BRACHIOPODA. 359 
four channels to the organs of reproduction and to the mantle ; 
and its flow-.. dbably assisted by a number of subsidiary 
pulsatile vesicles siti:ted on the main arterial trunks. It then 
courses through the plexus of lacunes in the pallial sinuses and 
lobes ; turns back through the lacunes of the parietes into the 
system of visceral lacunes. It probably enters the liver, and 
ultimately finds its way back into the heart through the 
branchio-systemic yein. There is, however, another and more 
important blood current, which traverses the whole length of 
the brachial canal, and penetrates to the extremities of the 
cirri, before it joins the current returning from the visceral 
lacunes and flows with it into the branchio-systemic vein. 
The blood which has passed through the brachial canal is 
far more highly oxygenated than the blood which has flowed 
through the pallial membranes. There seems to be strong 
evidence that the so-called arms are really the gills or respira- 
tory organs of the mollusc. They also serve to bring food 
to the creature’s mouth by the means before noticed. The 
mantle is an accessory breathing-organ. It attains its highest 
development as such in Lingula, but even in this genus the 
brachial apparatus performs the chief part in oxygenating the 
blood. 
There is another system of canals which take their rise from 
the visceral cavity. What its function is has not been deter- 
mined; it is not the blood system as was formerly imagined, 
and has no connection with it. The perivisceral cavity and the 
visceral lacunes which diverge from it may, it is thought, be 
homologous to the water-vascular system in Polyzoa, the 
function of which is probably to evacuate the effete nitro- 
genised products which have been eliminated from the blood. 
Consequently it would perform the offices both of the kidney 
and the renal organs. 
The generative organs occupy the great pallial sinuses, and 
probably both sexes are combined in one individual. In the 
articulated Brachiopods the ovaries and testes are placed in the 
mantle; but in Lingula and Discina they occur in the peri- 
visceral chamber. The ova escape into the oviducts (regarded 
by Cuvier and others as hearts), which open externally, and 
have nothing to do with the vascular system. In Rhynchonella 
there are four oviducts, but in most, if not all the other 
Brachiopods, there are only two. In Terebratulide they are 
divided into two portions, called the auricle and ventricle by 
Professor Owen. Mature eggs have been found in large numbers 
in the periyisceral chamber and in the oyiducts. Recent 
