57 



at the proximal end of the tentacle, within the tissue of 

 the bell-margin (Pig. ;^4) the cavity of the circular canal is 

 carried out along the axis of the tentacle towards the tip. 

 In this way the tentacle, which was originally imperforate 

 as in the lax-val condition, has becone hollow, 



r. The Sense-Organs . - The origin of the sense-organs is 

 very similar to that of the tentacles (Figs. 27 ^: 30), In fac" 

 it seems clear from a study of these processes in G onionema 

 that the sense-organs must necessarily be regarded as mod- 

 ified tentacles. In the case of these sensory clubs (S.*^.) 

 the endodermal tissue of the circular canal (End. ) grows 

 down in a plug into the ectodermal tissue of the bell-mar- 

 gin ( Ect . ) This latter becomes closely applied to the 

 outside of the plug, as a thin investigating epithelium, 

 and it also spi'eads out in a thin lamella over the inner 

 surface of the capsule which appears in the ectoderm in 

 front of the developing club. Figures 34 and 55 are draw- 

 ings by Professor Brooks from sections cut transversely 

 across the bell-margin, showing the early stage in the for- 

 mation of a sense-organ. I have not been able to demon- 

 strate the presence of sensory hairs in tlio cavity of the 

 capsule. The cells at the tip of the club soon begin to 



