266 BULLETIX: MUSEUM OF COMPAEATIYE ZOOLOGY. 



As a riile, variations in the mesenteries occur in both members of a 

 pair in the same way, but not infrequently one finds pairs in which the 

 two members are not equally developed. When this occurs amongst 

 the complete or nearly complete mesenteries, it may result in the forma- 

 tion of a pair one member of which is complete and the other incom- 

 plete (Fig. 2). The 131 specimens of Metridium examined possessed 

 in all 739 pairs of nou-directives, and, of these, 17 pairs (or about 2.4 

 per cent), distributed through thirteen individuals, possessed each an 

 incomplete member. Of the thirteen individuals exhibiting this vari- 

 ation, ten were of the monoglyphic type, and three of the diglyphic 

 type. In the monoglyphic type it is customary to assume that the 

 single siphonoglyph present corresponds to the so-called ventral one 

 of the diglyphic condition. This assumption is at least convenient, 

 for it allows us to distinguish in each pair of lateral non-directives a 

 dorsal and a ventral member. Admitting this distinction for the sake 

 of description, it may be said that seven of the ten monoglyphic 

 specimens had each a single pair of non-directives in which one mem- 

 ber was incomplete, and of these incomplete mesenteries four were 

 dorsal, two were ventral, and one was indeterminable (Fig. 8) ; and 

 that the three remaining monoglyphic specimens had each two such 

 pairs, of which in one instance both the incomplete mesenteries were 

 dorsal, and in two instances one was a dorsal and the other a ventral 

 mesentery (Fig. 7). Thus in the ten monoglyphic specimens, this 

 variation was observed in thirteen pairs of mesenteries, of which eight 

 presented incomplete dorsal members, four incomplete ventral members, 

 and one was indeterminable. It is evident that this variation is not 

 limited to either dorsal or ventral members, and is not con-elated with 

 the fact that in many actinians ventral members, as a rule, develop later 

 than dorsal ones ; in other words, this variation is probably not to be 

 regarded as atavistic. 



In the adult condition of the diglyphic type, I see no way of distin- 

 guishing dorsal from ventral, and the most that can be said of the three 

 cases of variation met with under this type is that in two of them only 

 one mesentery each was incomplete (Fig. 2), while in the third two were 

 incomplete. In the latter case the two mesenteries (as in Fig. 7) were 

 not on corresponding sides ; hence one of them must have been dorsal 

 and the other ventral, but exact determination could not be made. The 

 variations in the diglyphic type, then, present no essential features 

 not already met with in the monoglyphic type. 



Many pairs of mesenteries in which both members are incomplete 



