculiarities in the acepha- 
lous mollusca is, that in many species, a greater or less 
number of the fibres of the adductor muscles may be attached 
and agglutinated to foreign bodies, so as to serve for an ex- 
ternal point of attachment for the animal. It is this which 
constitutes the byssus in the pinna, and the mytilacea, and the 
tendinous foot of the tridacne, and. certain species of arca. 
This byssus is not really formed, as some authors have as- 
serted, of a mucosity secreted by a gland, and spun (like the 
silken productions of insects) in a groove in the foot. It is 
nothing but an assemblage of muscular fibres, dried up in 
a portion of their extent, still contractile and living at their 
origin, and which were so throughout their whole length, at 
the period in which they became attached. 
Another singularity, not less worthy of remark than the 
preceding, is the observation of the march, or change of place 
of the adductor muscles, in proportion as the animal grows, as 
well as its shell. In fact, if in a very young shell the muscle 
is subcentral, it must necessarily happen that, to retain this 
position when the shell is twice as large, it must have moved 
as the shell grew. It is admitted, that in aspiri-valve shells 
the muscle seems to descend with the animal in like man- 
ner as in a bivalve shell the subcentral muscle advances, not 
that it detaches itself all at once, but that an anterior rank of 
fibres is detached at the same time that a posterior rank is 
13 
