PALLIAL SINUSES OF THE BRACHTOPODA. 375 



cation just noticed is cut oif, and an intestine formed, with or without an anal aper- 

 ture. 



Let us now suppose tlie perivisceral cavity of the Folyzoa to be homologous with the 

 common cavity of the Coelenterata, 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 tliat of a covered-eyed Medusa. Nevertheless, having no dit'eet 

 communication with the stomach, its superiority may be asserted as a system so far 

 closed. Now, as this appears to be homologous with the perivisceral cavity of the Bra- 

 chiopoda, the ramifications of the palHal sinuses of the latter, with their ciliated lining 

 and contained corpuscles, present the highest form of this pseudo-circulatory s)'stem. 



The existence of a true vascular system in the Folijpi 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 

 cavity. May not the same doctrine be applied to the BracMopoda ? If not, the true 

 vascular system, as described by Mr. Hancock, is altogether distinct, both in its nature 

 and origin, from what has hitherto been regarded as the rudiment of the circulatory 

 system in Coelenterata. 



In the Tiinicata, however, a single and simple heart superseding the five vesicles of 

 the Braclilopoda 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 Tiinicata, Brachiopoda and Folyzoa, con- 

 stituting the Molluscoida of M. Milne-Edwards. 



EXPLANATION OE PLATE XXXV. 



Fig. 4. Portion of the pallial sinus-system of Lingula, showing the course of the ciliary currents. 



a, longitudinal septa dividing the ciliary currents. 

 Fig. 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 ; bat in 



this particular case it was present in all. 



intelligence, at the slightesit 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 well-known Polijxoon — a piece of dishonesty which, although its perpetration cannot be seriously recommended, 

 might challenge detection in this particular case. 



