50 bulletin: museum of compaeative zoology. 



tions. The arrangement of their siphonoglyphs and mesenteries is 

 shown in Figure 15 (Plate III.). So far as the anatomical peculiarities 

 of this and the five other pairs are concerned, they might well he con- 

 sidered as animals that had undergone division. 



A further important feature of the six pairs was that in all cases mem- 

 bers of the same pair were of the same sex. Of the six pairs, three were 

 female and three.male. This uniformity is precisely what would be 

 expected, if these pairs arose by longitudinal fission, but quite the re- 

 verse of what we should anticipate had they been the result of accidental 

 juxtaposition. This evidence seems to me to favor strongly the view 

 that longitudinal fission is a regular method of increase in M. margina- 

 tum, though it does not exclude the possibility of some of the double- 

 mouthed forms being true monstrosities. It is, however, not my belief 

 that the specimen which was watched by me some two months and which 

 did not progress in division was a monstrosity. I am rather inclined to 

 Torrey's opinion, that the process of longitudinal fission is an extremely 

 slow one. This accords with what we know of the length of life of 

 Actinians, some having outlived human beings. 



In all the double-mouthed specimens which I have examined the two 

 partial individuals were of about equal size, so that longitudinal fission 

 in M. marginatum may be justly described as equal. In no case have I 

 ever observed any evidence of distinctly unequal longitudinal fission, 

 though one of the specimens figured by Agassiz (Plate I. Figs. 2 and 3) 

 shows some considerable inequality. In this respect M. marginatum 

 seems to be strikingly different from M. fimbriatum, which according 

 to Torrey ('98) divides more usually by unequal than by equal longi- 

 tudinal fission. 



In longitudinal fission, obviously, certain fundamental changes must 

 occur : new siphonoglyphs and new complete mesenteries, both directive 

 and non-directive, must be developed. In the material which I have 

 studied I have been able to discover no trace of the formation of new 

 siphonoglyphs. The usual symmetrical arrangement of two of the sipho- 

 noglyphs in each double animal suggests a process of division by which 

 one original siphonoglyph gives rise to two by longitudinal splitting, as 

 described by Torrey ('98). This is very likely one of the initial steps 

 in longitudinal fission, though it is not necessarily essential, as the con- 

 dition of specimen H (Plate III. Fig. 13) shows where the original sipho- 

 noglyph obviously did not divide. 



Of the formation of new directives I have seen absolutely nothing. 

 The production of new complete non-directives ia possibly indicated in 



