774 PROCEEDINGS OF THE ACADEMY OF [DeC, 



Hargitt, in his paper, Variations Among Hydromedusce, discusses 

 the arrangement of tentacles in Gonionema. He approaches the 

 question as a student of variations, and unfortunately lacks the 

 young mat(!rial from which I have found it possible to educe very 

 definite rules in the arrangement of marginal organs and their 

 order of appearance. As a natural result Hargitt comes to the 

 conclusion that so much irregularity occurs as to render it impos- 

 sible to discover any definite order of appearance or ultimate 

 arrangement in these organs. It is true that the abnormal speci- 

 mens which he studied most'closely do show very little regularity, 

 as would indeed be expected. But in normal individuals quite a 

 remarkable degree of precision is manifest in the position and order 

 of appearance of tentacles and sense-organs, with reference to each 

 other and with reference also to previously arisen organs of the 

 same kind. This is particularly true in the younger stages. 



If we examine the eight-tentacled medusa the following points 

 are noticeable : First, (he tentacles are evidently 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 of equal size. These tentacles are 

 very similar in appearance and structure to the larval tentacles, and 

 there seems little reason why the larger perradials may not be the 

 permanent larval tentacles.^* 



Second, the sense-organs are four in number and placed in defi- 

 nite positions, relative to the tentacles. If we look at the bell- 

 margin from the oral side, the newly arisen tentacles in the four 

 quadrants have apparently crowded in between the sense-organ and 

 the perradial tentacle, which comes before it as the hands of a 

 watch go. Fig. 11 shows this stage, and is made from a camera 

 drawing of the eight-tentacled medusa. The relation which 

 is here exhibited in the youngest stage of the free-swimming 

 gonosome is the same throughout the growth of the medusa: 

 wherever a rudimentary or newly arisen tentacle lies on the bell- 

 margin, it will always, normally, be found to lie just in front of a 

 newly arisen sense-organ, and just after a larger tentacle, i.e., one of 

 an earlier cycle. 



Much has been written to show that coelenterates, and especially 



"This conclusion is strengthened by work done since this paper was 

 written. 



