420 SUPPLEMENT 
contains in its interior a soft pulpous body of the same form 
as itself. ‘This does not appear to be contractile. 
There is a considerable byssus of the same structure as that 
of the mussels. It appears to proceed from the adductor 
muscles, and not from the tube. 
The body of the animal exactly fills the two valves of the 
shell, and is so placed that one valve corresponds to the back, 
and the other to the belly. ° 
Seen from above, the body presents a posterior or visceral 
cavity, covered with a very thin, transparent membrane. In 
raising this from the back, a sort of regular figure is per- 
ceptible, anterior and surrounded with branchial lamine. 
One altogether similar is on the other side. 
The body, properly so called, is comprized between two 
cutaneous laminz forming the mantle, whose whole circum- 
ference, thicker and more evidently muscular, presents no 
trace of papille or tentacula. This membrane is very thin, 
and altogether adherent on the mass of the viscera, which it 
allows to be seen in almost the entire posterior half of the 
body. In all this half are the fasciculi of muscular fibres, 
which pass from one valve to the other, and which are five 
in number. From one of these seems to spring the byssus 
just mentioned. 
Beyond the first pair of muscles, the lobes of the mantle 
are entirely free, as far as their adherence tothe trunk. Their 
form is altogether that of the extremity of the shell. On their 
internal face is a disposition evidently branchial. 
On raising this part of the mantle from front to back, we 
find the mouth and tentacular apparatus. The mouth is 
very small, but quite visible, transverse, and at the extremity 
of a sort of point or flatted nipple. There are really four ten- 
tacula, to the first pair of which organs the name of arms has 
been given, whence the denomination brachiopoda for the 
class. 
