No. I.] THE SENSORY CLUBS. 29 1 



modified endoderm cells, derived from the endoderm of the 

 circular tube under the nerve ring ; and a covering of ecto- 

 dermal epithelium continuous with that of the nerve ring. My 

 osmic acid specimens do not show sense hairs, either on the 

 nerve ring or the club, although Haeckel assures us that in 

 some cases at least, if not in all, the clubs carry tactile bristles. 



Fig. 7 is a section through the base of one of the primary 

 tentacles of the same specimen, and comparison with Fig. 5 

 will show that the sense club in the one section occupies very 

 nearly the same position in relation to the velum and the upper 

 nerve ring that the ocellus occupies in the other. 



The stalk of the club is solid, but in the axis of the enlarged 

 portion there is a remnant of a cavity, and Figs. 5 and 6 show 

 that there is a deep pouch or diverticulum from the cavity of 

 the circular tube, pushing down among the large vacuolated, 

 endoderm cells nearly to the stalk of the club, which, undoubt- 

 edly, arises as a hollow tentacle. 



There is only one club on the bell-margin of the young 

 Laodice shown in Fig. 3, and in this the cavity reaches nearly 

 to the base of the stalk. 



The endodermal axis of the sensory club of the Narcomedusae 

 is solid, and it is not directly continuous with the endoderm of 

 the circular tube as it is in Laodice, for while the Hertwigs 

 have shown that this continuity is found during the early stages 

 in the development of the sense club, the connecting axis of 

 endoderm cells disappears before the club acquires its perfect 

 form. 



The sense clubs which the Hertwigs have studied in the 

 Narcomedusae and Trachomedusae differ greatly among them- 

 selves, although these authors show that they all may be 

 reduced to a common plan. 



Their account of these organs, which they regard as organs 

 of hearing, may be briefly summarized as follows : 



In the Aeginidae they are tentacle-like bodies attached to the 

 bell-margin in connection with the upper nerve ring, and pro- 

 jecting directly into the surrounding water. They consist of 

 two parts united by a slender stalk ; a basal portion which may 

 be regarded as a local enlargement of the nerve ring, and a 



