266 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
As a rule, 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 à 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 non-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 diglyphie condition. This assumption is at least convenient, 
for it allows ys to distinguish in each pair of lateral non-directives a 
dorsaland 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 correlated 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 diglyphie 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 incompleto (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 mot with in the monoglyphic type. 
Many pairs of mesenteries in which both members are incomplete 
