43 



possible to discover any definite order of appearance or 

 ultimate arranfrement in these organs. It is true that the 

 abnormal specimens which he studied most closely do show 

 very little regularity - as would be expected. But in nor- 

 mal individuals quite a remarkable degree of precision is 

 manifest in the position and order of appearance of tenta- 

 cles and sense-organs, with reference to each other and al- 

 so to previously arisen organs of the same kind. This is 

 particularly tn^e in the younger stages. 



If we examine the eight-tentacled medusae the follow- 

 ing points are noticeable: First, the tentacles are evi- 

 dently of two cycles, in order of appearance. The four at 

 the ends of the radial canals, or the perradials, are equal 

 in size, and larger than the four interradials , which are 

 also equal in size. These tentacles are very similar in 

 structure and appearance to the larval tentacles, and there 

 seems little reason why the four larger perradials may not 

 be the permanent larval tentacles. 



Second, the sense-organs are foux' in number and 

 placed in definite positions, relative to the tentacles. 

 If we look at the bell-margin from the oral side, the newly 

 arisen interradial tentacle in each interradial <iuadrant 



