SENSE-ORGAN OF PLACUNA 161 



forms in its general histological structure to those 

 previously described in other Lamellibranchs, and 

 I have had the opportunity of comparing it in 

 section with the paired, whitish, adrectal, semi- 

 lunar organs of Area rhombea, a species which 

 occurs in the same localities as Placuna in the 

 back-waters of the eastern province of Ceylon, 

 near Trincomalee. Transverse sections of the 

 sense-organ in Placuna, stained with alum car- 

 mine, show externally the dense layer of perfectly 

 preserved, stiff, sensory cilia, exceeding in length 

 the height of the sensory epithelium. At their 

 bases they perforate the stout, cuticular mem- 

 brane, which presents a double contour. Below 

 the cuticle there follows an outer prismatic layer, 

 then a middle layer of evenly placed elliptical, 

 nuclear bodies representing the layer of spindles ; 

 thirdly, an inner fibrillar layer, and finally a basal 

 plexus of nuclear bodies adjoining the basement 

 membrane which separates the epithelium from 

 the gelatinous conjunctive tissue upon which it 

 rests. 



Thiele emphasised the extraordinary resem- 

 blance between the epithelium of the Molluscan 

 abdominal organs and the Annelid lateral organs 

 as described by Eisig in the Capitellida^ ; and 

 satisfied himself by. actual comparison of prepara- 

 tions that in fact the similarity is very close. 



Recently another pair of sense-organs, histo- 

 logically resembling the abdominal sense-organs 



