52 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



about one fifth of the numbers of specimens ordinarily collected belong. 

 While I can confirm Torrey's ('98, p. 357) statement that diglyphic 

 and monoglyphic individuals may arise by the same mode of non-sexual 

 reproduction, I believe he has gone too far when he denies any possible 

 correlation between these structural types and the methods of repro- 

 duction. Since no mode of non-sexual reproduction has been shown to 

 give rise to a hexamerous diglyphic specimen, and since this type is rep- 

 resented by about one fifth of the number of individuals in the species, 

 it seems to me still possible that this may be the result of sexual i*epro- 

 duction. 1 The value of this suggestion must, however, await further 

 investigation. Should it prove true, much of the irregularity in the 

 arrangement of the mesenteries in Metridium would be associated with 

 non-sexual reproduction. 



Summary. 



The double specimens of Metridium examined had either two mouths 

 on one oral disk or two complete oral disks. In the former cases the 

 oesophageal tubes were incompletely divided (Y- or V-shaped) ; in the 

 latter, there were two completely distinct tubes. 



The mouths of the double specimens were usually monoglyphic, some- 

 times diglyphic, and in one instance aglyphic. Two siphonoglyphs were 

 usually placed symmetrically to the supposed plane of division. 



A pair of directive mesenteries was always attached to each siphono- 

 glyph. There were about twice as many non-directive mesenteries in 

 double specimens as in single ones. 



In any given case the assumed plane of division passed through either 

 two primary ectoccels or two primary entocoels, never through a primary 

 entocoel on one side and a primary ectocoel on the other. 



The double specimens were either male or female, and showed no evi- 

 dence of hermaphroditism. 



They are not due to fusion. 



1 In briefly discussing this question, Torrey ('9S, p. 357) has quoted me in a 

 somewhat misleading way. It is true that I suggested that the monoglyphic and 

 diglyphic types might have the value of varieties, and also that they might be cor- 

 related with the methods of reproduction, but tins was not done in one breath, as 

 might be inferred from Torrey's statement. I mentioned the possible interpretation 

 as varieties on page 269 of my former paper (Parker, '97), and, after stating my dis- 

 inclination to accept this interpretation, I suggested on page 270 the possibility of 

 correlation with the methods of reproduction. Why Torrey should have com- 

 bined these two suggestions as one, and why he should have used quotation marks 

 for a piece of composition which is not mine, remain to be explained. 



