504 MAVOR. 



c. Conclusions as to the Course of Development of the Mesenteries, 

 Mesenterial Filaments and the Gastrovascular Cavity. 



If the assumption be made that the mesenteries develop in the 

 order of their size, the order of their development in the larva of 

 Agaricia fragilis is indicated by the numbers given the mesenteries 

 (Fig. 2, E), with the exception, that numbers V and VI develop 

 simultaneously. So far as the first four pairs of mesenteries are con- 

 cerned the order of development is that found by Faurot ('95) in 

 Adamsia palliata and HalcamjM chrysanthellum; by Wilson ('88) in 

 Manicina arcolata; by McMurrich ('91) in Rhodactis sancti-thomae 

 and Aulactinia and by Duerden (:04) in Siderastrea radians. 



The writer believes that the larvae studied show that in the develop- 

 ment of the six pairs of primary mesenteries there may be recognized 

 three periods as follows: first, a period in which there are two pairs 

 (/ and //), a condition not represented by any of the larvae, but shown 

 probably to exist by the great development of pairs / and // and the 

 only slight indication of pairs /// and IV in larva A; second, a 

 period in which there are four pairs, represented in an early stage by 

 larva A, and shown probably to exist by the large size of pairs III and 

 IV and the small size of pairs V and VI in larvae B and C; third, a 

 period in which there are six pairs of mesenteries represented by larvae 

 B to I. By dividing the development of the mesenteries into these 

 periods the writer does not wish to deny the appearance of the pairs 

 of mesenteries in succession, but merely to show their association 

 into three sets of two pairs each. Further, it is to be noticed that 

 this association in sets of two pairs becomes more intimate as develop- 

 ment proceeds. In larva A pair I shows a very considerable advance 

 over pair // and this difference persists to some extent in the older 

 larvae. In the same larva pair III shows only a slight advance over 

 pair IV and in the older larvae such a difference between pairs /// 

 and IV is hardly to be seen. In larvae B to I pairs T" and VI seem to 

 have appeared and developed simultaneously. 



If this interpretation is correct, the ventro-dorsal order of develop- 

 ment which is evident in the first two pairs, becomes less marked in 

 the next two pairs (/// and IV) and has entirely disappeared in the 

 last two pairs (F and VI). In other words the bilateral symmetry- 

 of the larval mesenteries has begun to give way to a radial symmetry 

 while it is still free swimming. 



Evidence of close association in development between the first 

 two pairs of mesenteries is seen also in the development of their 



