MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 155 
thick and strong muscles which form the anterior and posterior pairs above noticed, 
do not decussate each other, but pass a little obliquely from one valve to the other. 
On the lower valve they are attached to the margin of the elevation caused by the 
oval depression noticed on the exterior of the shell. Some of the fibres of the large 
anterior pair pass through the chink in the perforated valve, and expand into the organ 
of adhesion. Within the space included by the above pairs of muscles, there are two 
slender pairs of muscles which decussate each other. The superior pair! take their 
origin from the anterior part of the strong membrane that circumscribes and protects 
the viscera below the stomach, and between the insertions of the anterior shell-muscles ; 
they then ascend, diverge on either side the alimentary canal, and are inserted into 
the opposite valve outside the posterior shell-muscles. The inferior pair? arise from 
the sides of the membranous circle, and converge, as they pass below the preceding, to 
be inserted into the perforated valve on the inner side of the posterior shell-muscles. 
While, therefore, the larger muscles have the more important office of guarding the 
animal by closing the shell, the smaller muscles would admit the water by sliding the 
margin of one valve over the other; and they are also calculated to produce a com- 
pression of the viscera. 
The labial processes, or brachia, are scarcely more adapted to protrude externally than 
in Terebratula Chilensis, the only parts that are free being the short spiral extremities ; 
but in the more muscular character of their basis or stem, they exhibit a closer affinity 
with Lingula. Considering the arms as a pair, the stems are then joined below the 
mouth, forming on that aspect a transverse semilunar fleshy basis, fringed and convex 
anteriorly. This is attached to the anterior part of the tendinous belt of the viscera. 
At the sides of this basis the arms make a sudden bend upon themselves towards the 
mouth, above and in front of which the extremities make a spiral turn and a half’. 
The bent portions are closely adherent to each other, not free as in Lingula. These 
parts of the arms, by contracting from the angle of flexion towards the mouth, would 
necessarily become thicker, and so press upon and open the shell a little way, in a 
manner analogous to what I have supposed to take place in the calcareous loop of Ter. 
Chilensis ; the arms in Orbicula are not, however, supported by an internal calcareous 
process. The muscular basis, when cut into, exhibits on each side a well-defined cylin- 
drical cavity®, which commences near the mesial plane in the transverse part below the 
mouth, and is continued into the spiral extremity. I injected these canals, but could 
not in that way perceive that they had any connexion with the vascular system: no 
part of the fluid entered the filaments composing the fringe. The parts being hardened 
by long maceration in spirits, prevented the unfolding of the arms by any force I could 
use ; but I conclude, nevertheless, that the canals serve to extend outwards the free 
spiral extremities, by being forcibly distended with fluid propelled along them,—a 
th. Figg. 7. 8. 24, Fig. 7. 8. 3K. Fig. 8. + 1. Figg. 9. 10. 5 m, Figg. 9. 12. 
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