Zool.— Vol. I.J TORREY— MONOGENESIS IN METRIDIUM. 357 



The formation of complete mesenteries in the bud from 

 incomplete mesenteries in the parent is again seen in fig. 

 n«, where pair c is formed from the incomplete pair imme- 

 diately above it in the parent. 



These colonies have manifestly no definite architecture 

 such as is possessed by Amphihelia and other of the Mad- 

 reporaria. Whether the great diversity of methods of 

 asexual reproduction in Metridium indicates a decline from 

 a former widespread colonial habit, future investigations 

 must decide. The percentage of monogenetic polyps is 

 too small to make it probable that monogenesis assists Me- 

 tridium in its struggle for existence. Nor does it seem 

 more probable that, with the possible exception of basal 

 fragmentation, it can be accounted for on purely physio- 

 logical grounds. 



Parker, 1897, has suggested the possibility that the mono- 

 glyphic and diglyphic types of Metridium may be of the 

 value of varieties, "the products, one of the sexual, the 

 other of the asexual mode of reproduction." My observa- 

 tions lead to an opposite conclusion. Both monoglyphic 

 and diglyphic polyps reproduce by fission : and both may 

 result from fission (figs. 3-7). Further, a bud from the 

 oesophageal region may possess either one or two siphono- 

 glyphs. One bud produced upon the foot disk had, at the 

 time of liberation, but one siphonoglyph. Again, as has 

 already been mentioned, two buds were found to have 

 arisen independently from a single basal fragment: the one 

 is monoglyphic, the other diglyphic. There can be little 

 doubt, then, that the monoglyphic and diglyphic types are 

 not of the value of varieties ; such variation is not correlated 

 with the process of asexual reproduction. 



VI. Summary. 



Monogenesis is a well established process in Metridium 

 fimbriatum; longitudinal fission, laceration, and budding 

 from oesophageal and foot regions occur. Monogenous 

 individuals may be sexually mature. 



