530 MEMOIRS OF THP: NATIONAL ACADEMY OF SCIENCES. 



six pairs of protocnemes, their occurrence suggesting some connection witli the first stages in the 

 formation of the skeleton (fig. 108). Thej' clearly correspond in position with the six septa of 

 2[unic!na (fig. 135), and it is conceivable that they represent invaginations of the basal wall 

 preceding the formation of the septa. No calcareous deposition however was observed to take 

 place within thorn; prol)ably the nnfavoral)le conditions under which the larva was kept interfered 

 with its normal dcNciopmcnt. 



Small polyps are sometimes found around the larger colonies of Favia, and represent 

 larvic whii'h have fixed themselves immediately or shortly after extrusion. From these 

 several further stages of development have been secured, one of which is represented in fig. 

 109. The polyp is in a partly expanded state, and only the discal region is indicated, as seen 

 under a low power of the microscope. The six pairs of primary mesenteries are present, the 

 fifth and sixth pairs still free from the stomodaHim; in addition to these a pair of mesenteries has 

 appeared in each dorsal exocoele, and a small pair in each of the middle exocceles. At present 

 the metacnemic pairs extend but a short distance over the margin of the disk, but are continued 

 farther down the column wall. No mesenteries have j'et appeared within the ventral exocceles. 

 The significance of the stages in the mesenterial development here represented has been 

 already noticed in discussing the appearance of the metacnemes in the Madreporaria generally. 



Of the tentacles six entoco^lic and six exocoelic members are already present, forming two 

 cycles, the inner tentacles a little larger than the outer, and both slightly knobbed. In addition 

 to these a tentacle has arisen in association with each pair of metacnemes. making sixteen in all. 

 At this stage it was impossil)le from their position to say whether the new tentacles were 

 entoccslic or exocoelic in relation to the pairs of metacnemi^s. I)ut it is significant that they follow 

 closely upon the development of the mesenteries. Most probal)ly they are the entocoelic 

 outgrowths which have appeared somewhat in advance of the exocoslic, following the sequence of 

 the prototentacles. 



The ditterent stages secured in the development of Fdvla fragum afl'ord a complete series 

 illustrating the order of appearance of the mesenteries in coi-als, and it is desirable that they 

 should be presented in their regular sequence. The series extends from larvse with only one pair 

 of complete mesenteries to young polyps in which fission is instituted. The earliest stage occurs 

 in non-extruded larva» ol)tained from a colony after decalcification (fig. 112). Three pairs of 

 . mesenteries are present, l)ut only one pair is complete, and this divides the ccelenteric cavity into 

 two unequal chambers. In the larger of these is a second pair of mesenteries, not complete as 

 yet, but bearing rudimentary mesenterial filaments; in the smaller chamber is a third pair of 

 mesenteries, which are very rudimentary. The first pair extends almost the whole length of the 

 larva, the filaments strongly developed all the way; the second terminates some distance in 

 advance of the al)oral end; while the third has only a very limited course. 



LarviB whi<li had been extruded a few hours when preserved reveal the next stage, 

 represented in fig. IIH. Two pairs of mesenteries are united with the stomod;eum, and, by 

 comparison with the previous figure, the new complete pair is evidently the dorsal pair, the 

 second of the mesenterial sequence. The ventral pair (III. Ill) is no lietter developed than in 

 the former figure, but in sections })elow the termination of the stomodanun a new pair has 

 appeared between the dorso-lateral pair (fig. 114). This is manifestly the fourth pair in the 

 mesenterial sequence, and it is inserted dorsal to the second pair. Also between the first and 

 second developmental pairs arc found the merest rudiments of another pair (V, V). 



Sections of larva', a little older, and in one cast- of a young polyp already settled, present 

 the next stage, wher(> three ])airs of mesenteries are inserted on the stomoda'um (tig. 11;")). 

 Comparison with fig. IIH indicates that it is the third pair in the mesenterial sequence which has 

 now reached completion. The fourth pair extends more upward, and the fifth pair has reached 

 the level of the stoinodanun; the sixth pair has not yet reached the stomodfval region, but is 

 present below (fig. Uti). Finally, in larv;e which have just settled (figs. 10.5, 106), four 

 mesenterial pairs have become complete, and the fifth and sixth pairs are well developed in the 

 upper part of the cohunn, but remain free from the stomoda'um. 



The sequence for the protocnemic pairs is thus complete. The first and second jiairs to 

 iirise become the ventro-lateral and dorso-lateral of the Edwardsian mesenteries, the third pair 



