Dimorphism and Regeneration in Aietridium. 233 



distinguished on account of the advanced state of regeneration 

 of the polyps. 



The frequent occurrence of asexual reproduction in Metridium 

 explains the prevailing asymmetry of individuals in this species, 

 regenerated diglyphic polyps in particular being rarely sym- 

 metrical. It is usual to find the mesenteries arranged with more 

 primaries and secondaries on one side of the plane passing through 

 the siphonoglyphs than on the other. This condition is to be 

 explained by the fact that fragments, either spontaneously pro- 

 duced or formed artificially by random cuts from the base of the 

 foot-disc, arise without any definite reference to the parental 

 mesenteries which traverse that region. Hence, if the directive 

 mesenteries chance to be nearer one end of a fragment than the 

 other or if the new directives are formed nearer one edge of the 

 regenerated area than the other, an asymmetrical polyp results. 



The idea which has been advanced in the foregoing pages is 

 capable of giving an explanation also of the great variation in the 

 numerical proportions of monoglyphic and diglyphic polyps in 

 different localities. (See Table I.) If we suppose that in cer- 

 tain localities, like Newport, R. I., or at particular seasons of the 

 year sexual reproduction is favored, regular hexamerous diglyphic 

 polyps should at such places or seasons be relatively more abun- 

 dant. Torrey ('02) correctly explains as due to asexual repro- 

 duction patches of similarly colored sea-anemones, but the occur- 

 rence of diglyphic individuals among polyps asexually produced 

 does not, as he supposes, show that the diglyphic character has 

 been inherited as such, but rather that in these particular cases 

 the parental fragments happened to include a directive system. 

 The diglyphic hexamerous polyp of Aiptasia, described by Andres 

 and cited by Torrey ('02) as "evidence to be explained," may be 

 explained on the same basis. The dimorphism which, according 

 to Torrey ('02), occurs in polyps produced by budding from the 

 column of sea-anemones is doubtless capable of explanation in a 

 similar way. 



The production in the experiments here described of polyps 

 with a divided oesophagus and perhaps, in other cases, of two 

 directive systems formed simultaneously in the regenerated tissue 

 are worthy of notice as giving a clew to the origin of double mon- 

 sters and of triglyphic polyps. Both of these abnormal conditions 

 well known in collections of polyps, doubtless arise in spontaneous 



