402 TORREY 



one siphonoglyph. Again, . . . two buds were found to have arisen 

 independently from a single basal fragment ; the one is monoglyphic, 

 the other diglyphic." The causes of such variation evidently lie 

 deeper than the method of reproduction. 



The same causes must account for triglyphic and polyglyphic forms 

 also. While monoglyphic and diglyphic polyps may arise by fission 

 and CESophageal budding and so derive their siphonoglyphs immedi- 

 ately from those of the parent, triglyphic and polyglyphic polyps 

 probably arise in two ways only, viz : from egg embryos and from 

 basal fragments. In Metridiuin only a fraction of one percent of 

 the total number of polyps is triglyphic, and polyglyphism is unknown. 

 Among seven specimens of Sagartia spoiiglcola^ according to Mc- 

 Murrich in 1S97, only one had two pairs of directives, irregularly 

 placed ; one had four pairs ; the others had three pairs each. Two 

 were hexamerous, two heptamerous, and three octamerous. In four 

 specimens of Bunodes thallia^ G. Y. Dixon and A. F. Dixon found 

 one siphonoglyph in one, two in another and four in another. In the 

 same connection Condylactis raitisayi is mentioned as having from 

 two to seven siphonoglyphs. 



It is not, however, upon the arrangement of mesenteries alone nor 

 the number of siphonoglyphs alone that the suggestion of a correla- 

 tion between variation and method of reproduction is based, but rather 

 upon the relation of these two sets of structures to each other. This 

 brings us to the third point in the discussion. 



(c) Under this head I hope to show that, with the possible excep- 

 tion of polyps with a non-typical number of mesenteries, it is not nec- 

 essary to assume a correlation between structural type and mode of 

 reproduction ; but that, if there is any correlation at all, it exists more 

 probably between the number of siphonoglyphs and the number of 

 mesenteries, based upon ancestral tendencies and irrespective of repro- 

 ductive processes. Polyps w^ith a non-typical number of mesenteries 

 are excepted because (i) many of them are produced by basal frag- 

 mentation and the cause of their non-typical condition is obvious ; (2) 

 they are not actually known to be produced from the egg. Whether 

 the exception will persist when the development of ATetridhuti is 

 fully known remains to be seen. 



That diglyphic hexamerous polyps develop not only sexually but 

 non-sexually is established, I think, by the following considerations. 



In the first place, I have already referred (p. 39S) to a case of 

 fission in which one member of the group was hexamerous and di- 

 glyphic, and approximately regular. I shall not insist upon this case 



