HORNELL ANATOMY OF PLACUNA 83 



Two other small ducts also open into the nephridia, one on each side from 

 a compact reddish-brown fleshy gland situated on each of the common efferent 

 branchial trunks immediately between the ventral margin of the palps and the anterior 

 ventral corner of the visceral mass. These two glands (figs. 4, 6-8, Acc.ex.o.} are 

 sub-triangular in lateral view, and measure about three millimetres along each side. 

 They are made up of closely-packed tubules lined with large cells which leave but a 

 small lumen (fig. 29, Acc.ex.o.) ; compact though they arc, there is little doubt that we 

 have here a highly-specialised form of Keber's organ, or so-called pericardial glands. 



It has been noted that no pericardium appears to be present in Placuna. The 

 same statement was formerly made of Anomia, but although Pelseneer argued that 

 the vestige of a pericardium does persist in a much reduced and vestigial form as an 

 obscure connection between the hinder portions of the nephridia, which in Anomia lie 

 on either side of the rectum, Sassi (loc. cit.) controverts this, and denies the existence 

 of any vestige of a pericardium. 



In Placuna I venture to suggest that the two fleshy glands which open right and 

 left into the most dorsal part of the right and left nephridia respectively represent the 

 reno- pericardial tubes of Lamellibranchs more normal in the structure of this organ, 

 for example, Margaritifera vulgaris, where each of these tubes is seen as a wide sleeve 

 proceeding from the pericardium on each side to insertion in the wall of each lateral 

 nephridial chamber. In this type the ventral surface of the pericardium is beset with 

 excretory glands a Keber's organ. In Placuna it would seem as if these glands 

 had shifted outwards to the distal extremities of the reno-pericardial ducts and 

 there massed compactly, a change accompanied, if not indeed produced, by the gradual 

 obliteration of the central or true pericardial chamber. If this hypothesis be correct, 

 then the dark compact glandular bodies, seen one on each lateral wall of the renal 

 organ, represent all that is left in Placuna of the pericardium and reno-pericardial 

 tubes. 



NERVOUS SYSTEM. 



The central nervous system of Placuna, although it consists of the same 

 components as in other Lamellibranchs, suffers extreme secondary modification due to 

 torsion, fusion, and suppression of parts consequent upon the asymmetry of the other 

 organs and their general concentration and excessive development upon the right 

 side of the body. 



It is constituted of the usual three ganglionic centres, (a) the cerebral, consisting 

 of two separate paired ganglia widely separated and asymmetric (figs. 10 and 11, 

 Cer.g.); (b) an apparently single pedal ganglionic mass (Ped.g.) formed by the fusion 

 of originally paired pedal ganglia, which are still resolvable as such on minute 

 examination ; and (c) a single median parieto-splanchnic ganglion (Par.sp.g.), showing 

 little trace of ever having been paired except in the paired nature of the nerves it 

 gives off. 



