Zool.-Vol. I.] TORRE Y—MONOGENESIS IN METRIDIUM. 347 



II. Longitudinal Fission. 



(a.) Equal. — The equality here is only approximate. No 

 perfectly equal fission has been observed, which is hardly 

 to be wondered at in such a variable genus as Metridium. 



Three cases of equal fission have been seen. The best 

 example is shown in section in fig. 1. This figure is drawn 

 to show the arrangement of the mesenteries and siphono- 

 glyphs in the oesophageal region, looking toward the mouths 

 from within. The six mesenteries diverging from B, three 

 running to each oesophagus, and the two mesenteries, one 

 bearing a mesenterial filament, which run between the two 

 oesophagi, are below the plane of the section. 



While the symmetry is not absolute, it is still striking. 

 At D and D' are the two pairs of directives, indicating the 

 siphonoglyphs, symmetrically placed to the plane of division 

 passing through A-B . There are eleven pairs of complete 

 mesenteries symmetrically placed with respect to the plane 

 of division; five pairs to each oesophagus, and one pair 

 divided, one member going to each oesophagus. The in- 

 comple ssenteries comprised between every two adja- 



cent p.' jf symmetrically placed complete mesenteries 



are, with the exception of the two sections 1-2 and 1'— 2', 

 remarkably symmetrical for such a variable species. 



(b.) Unequal. — This is the usual mode of division. Its 

 later stages cannot always be distinguished with certainty 

 from budding. Fifteen undoubted cases have been ob- 

 served. The progress of the division is as a rule from the 

 oral toward the foot disk. One specimen has been taken, 

 however, which was dividing in the reverse direction, as 

 described by McCrady, 1858, for A. cavernosa. The foot 

 disk had divided completely, and the plane of division had 

 parted the column for one-third of its length. The animal 

 died soon after its capture, which prevented dissection. 1 



1 Since this was written, the pedal disk of a polyp in the laboratory was accidentally 

 torn into two portions. These, upon healing of the surfaces of the wound, have 

 remained distinct, two apparently well-formed pedal disks being the result. The 

 exception mentioned in the text may have been due to a similar cause. 



