PARKER LONGITUDINAL FISSION IN METRIDIUM MARGINATUM. 47 



lives is so placed that, while they start from nearly the same place on the 

 outer wall, they pass to different oesophageal tubes. The pair opposite 

 these has similar connections, so that at this stage the assumed plane of 

 division may be said to pass through primary entocals on both sides. 

 The same is true of G (Fig. 12), and Turrey ('98, Plate XXI. Fig. 1) fig- 

 ures what seems to be a similar case, though in his description (p. 347) 

 he mentions only one pair of mesenteries as divided between the oesopha- 

 geal tubes. Apparently there is some discrepancy here, but whether it 

 is the description or the figure which is faulty, it is impossible to say. 

 In the remaining specimens examined by me, the division plane always 

 lay in primary ectocoels. It is noteworthy that in the nine cases 

 studied none showed the plane of division passing through a primary 

 ectocoel on one side and a primary entocoel on the other. In this re- 

 spect the specimens were always perfectly symmetrical, a condition 

 which may hold for other Actinians, as suggested by the symmetrical 

 division of Zoanthus thomensis as described by Koch (^^^, p. 33). 



The incomplete mesenteries formed what were usually irregular groups 

 in the primary ectocoels. Their arrangement seemed to bear so little 

 on the question of division that I have not attempted a description of 

 them. In the figures of transverse sections the incomplete mesenteries 

 are either represented in full in a given primary ectocoel or their pres- 

 ence is indicated by x. Primary ectocoels not marked in one or other 

 of these ways contained no incomplete mesenteries. 



Of the double-mouthed specimens whose sexes could be determined, 

 four were females and three were males. Xo evidence of hermaphro- 

 ditism was observed, though the specimens were carefully scrutinized in 

 this respect. Obviously the double-mouthed condition is not peculiar to 

 either sex. 



The interpretations that have been placed upon these double-mouthed 

 specimens have already been stated. Johnston's idea that they have 

 arisen by the fusion of two originally independent individuals seems to 

 me entirely unwarranted. M. marginatum is represented by individuals 

 showing extreme variations in color and markings, and yet the two mem- 

 bers of all double specimens seen by me have been strikingly similar. 

 If fusion were the means of forming double animals, particolored combi- 

 nations ought occasionally to occur, but such I have never seen. 



There remain then the two suggestions of monstrosities and of stages 

 of fission. To test which of these was the correct interpretation, I at- 

 tempted to watch the process in what might be assumed to be a dividing 

 individual. This animal was found at Wood's Hole, August 6th, 1898, 



