PARKER: METRIDIUM MARGINATUM. 265 
differences are due much more frequently to fundamental differences in 
the plans on which the mesenteries of different individuals are laid 
down than to the more easily conceived relation between complete and 
incomplete mesenteries. 
The incomplete mesenteries have not been exhaustively investigated. 
Their great number, variability in size, and the frequent difficulty met 
with in attempting to classify them, render such a task nearly impos- 
sible. In what are generally assumed to be the more typical specimens 
of Metridium (Fig. 1), an exocol may contain one pair of secondary 
mesenteries, two pairs of tertiaries, four pairs of quarternaries, and 
evidences (ridges) of eight pairs of quinaries. Though this condition 
was occasionally realized, in the great majority of cases irregularities in 
what are presumably secondaries and tertiaries, not to mention higher 
orders, were so numerous that consistent tabulation was out of the 
question. So far as size and position were concerned, what seemed to 
be secondaries showed such variations that no two specimens in which 
the arrangement of the complete mesenteries agreed, had similar arrange- 
ments of the secondaries, except in six instances of the 40 typical 
diglyphic specimens; and each of these six instances showed variations 
in the tertiaries characteristic of it as an individual. So far, then, as 
the incomplete mesenteries are concerned, we soon reach groups of vari- 
ations by which individuals may be characterized ; in other words, if the 
variations of the primaries (complete mesenteries), secondaries, and 
tertiaries be considered together, it will be seen that no two of the 
131 specimens examined were alike, each one having a combination 
of variations peculiar to itself. This is, perhaps, the most important 
feature in the variations of the incomplete mesenteries. 
That variations in the number of mesenteries, such as have been 
pointed out in the preceding paragraphs, occur in other actinians: is 
well known. Thus Carlgren (793, p. 106) states that in Metridium 
dianthus, in addition to a single pair of directives, six, seven, on even 
nine pairs of non-directives may occur, and F. Dixon (88) has shown 
that in several species of Sagartia the number of non-direotives may 
reach twelve or even sixteen pairs. Further, in four speeimens of 
Bunodes thallia, G. Y. amd A. F. Dixon (89, pp. 317, 318) found 
respectively 15, 19, 21, and 26 pairs of non-directives, These citations 
suffice to show that extensive variations in the mesenteries may occur 
in other actinians than Metridium marginatum, but the cases recorded 
for any one species are so few that generalizations cannot be drawn 
from them. 
