PALLIAL SINUSES OF THE BRACHTOPODA. 375 
cation just noticed is cut off, and an intestine formed, with or without an anal aper- 
ture. 
Let us now suppose the perivisceral cavity of the Bolyzoa to he homologous with the 
common cavity of the Ccelenterata, but giving off" no canals, and the oscillation or motion 
of the contained blood (?) depending upon the activity of the animal's movements as well 
as upon ciliary action, the scheme of circulation exhibits an almost retrograde simplicity 
when compared with that of a covered-eyed Medusa. Nevertheless, having no direct 
communication with the stomach, its superiority may be asserted as a system so fax 
closed. Now, as this appears to be homologous with the perivisceral cavity of the Bra- 
chiopoda, the ramifications of the pallial sinuses of the latter, with their ciliated lininu 
and contained corpuscles, present the highest form of this pseudo-circulatory system. 
The existence of a true vascular system in the Polypi is intimated by M. Milne-Ed- 
wards ; but it is not improbable that the vascular ramifications observed by him, and which 
undoubtedly exist, are in reality derived, or, as it were, dismembered from the common 
. May not the same doctrine be applied to the Brachiopoda? If not, the true 
vascular system, as described by Mr. Ilancock, is altogether distinct, both in its nature 
and origin, from what has hitherto been regarded as the rudiment of the circulatory 
system in Ccelenterata. 
In the Tunicatd, however, a single and simple heart superseding the five vesicles of 
the Brachiopoda offers some little approach to the more perfect organs of the Mollusca 
proper, though there can as yet be no distinction between arteries and veins, as the current 
of the blood is reversible in the same channels. Finally, it may be affirmed, that a heart 
with a receptive and a propulsive cavity, furnished with a valvular apparatus to deter- 
mine an irreversible path for the circulation, as well as the distinctive offices of artery and 
vein, distinguishes the true Mollusca from the Tunicate, Brachiopoda and Polyzoa, con- 
o 
the Molluscoida of M. Milne-Edwards 
EXPLANATION OF PLATE XXXV. 
F »g. 4. Portion of the pallial sinus-system of Lingula, showing the course of the ciliary currents. 
a, longitudinal septa dividing the ciliary currents. 
Fi g- 5. Violet-tinted corpuscles which circulate in the pallial sinuses of Lingula. The little pit or 
depression, observable on one side of these corpuscles, is probably due to exosmosis; but in 
this particular case it was present in all. 
intelligence, at the slightest touch, are represented as possessing the rigidity of the teeth of a comb, or, at least, of 
bristles. A very far more correct idea of them might be given were the artist to copy the tentacula of Vesicularia, 
or any W eli_ known Polyzoon— a piece of dishonesty which, although its perpetration cannot be seriously recommended, 
might challenge detection in this particular case. 
