430 " THETIS " SCIENTIFIC RESULTS. 



seems unlikely in the extreme that the vena-cephalica also 

 conveys blood to that region. 



3. Since the pericardial branch of the vena-cephalica is smaller 

 than the main trunk, it is highly improbable that the nephridial 

 empties into the other branch, because, in that case, the trunk 

 must also empty into the pericardial branch. 



In view of these facts it is evident that neither of the nephridial 

 are efferent veins. 



There are, it seems, three circuits of circulation in this mollusc: 

 one branchial and two nephridial. 



(1) The branchial circuit is that of portion of the blood carried 

 forward by the cephalic artery ; its course is from the aorta- 

 cephalica by lacunte to the efferent branchial vein, and by this 

 vessel back to the heart. 



(2) The cephcdicu-ne])hridial circuit is also that of a portion of 

 the blood carried by the aorta-cephalica ; from this vessel it 

 collects into the sinuses in communication with the vena- 

 cephalica and rectal sinus, and by these vessels is conveyed 

 partly to the nephridium, partly to the pericardium. From the 

 cavity of the nephridium it passes through the pores into the 

 nephridial gland and is carried thence to the heart by the vena- 

 cava. 



(3) Viscero-ne2ohridial circuit. That portion of the blood 

 carried posteriorly by the vena-cephalica, which enters the peri- 

 cardium, passes through that cavity to reach the visceral coil by 

 the vense-pericardiales. From the lacunae of the visceral coil it 

 passes through the vena-maxima, accompanied by the blood from 

 the aorta-visceralis, to reach the nephridium ; thence its course 

 is as in the cephalico-nephridial circuit, 



5. Nervous System. 



No trace of this system was found. The central ganglia, i.e., 

 the cerebral, pleural and pedal, had rotted away, and the nerves 

 were washed about and became indistinguishable from the 

 numerous muscle strands lying in the body cavity before it was 

 realised that they were no longer attached to their respective 

 ganglia. 



6. Reproductive System. 



(PI. xxxix., fig. 1.) 



As stated in the introductory remarks, the single specimen 

 was a female ; nothing is therefore known of the male organs, and 

 of the female only the macroscoi:)ic aspects of the gonad and uterus. 



The gonad {go.) is rather small ; it is situated near the apex of 

 the visceral coil, there it overlies the digestive gland, and it is of 

 a yellow colour. 



