I 68 ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



In this one genus, then, we have every transition from the proso- 

 branchiate to the opisthobranchiate type of organisation. 



The aorta is too dehcate to be readily traced in C. virgulata and 

 C. aciciilata. It may, however, be seen passing through the nervous 

 ring and bifurcating for the epipodia, fig. 7 w. There is no rudiment 

 even of a venous system. 



The Cleodora curvata (figs. 4 and 5) (one of the true Cleodorce as 

 formerly defined) forms a transition from the preceding species to 

 Hyalcea. It has the general organization of the former, with the flat- 

 tened shell more or less fissured laterally, and the filiform appendages 

 to the mantle, of the latter. 



The alary expansion forms a more rounded disc than in C. aciadata 

 and virgulata, the metapodium having become widened out and almost 

 undistinguishable from the epipodia. The triangular lobes, rudiments 

 of the mesopodium, have disappeared. 



The intestinal canal resembles that of the two former species ; its 

 flexure is ventral, and the anus opens into the cavity of the mantle on 

 that side. The long ca;cum has orange-coloured glandular parietes. 



The position of the heart has been described. It is comparatively- 

 large, and the aorta may be readily traced from it, passing forwards 

 over the stomach and through the nervous ring, and eventually dividing 

 into two branches, one for each epipodium. 



There is a more distinct rudiment of a venous system in this 

 mollusk than in Firolo'ides or Atlanta. A wide canal traverses the 

 mantle towards its upper part ; it is crossed by various muscular 

 bands. Another distinct canal can be traced from the auricle towards 

 the right side, skirting the lower border of the branchial chamber. 

 Whether it becomes continuous with the right extremity of the pre- 

 vious canal or not, I could not certainly determine. The blood flows 

 from both ends of the first-mentioned canal towards the auricle, but 

 on the left side there does not appear to be so distinct a venous canal 

 as on the right. 



In all species of Cleodora there is an elongated sac, in its structure, 

 contractions, and position relatively to the heart, exactly resembling 

 that of the Heteropoda. It communicates by a small aperture with 

 the cavity of the mantle. 



The nervous system in all these species consists of three ganglia on 

 each side of the oesophagus. Four of these form a mass, placed en- 

 tirely below the oesophagus, and the other two, placed in contact with 

 and immediately above them at the side of the oesophagus, are united 

 above by a broad flattened supracesophageal commissure. 



The upper {cephalic) ganglia give off two principal branches to the 



