222 University of California Publications. [zoology 



But while discontinuity of some sort is probably at the basis 

 of the phenomenon of fission in 8. davisi, it is apparent that the 

 experiments go no farther than to point out this fact. Why the 

 mesenteries in an unharmed polyp begin to group themselves 

 about two renters, and why opposite areas in the foot disk move 

 away from each other constantly only in polyps which are to 

 divide, are problems which still await solution. A possible 

 explanation of the direction of the fission plane may be suggested, 

 however. This plane passes perpendicular to the line of greatest 

 strain. This line of strain is parallel with the major month 

 axis, and at the ends of the mouth lie the directive mesenteries. 

 There can be little question that the arrangement of the muscle 

 bands on the outer sides of the directives and near the oesophagus 

 leads to mechanical results which are different from those 

 achieved by all the other muscle bands, which lie on the inner 

 sides of the non-directive mesenteries and farther from the 

 oesophagus. This mechanical difference is always correlated 

 with the shape of the mouth and may be sufficient to determine 

 the direction of the line of greatest strain and consequently the 

 direction of the fission plane in a dividing polyp. The uniform 

 failure to divide of polyps which were cut perpendicular to the 

 direction of the normal fission plane lends support to this view. 



III. HETEROMORPHOSIS. 

 Until the last few years heteromorphosis has been quite 

 unknown among the Anthozoa. A typical example was recently 

 obtained by Wilson (:03) in Henilla. a new hydranth regenerat- 

 ing on the aboral end of an extirpated axial polyp of a young 

 colony. Hazen ( :02), in discussing the factors which determine 

 the orientation of regenerating pieces of 8. luciae, says that a 

 pedal disk is produced at the point of contact with the substra- 

 tum, no matter how the piece falls, provided it is not subse- 

 quently disturbed. There is no specific statement that a pedal 

 disk was ever regenerated at the oral end of a piece, and the 

 brevitv of the account leaves this in doubt. There appear to be 

 no published observations on the appearance of a hydranth on 

 the aboral end of a regenerating anemone, though attempts have 

 been made on several species, notably by Loeb, to bring about. 



