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peripheral portion which is placed, like a small tentacle, on this 

 enlargement. They call the first, the auditory pad, " Hor- 

 polster," and the second, the auditory club, " Horkolbchen." 



Some of the Trachynemidae, Aglaura, for example, have 

 similar organs, while in others the sensory pad is folded 

 upwards around the club in such a way as to shut it in to a 

 sensory vesicle, which may be open distally or completely 

 closed. 



In the Geryonidae the sensory vesicle which is thus formed 

 is imbedded in the gelatine of the bell, and is thus shut off 

 from all contact with the surrounding water. 



Greatly as these various organs differ among themselves, the 

 Hertwigs show that all are constructed on a common plan, 

 and that they present three successive stages in the evolution 

 of a peculiar sense organ which attains to its greatest com- 

 plexity and perfection in the Geryonidae. 



Among the forms which these authors studied Cunina lativen- 

 tris has the organs in the simplest and most primitive con- 

 dition. In this species the sensory pad is an insignificant 

 thickening of the marginal nerve ring without any well marked 

 boundary. It is wider tangentially than radially and forms a 

 sort of pedestal, from the centre of which the sensory club 

 arises. This is a cylindrical body like a rudimentary tentacle. 

 Its basal end is much constricted to form a short stalk which 

 joins it to the pad. Its free end is, on the contrary, somewhat 

 enlarged, and it usually contains two concretions, the periph- 

 eral one largest. The sensory pad is nothing more than a 

 thickening of the nerve-ring. The sensory club is made up of 

 two sharply contrasted parts : a cylindrical axis, and a sensory 

 epithelium with long, stiff hairs. The sensory epithelium is 

 continuous at the base of the club with the epithelium of the 

 nerve-ring, and it is separated by a supporting layer from the 

 axial portion which consists of a single row of large flattened 

 cells, like those in the axis of the tentacle of a hydroid. The 

 early stages in the development of the club show that these 

 axial cells are derived from the endoderm of the circular tube 

 or its equivalent, although the fully formed club is not imbedded 

 in the pad but attached to its surface, and the only trace of the 



