310 BRACHIOPODA. 
are unfolded is simple and beautiful. ‘The stem of 
the process is hollow, and partially filled with a 
fluid, which being forcibly injected towards the 
extremity, by the contraction of a double series of 
muscles behind, the whole of the lengthened organ 
is straightened and projected. 
Most of the species have a shelly frame-work 
within one of the valves, consisting of slender 
loops and arches, variously arranged, and more or 
less complex. This is intended to support the 
fringed arms, and to keep the valves open, or even 
to assist in opening them; for there is in this 
class nothing corresponding to the hinge-cartilage, 
which performs the latter function in the Con- 
chifera. 
Respiration in these animals seems to be per- 
formed by the mantle itself; the long fringed arms 
having apparently nothing to do with this office, 
notwithstanding their gill-like structure. 
Some of the species are found in the shallows of 
sandy shores; but others inhabit the darkness and 
solitude of the deep sea; some of the Terebratule 
dwelling in water from sixty to ninety fathoms 
deep; while Crania personata has been dredged 
up from a depth of 255 fathoms. The respiration 
and nutrition of animals that can subsist beneath a 
pressure so enormous, are subjects, as Professor 
Owen remarks, “ suggestive of interesting reflec- 
tions, and lead one to contemplate with less sur- 
prise the great strength and complexity of some of 
the minutest parts of the frame of these diminutive 
creatures. In the unbroken stillness which must 
pervade those abysses, their existence must depend 
upon their power of exciting a perpetual current 
around them, in order to dissipate the water already 
