500 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



adjacent. On the right upper side of the directive axis the fifth and sixth protocnemes, 

 incomplete from the beginning, have now disappeared, as is also the case with the pair of 

 metacnemes on the same side. Mesenterial filaments are no longer present on the dorsal directives 

 (IV, IV), and the pair of metacnemes has also disappeared from the middle exocoele on the left 

 side, but the dorsal pair (A), though very feeble, still persists. 



The arrangement of the septal ingrowths, present as yet only on the left lower side, is very 

 instructive. The largest occur within the entoeoele of the pairs of directives; two are found 

 between the left dorsal mesentery and the mesentery of the fii'st bilateral pair (IV-II); two 

 between this last mesentery and the mesentery of the second bilateral pair (II— I), while only one 

 appears between this and the left ventral directive (I— III). Three of the septal invaginations 

 are exocoelic, the exocoele of the dorso-lateral pair of protocnemes (II, V) is provided with one, 

 but the septum of the exocoele of the ventro-lateral pair (I, VI) is as yet undeveloped. The 

 entoeoele of the persistent pair of metacnemes (A) also contains an invagination. 



In the section from a still lower region, represented in fig. 89, fully developed filaments 

 are retained only on the first three bilateral pairs of protocnemes. Only nine mesenteries are 

 present, the mesentery remaining in addition to the Edwardsian mesenteries is the fifth member 

 of the left side. A septum occupies each chamber between any two mesenteries, whether the 

 two constitute a pair or not; the septum in the entoeoele of the left dorso-lateral pair is small, 

 and soon disappears along with mesentery V. The. skeletal fragments within the middle of the 

 gastro-ccelomic cavity represent the first appearance of the columellar projections. 



In lig. 90 only mere traces of six of the mesenteries persist, the first pair being the strongest; 

 the number of septa is also six. corresponding with the number of mesenteries. With one 

 exception the septa are all united in the middle, otherwise the gastro-ccelomic cavity is divided 

 into as many separate chambers as there are septa. 



Another bud of about the same age presents a different condition with regard to the 

 appearance of the first two pairs of metacnemes from that just described (fig. 82). The 

 protocnemes are at the JSdwardsia-Bts,ge, and within each of the median lateral exocosles are the 

 rudiments of a bilateral pair of metacnemes (A, A), which are somewhat better developed in 

 sections below the stomodseum. 



The tentacles in the bud have already appeared, and exhibit an interesting stage. Fig. 83 

 is taken from a slightly oblique section, the upper half through the free portion of the tentacles, 

 and the lower through their attachment to the periphery of the disk. The polyp was expanded 

 to such a degree that the peristome projected in a cone-like manner beyond the zone of tentacles, 

 and, as seen in the middle of the section, the Edwardsian mesentaries extend as far as the 

 stomoda?al walls. Ten of the tentacles show a regular alternation of large and small members, 

 and, superposing the section on that in fig. 82, the tentacles arising from the entocoeles are seen 

 to be the larger, and those from the exocceles the smaller. In place of the single median lateral 

 prototentaele on each side are three tentacles, as yet incompletely formed. These will be found 

 to correspond in position with the incipient pair of metacnemes in the median lateral exocceles, 

 and serve to establish that the metatentacles arise practically simultaneously with the metacnemes, 

 an entoccelic and an exocoelic member together. 



A third bud, somewhat older than either of the two described, was also studied, and the 

 details of its lower region are represented on PI. XIII, fig. 91. The upper right side is the border 

 toward the centre of the colony, and the lower left side is the outer aspect. The relationships of 

 the mesenteries, as before, show the inner border to be the sulcular aspect, and the outer border 

 the sulcar. In the stonioda>al area six pairs of protocnemes and six pairs of metacnemes occur, 

 and of the former all the pairs are complete on one side, hut the members of the fifth and sixth 

 pairs are incomplete on the other side. 



In the region represented only four pairs of the metacnemes occur, the two pairs on the 

 upper left side having disappeared some distance; above. Mesenterial filaments occur on the 

 Edwardsian mesenteries, but not on the fifth and sixth bilateral pairs; and septal invaginations, 

 both entoccelic and exoeodic, appear within practically all the mesenterial spaces. 



The lower sections demonstrate the same relation as in the previous bud, namely, that as the 



