BRACHIOPODA. 303 
liver; the intestine of Lingula is reflected dorsally, slightly 
convoluted, and terminates between the mantle-lobes on the right 
side. In Orbicula it is reflected ventrally, and passes straight 
to the right, ending asin Lingula. In Terebratula, Rhynchonella, 
and probably all the articulated Brachiopoda, the intestine is 
simple and reflected ventrally, passing through a notch or fora- 
men in the hinge-plate, and ending behind the ventral insertion 
of the adductor muscle. 
The circulatory system is far less complex than was formerly 
supposed, and does not differ greatly from the same system in 
the Tunicata. The heart is placed on the dorsal surface of the 
stomach, and consists of a simple, unilocular, pyriform vesicle 
without any auricle. From it the blood is propelied through 
four channels to the organs of reproduction and to the mantle; 
and its flow is probably assisted by a number of subsidiary 
pulsatile vesicles situated 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 vein. 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, which serve to bring food to the creature’s mouth by the 
means before noticed, also subserve the purpose of respiratory 
organs. 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 oxygen- 
ating 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 nitrogenized 
products which have been eliminated from the blood. Conse- 
quently it would perform the offices both of the kidney and the 
renal organs. 
The generative organs occupy the great pallial sinuses, and 
the sexes are separate. In the articulated brachiopods the 
