452 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



( /') The .sixth jiair ))ccomes the veiitr.il moiety of tlio vcntro-hitcnii puirs of iiieseiitei'ies on 

 each side of the polyp. 



Among the muiiy larva' .sectionizod very few irregularities have been met with. Sometimes 

 one member of a pair will appear in advance of the other; in one larva only five complete mesen- 

 teries occurred, one memt)er of the third pair having lagged behind. A young polyp settled for 

 some time still presented only three complete jjairs along with three incomplete pairs. 



Although none of the other larvte which have been studied present so comjjlete a series as 

 F. fra(iui)i. yet all the evidence from them goes to support the sequence just established. They each 

 represent Facia at one or other of its developmental phases. Newl\--hatched larva- of lsophi///iii 

 dlpsnce-a contain three pairs of mesenteries, the pairs differing greatly in the extent of their 

 development (Pis. XVILXVIII). The middle pair again extends nearly the whole length of the 

 larva, and bears filaments which are strongly developed, especially at their lower extremity'. Of 

 the two smaller pairs, one is very rudimentary, while the other extends a short distance below the 

 stomodwum. The stage very closely corresponds with tliat in the earliest available larva of 

 F. fragum (fig. 112). 



The non-exti"uded larva' of J'orltes i-lamria also reveal a phase with three pairs of mesente- 

 ries, while the mature polyps never get beyond the mesenterial stage with four j)airs complete 

 and two pairs incomplete — a stage represented by F. frayiuu at the time of Hxation. 



The lar\;e of A(jarlcta agaricites on extrusion alreadj- possess the six pairs of primary 

 mesenteries, all extending nearly the full length of the larva, but oidy the first four pairs are 

 united with the stomoda'um (PI. XXV). I5elow the stomoda-al region all the twelve mesenteries 

 also l)ear well-developed me.senterial filaments. In this species, then, the stage reached by 

 the larva on hatching is directly compai'able with that in Faria fragum. Maiiicrna areoJata^ and 

 VaryaphyUhi cyatlniK at or about the time of fixation, as well as with the adult polyps of J'orifrs 

 and Madrepwa. 



The earliest larva' of Slderasfra'a radiunx sectionized rexeal eight mesenteries arranged in 

 four l)ilateral pairs. The two lateral pairs, representing the first and second in the sequence, are 

 united with the stomodseum, while the dor.sal and ventral axial pairs, representing the directives, 

 are free. Of the two directive pairs, the ventral pair (III) is slightly larger than the dorsal pair 

 (IV), and in larva a little older the former becomes united with the stoniodaMu, while the latter 

 is still free. In larvw of about this age the fifth and sixth pairs make their appearance, and the 

 dorsal directives uiuting with the stomoda'um the larva has reached the 7r<7/'v//Y/.svV/-stage of 

 mesenterial development. Atal)outthis stage the lar\a undergoes fixation. Filaments do not 

 appear on any of the mesenteries until their eoiuiection with the stomoda?um has been fully 

 established, Init in most other sjx'cies they are formed while the mesentery is still free. 



The order of appearance and su])sequent development of the primary twelve mesenteries, 

 within the sexually produced larvse and young polyps of the Madreporaria, thus appears to be 

 \ery uniform, for no exception to the se(iuence first established b\' Wilson and von Koch has yet 

 occurred. In the extent of its de\'elopment. and also in its strong mesenterial filaments, the first 

 pair to arise usually assumes predominance, and retains it until most of the other mesenteries 

 lieconie fully established. While the second and third pairs are scarcely apparent the first pair 

 may have grown nearly the full length of the larva, each niem1)er tipped with the mesenterial 

 filament all the way. The second, third, and fourth ])airs follow one another in regular succes- 

 sion, uniting with the stomodieum in the order of their aj^pearance. 



In most Actinological studies the fifth and sixth pairs are stated to arise simultaneously, and 

 H. V. Wilson observes the same for Manicina. Though such may often be the case, instances 

 occur in which one pair a{)pears in advance of the other, and where the same pair becomes united 

 with the stomoda'um l)efore the other. Young polyps of a Sagartia from Beaufort, for which 

 I am indebted to Dr. C. Grave, all show in section that the liilateral pair between the first and 

 second Edwardsian pairs becomes inserted on the stomodieum in advance of the pair between the 

 first and third Edwardsian pairs. Another such instance occurs in the bud polyp of L'Jadoeoni 

 arliuscida, represented in transverse section on PI. VIII, lig. 6<t. The polyp was preserved in 

 a fully distended condition, the disk protruding in a cone-like manner above the zone of tentacles. 



