66 SIDERASTREA RADIANS. 



from their relation to the longer oral axis and from the known relation of 

 the complete and incomplete mesenteries at the Edwardsia stage of develop- 

 ment ; the two terminal chambers which include the longer oral axis are 

 always the directive entocceles, and the two alternating chambers on each side 

 between these are the lateral entocceles, while all the intermediate divisions 

 are exocoeles. Most of the young polyps were so situated as to permit of all 

 these details being clearly observed, and in no instance were the first six 

 tentacles developed as outgrowths of the entocoeles, but were always disposed 

 as in plates i and 2, figs. 7-9. 



As shown in plate i, fig. 7, the six tentacles first to appear alternated 

 with the first septa, the latter being entocoelic in position ; the smaller septa 

 which arise later (plate 2, fig. 9) correspond with the first tentacles, that is, 

 are exosepta. 



FIRST CYCLE OF ENTOTENTACI.ES. 



A long interval elapsed between the appearance of the primary cycle of 

 six exotentacles and the development of any others. Toward the close of 

 the fourth week additional small protuberances of the disc began to appear 

 in the interspaces between the primary series and a little nearer the center 

 of the disc. Like the first they soon showed a distinction into apical knob 

 and stem. In most polyps the six new tentacles arose simultaneously, 

 constituting a second inner cycle and alternating with the first ; but in some 

 cases the members developed successively, though no dorso-ventral or other 

 regular sequence could be established. Any intervals which occurred in 

 the appearance of the different members were very brief, except in one 

 instance where a fortnight passed between the appearance of the dorso-lateral 

 and the ventro-lateral pairs. 



There were now twelve tentacles present, arranged in two alternating 

 cycles of six each ; an outer cycle of six large exotentacles, the first to arise, 

 and an inner cycle of six smaller entotentacles appearing later (plate 2, fig. 1 1). 



Close examination soon revealed that the individual members of the inner 

 cycle were not disposed symmetrically midway in the interspaces between 

 those of the older cycle, but, as represented in plate 3, fig. 13, were a little to 

 one side. Kven the tentacles arising from the axial entocceles were some- 

 what to the one or the other side of the principal axis of the polyp. In most 

 cases the four small lateral tentacles were all situated toward the dorsal 

 aspects of their corresponding interspaces, while the tentacles over the direc- 

 tive entocceles might be to the right or to the left of the directive axis. This 

 peculiar arrangement of the young entotentacles was best observed when 

 the polyps were only partly extended ; during full expansion they appeared 



