178 THE ORGANIZATION 



prolongations of the wall of the hollow tentacles ; so that the 

 space between each pair is in direct continuation with the 



biy. 100. 



cavity of each feeler. The second set of tentacles (figs. 106, 

 107, II) originate at alternate points with the first six, (I,) but 

 behind them ; and oftentimes, as if to exemplify their distinct- 

 ness from the former, they are held in a different position, turned 

 back, for instance, as represented in this figure, (fig. 106, II). 

 The third set (III) alternate again with both the first and sec- 

 ond, and therefore are twelve in number; and originate still 

 further back from the mouth. In these figures they are as yet 

 quite small, (III,) and you can see but a part of the set here, 

 because, as I have said on a previous occasion, (p. 60, note f,) 

 they do not all develop at an equal rate. When, now, the 

 number of tentacles has increased by another set, which adds 

 twelve more, there are then twenty-four in all, and we find 

 no difficulty in ascertaining their relations ; but upon the next 



Fig. 106. Metridium marginatum. M. Edw. 6 diam. A profile view of a 

 very young individual, seen in the direction A, B, of fig. 107. m, mouth; st, 

 stomach ; p, bottom of st, where it opens into the general cavity ; p to p l , the 

 free edge of the semi-partitions ; I, II, III, the different sets of tentacles. Orig- 

 inal. 



Fig. 107. A foreshortened view of the head of fig. 106. A, B, the median 

 plane which divides the body into right and left halves. Original. 



