70 SIDERASTREA RADIANS. 



successively from the dorsal to the ventral aspect. The six new pairs of 

 mesenteries have given rise to six additional entoccelic and six additional 

 exoccelic chambers, from which further tentacles may arise ; so that mani- 

 festlv the mesenteries with their mesenterial chambers are formed in advance 

 of their tentacular prolongations. At the stage represented in plate 3, fig. 15, 

 all the primary entotentacles are yet simple except the two dorso-lateral 

 members, each of which consists of a broad stem uneqvially bifurcated at 

 the dorsal extremity. The dorsal and ventral exotentacles are simple, and 

 the broad base of each communicates with the second-cycle entoccele below 

 as well as with the exocoele on each side of it. An additional tentacle is 

 seen on the ventral aspect of the middle primary exotentacle on each side of 

 the polyp, and as vet they are smaller than the exotentacles first formed. 

 The new tentacle has pushed the older a little to one side, and both are 

 obviousl}- exocoelic in position, though in the figure appearing to extend 

 over into the entocoele. There are now eight exocoelic tentacles forming an 

 outermost cycle. 



A somewhat older stage is represented by the next figure (plate 3, fig. 

 16). The two middle exotentacles are equal in size and quite free from one 

 another, and both are now seen to be wholly unconnected with the median 

 second-cycle entocoeles. Another exocoelic member has also appeared on the 

 ventral aspect of the dorso-lateral exotentacle, on both the right and left sides 

 of the polyp, but as yet they are a little smaller than the primary exoten- 

 tacles. The four new exotentacles are situated at the same distance from the 

 center of the disc as the primary cycle of six exotentacles, and therefore, 

 instead of forming a new cycle, they merely add to the number of members 

 in a cycle already established. The outer cycle of exotentacles thus consists 

 of ten practically equal tentacles ; the two wanting to complete the hexameral 

 plan belong to the ventral system, which region is usually found to lag 

 behind the dorsal and middle systems in the extent of its development. 



The two stages (plate 3, figs. 15, 16) demonstrate the important fact 

 that the primary six exotentacles remain exotentacles, and always constitute 

 part of the outermost cycle ; further, any new exotentacles formed become 

 equal in size with the primary exotentacles and are so disposed as to be 

 members of the same cycle. The primarj^ exotentacles do not become 

 entotentacles when new pairs of mesenteries arise in the same radii, as from 

 their original position might have been expected. On the appearance of the 

 second cycle of mesenteries, they seem for a time as if communicating with 

 their entocoeles (plate 3, fig. 15), but this condition is onlj' temporary. When 

 additional exotentacles arise and the mesenteries are further developed, both 



