786 PROCEEDINGS OF THE ACADEMY OF [Dec, 



C. — Sense-organs. — The origin of the sense-organs is very simi- 

 lar to that of the tentacles (PI. XXXIV, figs: 24 and 25). In 

 fact, it seems clear from a study of these processes in Gonionema 

 that the sense-organs must necessarily be regarded as modified ten- 

 tacles. In the case of these sensory clubs (PI. XXXIV, fig. 25, 

 S. C. ), the endodermal tissue (End. ) of the circular canal grows 

 down in a plug into the ectodermal tissue (Ed.) of the bell-margin. 

 This latter becomes closely applied to the outside of the plug, as 

 a thin investing epithelium, and it also spreads out in a thin lamella 

 over the inner surface of the capsule which appears in the ectoderm 

 in front of the developing club, PI. XXXIV, fig. 25, is a draw- 

 ing by Prof. Brooks from a section cut transversely across the bell- 

 margin, showing the early stage in the formation of a sense-organ. 

 I have not been able to demonstrate the presence of sensory hairs 

 in the cavity of the capsule. The cells at the tip of the club soon 

 begin to secrete the solid concretion which later attains a considerable 

 size. The concretion is invested with a Ihin membranous ectoder- 

 mal covering. In Oonionema the concretions correspond with the 

 composition which has been given for similar structures in other 

 medusae — a calcium salt deposit in an organic matrix. Thus it 

 is seen that both tentacle and sense-organ consist of an endodermal 

 core which appears as a plug of tissue growing out from the lining 

 of the circular canal. In each case this core becomes invested 

 with a tunic of ectoderm which remains associated with it. 



Xematocysts. 



' In the hydra-stage the earliest appearance of nematocysts was as 

 interstitial cells arising from either tissue-layer. Their growth in 

 Oonionema is much the same as in Cordylophora lacustris as de- 

 scribed by Morgenstern (1901). They are carried out on the 

 tentacles by migration along with the ectodermal layer in which 

 they are set. The extreme attenuation of the tentacle as it is 

 when fully extended (fig. 14) gives an admirable chance to study 

 the cell-elements, especially the nematocysts. The tentacle appears 

 as a delicate rod of translucent substance, partitioned off at inter- 

 vals by the transverse walls of the endoderm cells, and studded 

 along its length with numerous glistening beadlike bodies, the 

 nematocysts. Above each of these thread cells a palpocil projects 

 like a thorn (PI, XXXIV, fig. 26). The capsule has an unusual 



