PARKER: METRIDIUM MARGINATUM. 267 
show variation of the kind indicated above, in that one member is 
larger than the other (Fig. 4), but because of the extreme variability of 
these parts no record has been kept of such variations. 
In a few cases single mesenteries have been observed (Fig. 2). These, 
as the arrangement of the longitudinal muscles of their neighbors shows, 
have absolutely no trace of a mate. In the instance figured, it is diffi- 
cult to decide which of the two mesenteries, the complete (y) or the 
incomplete (x), is the single one. One or other must be. Single mesen- 
teries as exceptions have already been recorded by F. Dixon (88, p. 138) 
in Sagartia, and by Carlgren (793, p. 106) in Metridium. 
Among the complete mesenteries, two cases of union by what would 
have been the median margins of the participants have been observed 
(Fig.8). An instance of this kind has already been recorded by R. 
Hertwig (82, p. 37) in Tealia, and in this, as in Metridium, the united 
mesenteries were not members of the same pair, but of adjacent pairs. 
No instances of the occurrence of longitudinal muscles on both the 
exoool and the endoccel face of the same mesentery, as observed by 
McMurrich (89, p. 30) in Aulactinia, have been noticed. 
So far as the mutual arrangement of complete and incomplete 
mesenteries is concerned, the monoglyphie and diglyphic types show 
rather characteristic differences. In the diglyphie type the complete 
mesenteries usually show no special tendency to collect at one pole or 
the other of the animal (cf. Fig. 1). In the monoglyphic type there is 
often a marked tendency for all but two pairs of the non-directives 
to collect opposite the directives (cf. Fig. 5) ; consequently the half of 
the animal centering about the directives has an arrangement of parts 
like that found in the corresponding half of a diglyphic animal, while 
the other half contains a more or less orowded group of non-directives. 
In this respect Metridium seems to differ from Sagartia, in which, 
according to the figures given by F. Dixon ('88, Plate I.), such a 
crowding of non-directives is not noticeable, This condition recalls in 
a superficial way that found in Cerianthus, in which an active growth 
of mesenteries takes place opposite the siphonoglyph. 
The characteristic arrangements of the mesenteries in connection with 
the monoglyphic and diglyphic types probably recur under similar con- 
ditions in M. dianthus; for such arrangements have been figured by 
Thorell (59, Tab, I. Figs. 1 and 2) and briefly described by Carlgren 
(793, p. 106). 
While the crowding of the mesenteries occurs as a rule only in mono- 
glyphic specimens of Metridium, one instance of it has been observed 
