PARKER : LONGITUDINAL FISSION IN METRIDIUM MARGINATUM. 51 



several cases. In specimen C a curious condition was observed, which is 

 shown in the diagram (Plate III. Fig. 8). Two complete mesenteries 

 start from the column wall, but unite before they reach the oesophageal 

 wall. The cavity which they enclose may be called a primary entoccel 

 so far as their longitudinal muscles indicate, but it contains two incom- 

 plete mesenteries which pair off with the united complete ones. The 

 condition might be interpreted as due to the splitting of a complete me- 

 sentery and the supplementary development of incomplete ones, though 

 this interpretation is not without objections. I am more inclined to look 

 on the condition as purely abnormal. In another case, the mesentery 

 marked 1 in Figure 7 (Plate II.), whose mate is incomplete, is itself 

 complete only through a small portion of its oral length. This might be 

 a mesentery originally incomplete, which by active growth had completed 

 itself in the oral region. A second similar case was observed among 

 the six natural pairs already mentioned. Beyond these meagre facts, I 

 observed nothing suggestive of the methods by which new complete 

 mesenteries may be formed. 



In all the specimens examined by me the fission was progressing from 

 the oral toward the pedal pole of the animal, never in the reverse direc- 

 tion, as described by McCrady ('59, p. 275) for Actinia cavernosa, by 

 ('arlgren ('93, p. 31) for Protanthea simplex, and as may also occur in 

 Cjreactis as described by "Wilson ('89, p. 38), who however did not 

 place this interpretation on what he saw. The direction of fission in 

 M. marginatum corresponds to that generally found in M. fimbriatum 

 as described by Torrey ('98). 



The kind of individuals produced by equal longitudinal fission in M. 

 marginatum can be easily inferred from the foregoing account. They 

 are usually monoglyphic, though occasionally diglyphic, possibly on rare 

 occasions aglyphic. A pair of directive mesenteries is regularly attached 

 to each siphonoglyph. The animals possess the usual number of com- 

 plete non-directive mesenteries. These, however, do not show the regu- 

 lar hexamerous arrangement often met with in this species, but are as 

 a rule very irregularly disposed. As I have elsewhere indicated 

 (compare Parker, '97, pp. 263, 264), M. marginatum is represented 

 by two chief types : a diglyphic one often characterized by a strictly 

 hexamerous arrangement of its mesenteries and a monoglyphic one 

 in which amongst other forms three sub-types, distinguished respectively 

 by having five, six, and seven pairs of non-directives, may be recognized. 

 The animals produced by longitudinal fission may come under any of 

 these heads except the regularly hexamerous diglyphic form, to which 



