CASSIOPEA XAMACHANA. 



219 



should each give rise to a single tentacle, while the pouches in the short diameter, r' r', 

 should each produce three. From a glance at Figs. H and J it will be seen that this is 

 not the case in Cassiopea. To 



be sure, most of the tentacles at H I ^ 



this stage are intcrseptal in 

 position, but the interradial ten- 

 tacles arise as often from pouch- 

 es in the long diameter, r r, as 

 from those in the short diame- 

 ter, r' r' ; and in both Figs. H 

 and I there is one interradial 

 tentacle that is distinctly septal 

 in position. The septa are still 

 complete, so that there can 

 hardly be any chance of a shift- 

 ing of position. In the sixteen- 

 tentacle stage, there is a ten- 

 tacle in the plane of each septum 

 (Figs. K and L) ; but here the 

 perforation of the septa has 

 commenced, and a shifting of 

 relative position is possible. 



Even at this stage irregularities are common. For example, one of the septa in Fig. K is 

 placed asymmetrically with relation to the tentacles, and two tentacles are wanting in 

 Fig. L. 



The Strdbila, Development of the Rhopalia. When the scyphistoma has reached a 

 diameter of about two millimeters, there appear the first characters that are distinctive of 

 the strobila. The first noticeable change in this direction takes place at the bases of the 

 tentacles of the more erect series. This change may be regarded either as the outgrowth of 

 a conical lobe from the margin of the circumoral disc bearing the tentacle at its tip, or as 

 a conical widening of the basal portion of the tentacle. The former view is probably the 

 better. At about this time there appear in the tentacle, just beyond the apex of the cone 

 from which it springs, a few glistening white bodies. These are the so-called otoliths, and 

 mark the beginning of the formation of the rhopalinm (Figs. 17 and 18). The tentacles 

 containing these will be called the rhopalial tentacles. 



These concretions, or otoliths, increase in number until they form a conspic- 

 uous mass, while the basal cone begins to broaden laterally. This is now distinctly non- 



Figs. H-L. Diagrams illustrating the space relations between the septa 

 ;n ul the tentacles, observed in five young scyphistomas of Cassiopea xama- 

 chana. r radii of the long diameter, r' radii of the short diameter. 



