Zool.— Vol. 1.1 TORREY—MONOGENESIS IN METRIDIUM. 349 



the division in the first two cases. The remaining two cases 

 were essentially similar to that shown in fig. 3c, though 

 more nearly equally divided. Figure 4 represents a di- 

 glyphic polyp. Figure 6 in all probability represents the 

 result of the division of another diglyphic polyp. This is 

 an unique case. The plane of division seems to have 

 passed through both siphonoglyphs of the original polyp, 

 as indicated by the division of the two pairs of directives. 

 The siphonoglyphs, immediately upon their division, were 

 probably approximated by the longitudinal constriction of 

 the oesophagus which usually follows the division of a 

 siphonoglyph. Opposite halves of the divided siphono- 

 glyphs could then unite by their inner edges to form the 

 two derivative siphonoglyphs, one on either side of the 

 plane of division. 



There are reasons for believing that this method of divi- 

 sion so plainly exhibited in the foregoing six cases is the usual 

 one. In each of the eight remaining cases of longitudinal 

 fission (excepting the single instance of division transversely 

 to the mouth) there is a marked symmetry in the position 

 of the siphonoglyphs with respect to the plane of division. 

 This has been alluded to already in connection with fig. 1, 

 where the greater axes of the mouths, passing through the 

 siphonoglyphs, form with the plane of division angles of 

 25 degrees. In fig. 7 these angles are about 15 degrees; 

 in fig. 8, 45 degrees. In five other specimens the angles 

 are respectively 45, 35, 35, 15, o degrees. 



The smallness of these angles suggests the conclusion 

 that each pair of diverging siphonoglyphs has arisen from 

 a single siphonoglyph by longitudinal fission. This con- 

 clusion is supported by the conditions found in figs. 7 and 

 8. The triglyphic type is exceedingly rare, two instances 

 having been counted among 451 individuals. This decid- 

 edly lessens the probability that these two cases were orig- 

 inally triglyphic polyps. Moreover, each could be derived 

 from fig. 4 by the completion of the directives and the 

 introduction of two pairs of non-directives between the pairs 

 of directives. Such cases as the one represented in fig. 1 



