158 MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 
left exposed by the retreating tide were it not buried in the sand of the shore, must 
meet with a greater variety and abundance of animal nutriment than can be found in 
those abysses in which Terebratula is destined to reside. Hence its powers of pre- 
hension are greater, and Cuvier suspects it may even enjoy a species of locomotion 
from the superior length of its peduncle. The organization of its mouth and stomach 
indicates, however, that it is confined to food of a minute description ; but its con- 
voluted intestine shows a capacity for extracting a quantity of nutriment proportioned 
to its superior activity and the extent of its soft parts. A more complex and obvious 
respiratory apparatus was therefore indispensable, and it is not surprising that the 
earlier observers failed to detect a corresponding organization in genera destined to a 
more limited sphere of action. 
The respiration, indeed, as well as the nutrition of animals living beneath a pressure 
of from sixty to ninety fathoms of sea water, are subjects of peculiar interest, and 
prepare the mind to contemplate with less surprise the wonderful complexity ex- 
hibited in the minutest parts of the frame of these diminutive creatures. In the still- 
ness pervading these abysses they can only maintain existence by exciting a perpetual 
current around them, in order to dissipate the water already loaded with their effete 
particles, and bring within the reach of their prehensile organs the animalcula adapted 
for their support. The actions of Terebratula and Orbicula, from the firm attachment 
of their shells to foreign substances, are thus confined to the movements of their 
brachial and branchial filaments, and to a slight divarication or sliding motion of 
their protecting valves; and the simplicity of their digestive apparatus, the corre- 
sponding simplicity of their branchie, and the diminished proportion of their soft to 
their hard parts, are in harmony with such limited powers. The soft parts in both 
genera are, however, remarkable for the strong and unyielding manner in which they 
are connected together: the muscular parts are in great proportion, and of singular 
complexity as compared with ordinary Bivalves; and the tendinous and aponeurotic 
parts are remarkable for the similarity of their texture and appearance to those of the 
highest classes. By means of all this strength they are enabled to perform the requi- 
site motions of the valves at the depths in which they are met with. Terebratula, which 
is more remarkable for its habitat, has an internal skeleton superadded to its outward 
defence, by means of which additional support is afforded to the shell, a stronger 
defence to the viscera, and a more fixed point of attachment to the brachial cirri. 
The spiral disposition of the arms is common to the whole of the Brachiopodous 
genera whose organization has hitherto been examined; and it is therefore probable 
that in that remarkable genus Spirifer the entire brachia were similarly disposed, and 
that the internal calcareous spiral appendages were their supports. If, indeed, the 
brachia of Ter. psittacea had been so sustained, this species would have presented in a 
fossil state an internal structure very similar to that of Spirifer. 
Tn considering the affinities of the Brachiopoda to the other orders of Mollusca, 1 
