200 
T. Davidson — What is a Brachiopod ? 
duce a large quadruple impression on tlie internal surface of the 
small valve, and a single divided one towards the centre of the large 
or ventral valve. The function of this pair of muscles is the closing 
of the valves. Gratiolet, who has likewise described with great 
minuteness the muscles of the Brachiopoda, informs us that those 
which close and open the valves were the only ones known to Pallas, 
but that he clearly defined their positions and functions. The same 
was done by Blainville and Quenstedt, but the absence of good 
figures caused much uncertainty to prevail. This deficiency was 
subsequently supplied by Hancock’s and Gratiolet’s admirable illus- 
trations. 1 Two other pairs have been termed divaricators by Hancock 
or Cardinal muscles (= muscles diducteurs of Gratiolet), and have 
for function the opening of the valves. The divaricators proper are 
stated by Hancock to arise from the ventral valve, one on each side, 
a little in advance of, and close to the adductors, and, after rapidly 
diminishing in size, become attached to the Cardinal process, a space 
or prominence between the sockets in the dorsal valve. The accessory 
divaricators are, according to the same authority, a pair of small 
muscles which have their ends attached to the ventral valve, one on 
each side of the median line, a little behind the united basis of the 
adductors and again to the extreme point of the Cardinal process. 
Two pairs of muscles, apparently connected with the peduncle and 
its limited movements, have been minutely described by Hancock as 
having one of their extremities attached to this organ. The dorsal 
adjustors are attached to the ventral surface of the peduncle, and are 
again inserted into the hinge-plate in the smaller valve. The ventral 
adjustors are considered to pass from the inner extremity of the 
peduncle, and to become attached by one pair of their extremities 
to the ventral valve, one on each side of, and a little behind, the 
expanded base of the divaricators. The function of these muscles, 
according to the same authority, is not only that of erecting the 
sliell, but also that of attaching the peduncle to the shell, and thus 
to effect the steadyiDg of it upon the peduncle. 
Gratiolet describes the peduncle with great care, and States that it 
is composed of two portions : — 1. A horny slieath formed of con- 
centric epidermic layers, very analogous to that which Prof. Vogt 
described in Lingula. 2. A fibrous stem enveloped by the sheath. 
This stem, formed of tendinous fibres, is fixed by its free extremity 
to different marine objects ; the other end passes through the foramen 
into the shell, and ends in a bulbous-shaped extremity. 
Such is the general arrangement of the shell muscles in the division 
composing the articulate Brachiopoda, making allowance for certaiu 
unimportant modifications observable in the Shells composing the 
different families and genera thereof. 
In the Tretenterata, of which Lingula may be cited as an example, 
the myology is much more complicated, and anatomists have differed 
considerably in their respective views concerning the function of 
some of the muscles. They have been described by Vogt, Hancock, 
Gratiolet, and others, and more recently by King, whose views I 
1 Journal de Conchyliologie, Octobre, 1857. 
