POSTLARVAL DEVELOPMENT. 77 



and unilateral pairs being established. On the dorsal and ventral axial 

 pairs the musculature is on the faces of the mesenteries turned away 

 from one another, which, by this means, are recognizable as directives. 

 The musculature on each complete lateral mesentery is on the ventral 

 face turned towards the incomplete mesentery, while this bears its muscle 

 fibers on the dorsally directed face, so that each unilateral pair of mesen- 

 teries at this stage is made up of a complete and an incomplete moiety 

 (anisocnemic). 



On the reasonable assumption that the comparative sizes of the mesen- 

 teries indicate their order of appearance in the larvae, the two bilateral pairs 

 which first unite with the stomodseum (plate i, fig. 3) are the first and second 

 pairs to arise. At the stage available little difference in size is represented 

 by these two pairs, so that it could not be readily determined which is the 

 first pair and which the second. The results of Von Koch (1897), Wilson 

 (1888), and others who have studied earlier stages of coral larvae, leave no 

 doubt, however, that the ventral of the two pairs is the first, and that the 

 dorsal pair is the second. Frequently in other corals the first pair is for a 

 time much better developed than the others, and bears well-defined filaments 

 (1902, p. 528). From the relationships shown on plate i, fig. 4, and 

 plate 8, fig. 51, there can be no uncertainty as to which are the third 

 and fourth niesenterial pairs. The ventral pair of directives is larger, 

 and unites with the stomodseum in advance of the dorsal pair ; it is there- 

 fore the third pair in the rnesenterial sequence, and the dorsal directives will 

 be the fourth. 



The incomplete mesenteries on plate i, fig. 6, representing the fifth and 

 sixth bilateral pairs, arise practically at the same time, and for the most part 

 remain equally developed. The more dorsal of the two in some instances 

 outstrips the other, and in other corals and actinians is frequently found to 

 unite with the stomodseum in advance of the more ventral. Whatever differ- 

 ences exist in size or time of union with the stomodseum support the view 

 now generally held that the incomplete pair of mesenteries between the first 

 and second pairs is to be regarded as the fifth, and hence the pair between 

 the first and third pairs is the sixth. 



The sequence of the protocnemes thus established for Siderastrea 

 radians is in strict agreement with that ascertained by H. V. Wilson (1888) 

 for Manicina arcolata, and by Von Koch (1897) for Caryopliyllia cy a tints, 

 both of whom had a complete series of larval stages for investigation. It is 

 also supported by the various early stages in other corals which I have had 

 under observation (1902, p. 450). The same order is followed by nearly all 



