76 SIDERASTREA RADIANS. 



4. Additional tentacles are now intercalated between the inner and outer 

 cycles already established. They arise, apparently without any regular 

 sequence, from the six entocoeles of the second pairs of mesenteries, and, like 

 the members of the first cycle of entotentacles, are at first simple, but after- 

 wards bifurcated. When the cycle is completed it constitutes the second 

 tentacular cycle of the mature polyp ; the original second cycle of six 

 exotentacles, now consisting of twelve members, becomes at this stage the 

 outermost or third cycle (law of substitution). 



5. As the pairs of third-cycle mesenteries develop, two tentacles, one 

 entocoelic and one exoccelic, arise in connection with each. The entocoelic 

 representatives appear between the second and outermost cycles of the previous 

 stage, and thus constitute a new third cycle ; the exocoelic representatives 

 arrange themselves among the members of the outermost cycle, which now 

 becomes the fourth of the tentacular cycles. Here, as in the two previous 

 cycles, the entotentacles are at first simple, then another moiety appears, and 

 finally a common stalk. 



6. The hexameral cyclic plan of the third-cycle entotentacles in 5. 

 radians being usually incomplete, the morphological tentacular formula for 

 the whole polyp with four cycles is 6, 6, x, 6 + 6 -f x, where x may be any 

 number from one to twelve. 



MESENTERIES. 

 FIRST CYCLE OF MESENTERIES (PROTOCNEMES) . 



The earliest larvae reveal, both externally and in sections, only eight 

 mesenteries, arranged in four bilateral pairs, two axial and two lateral. Of 

 these the two lateral pairs are already united with the stomodseum, but the 

 dorsal and ventral axial pairs are free, the ventral pair being a little larger 

 than the dorsal (plate i, fig. 3). In other larvae, only a day or two older, the 

 two ventral mesenteries have become united with the stomodaeum, but the 

 dorsal are still free, and two new pairs, the fifth and sixth, are beginning 

 to make their appearance (plate i, fig. 4, and plate 8, fig. 51). At the stage at 

 which the larva settles four pairs of mesenteries, two lateral pairs and the 

 dorsal and ventral axial pairs, are complete, and the fifth and sixth pairs have 

 become a little larger, but remain free from the stomodseum (plate i, fig. 6). 

 This is the stage represented by all the larvae at or shortly after fixation, and 

 is evidently one of importance in the ontogeny of the polyp, for no further 

 mesenterial development took place for a period of three or four weeks. 



In transverse sections of the larvae the vertical muscle fibers of the 

 mesenteries are already sufficiently well developed to permit of the directive 



