152 
T. Davidson — What is a Brachiopod? 
mantle ; the two central ones run direct to the front, near to which 
they bifurcate, while the outer ones give off, at intervals, on the side 
facing the lateral margin of the valves, a series of branches which 
bifurcate several times, as may be seen in the figures here reproduced 
from Hancock’s admirable work (PI. YIII. Fig. 9, PI. X. Fig. 4). 
That eminent zoologist observes that the inner lamina of the mantle, 
and more particularly that portion of it forming the floor of the great 
pallial sinuses, undoubtedly assist in purifying the blood. 
In 1854, in his review of my Monograph on British Fossil 
Brachiopoda, Oscar Schmidt called attention to an important 
anatomical omission, namely, the existence of a vast number of 
microscopic flattened, calcareous, denticulated plates or spiculse on 
certain parts of the surface of the mantle, which serve, no doubt, 
to stiffen the portions that contain tliem. It was, moreover 
remarked by Hancock, and afterwards by Deslongchamps, that 
these calcareous plates “are not spread over the entire surface of 
the mantle, but only over the large vascular sinuses, the arms, 
and the perivisceral cavity.” These spiculaa do not, however, ap- 
pear to be present in every species, being totally absent in 
Lingula, Bhynehonella, etc . ; but Deslongchamps again observes 
that, “ after an examination of the genera Kraussina, Terebratula, 
Terebratulina, Megerlia and Morrisia ( Platidia ), we find a series in 
which the number and consistency of the calcareous portions increase 
in a very rapid münner, the spicules forming several layers, leading 
the observer by a series of gradations to the genus Thecidium, in 
which the spicules are soldered together, occuj:>ying the whole of the 
mantle, and incrusting it to such an extent that the mantle is no 
longer distinguishable from the shell itself. The mantle forms that 
calcareous mass, which equals in consistency the brachial append- 
ages, a fact of which palseontologists have often, and in vain, sought 
the explanation.” 
The Brachial or Labial Appendages are a pair of singulär organs 
eminently characteristic of the Brachiopoda. They are often, more 
correctly, termed labial appendages on account of each member being 
a Prolongation of the lateral portion of the lips or margin of the 
mouth. The lamellibranchs, or conchifera, have aualogous append- 
ages, but very much less developed. They assume different shapes 
in different genera, and are supported, or otherwise, by the more or 
less complicated calcified skeleton already described. The brachial 
or labial appendages, whether they form a pair of long, beut, or 
spirally convoluted organs, occupy the larger portion of the cavity 
of the shell, in front of the visceral chamber ; they therefore are 
mainly composed of a membranous tube fringed on one side with 
long flexible cirri (PI. IX. Figs. 4, 5, 6), which were not capable of 
being protruded in those families and genera in which they were folded 
back upon themselves, and supported by a calcareous skeleton. In 
Bhynehonella, where the elongated spiral arms are only slightly sup- 
ported at tlieir origin by two short projecting calcareous processes, 
they could be unrolled at the will of the animal and protruded to 
some distance beyond the margins of the valves. When forcibly 
