BEICHIOPODA. 359 



fom' cliannels to the organs of reproduction and to tlie mantle ; 

 and its flow is probably assisted by a number of subsidiary 

 pulsatile yesicles situated on the main arterial trunks. It then 

 courses throuch. the plexus of lacunes in tli^' pallia! sinnsr'S 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 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 Lrngula, 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 efiete nitro- 

 genised products which have been eliminated from the blood. 

 Consequently it would perform the ofiices 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 RhynchonelJa 

 there are four oviducts, but in most, if not all the other 

 Brachiopods, there are only two. In Terebratulidce they are 

 divided into two portions, called the auricle and ventricle by 

 Professor Owen. Mature eggs have been found in large numbers 

 in the perivisceral chamber and in the oviducts. Eecent 



