No. I.] THE SENSORY CLUBS. 297 



As the surface of the ocean is the chief source of light, a 

 medusa with eyes may guide itself so as to approach or recede 

 from or move parallel to the surface. 



Ocelli too simple to form images may serve this purpose, 

 which has no doubt been an important factor in the early 

 history of visual organs, although eyes are not very well 

 adapted for this use, whether they are simple or highly per- 

 fected, for it is difficult to believe that slight movements in 

 the transparent water of the ocean can make any perceptible 

 difference in the amount of light. 



If the problem of giving a sense of direction to a disk-shaped 

 or bell-shaped animal which always moves towards one pole of 

 its central axis in a medium of its own density and which has 

 its central nervous system placed around its edge, were to be 

 solved, the simplest and most effective plan would be to attach 

 movable weights, at regular intervals, to the nerve-ring, so that 

 when one edge of the bell is raised and the other depressed the 

 nerve-ring shall be differently affected on the upper and lower 

 sides while movement upwards shall give one sensation and 

 movement downwards a different one to the whole nerve-ring. 



The so-called "auditory clubs" are, obviously, adapted for 

 giving to the medusae this all-important sense of direction — a 

 sense which they show, by every movement, that they possess. 



Among the veiled medusae the sense clubs are found in 

 three stages of perfection : 



1. In the Thaumantidae and Cannotidae they are simple 

 clubs, with an enlarged tip, united by a narrow stalk to a 

 sensory eminence on the nerve ring; 



2. In the Narcomedusae (Haeckel) and in the Aglauridae 

 among the Trachomedusae (Haeckel), the enlarged club-shaped 

 tip of the projecting club is loaded with calcareous concretions; 



3. In most of the Trachynemidae the sensory eminence is 

 raised up around the club in such a way as to inclose it in a 

 sensory vesicle, which is imbedded, in the Geryonidae, in the 

 gelatine of the bell. 



Comparison of the figures here given with those which the 

 Hertwigs give will show that we here have the same organ in 

 three successive stages of perfection. It is also clear that 



