ABDOMINAL SENSE-ORGANS 159 



between the hinder end of the foot and the vent, 

 a transverse, undulating line of greenish-brown 

 pigment interrupted in the middle line by a 

 narrow interval. That line proved to mark the 

 position of the pigmented sense-organs for which 

 he was searching. In the figure which he gave 

 to elucidate the topography of these organs, 

 another pair of organs, on either side of the vent, 

 was indicated with great distinctness, although no 

 reference was made to them in the text. These 

 latter organs were, eight years later, shown by 

 J. Thiele 1 to represent a pair of abdominal 

 sensory papillae having a highly characteristic 

 structure and innervated by a fine nerve pro- 

 ceeding backwards on each side from the visceral 

 ganglion. 



When examined in toto in a fresh preparation 

 the abdominal sense-organs are chiefly char- 

 acterised by the possession of a dense coating 

 of long, motionless, stiff cilia or sense-hairs. In 

 transverse section these cilia are seen to be 

 carried by a very high epithelium containing 

 numerous basal nuclei at different levels, an 

 intermediate layer of nuclei at one level, and a 

 clear peripheral zone. Thiele described the 

 organs in a number of bivalves, including a 

 species of scallop, Pecten varius, where he found 

 a sensory ridge on the right side only, there 

 being no corresponding ridge on the left side 

 1 Zeitschr. wiss. Zool^ xlviii., 1889, pp. 47-59. 



