262 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
two siphonoglyphs (occasionally one), thereby recognizing the mono- 
glyphic and diglyphic types as normal. 
Mesenteries. — In Metridium marginatum the pairs of mesenteries are 
attached lengthwise to the wall of the column, and either reach the 
cesophagus and unite with it (complete mesenteries) or fall short of 
that structure (incomplete mesenteries) Of the pairs of complete 
mesenteries the two usual kinds can be distinguished: those whose 
longitudinal muscles face the exocols (directive mesenteries) and 
those whose longitudinal muscles face the endocods (non-directive 
mesenteries). 
The dérective mesenteries are remarkable for the constancy of their 
relations to the siphonoglyphs. To each siphonoglyph is attached a 
single pair of directives, and in no instance among the 131 specimens 
examined was an exception to this rule found. In the monoglyphic 
type (Figs. 3, 4, 5, 7, and 8) one pair, and only one pair, of directives 
was present; in the diglyphic type (Figs. l and 2) two pairs were 
invariably observed ; and even the single specimen with three siphono- 
glyphs (Fig. 6) formed no exception, but exhibited three pairs of 
directives. 
This exact correlation between the number of siphonoglyphs and of 
directives, which probably also obtains in other species of Metridium 
(cf. Carlgren, 93, p. 106), as well as in the allied genus Sagartia (cf. 
F. Dixon, ’88, p. 136), is rather striking, because .the two sets of 
structures concerned are not invariably thus associated in all actinians. 
For instance, in Peachia and Oractis (ef. MeMurrich, ’91, pp. 135, 137)» 
though two pairs of directives are present, only one siphonoglyph 
occurs ; and in Ptychodactis (cf. Appellóf, 94, pp. 5, 7), though two 
pairs of directives can be seen, no siphonoglyphs are observable. These 
show that in some actinians directive mesenteries 
and thus they render more striking 
s of the directives and of the 
instances serve to 
may occur without siphonoglyphs, 
the correlation between the variation 
siphonoglyphs in Metridium marginatum. 
The non-directive mesenteries vary so much in their number and ar- 
rangement that they can best be considered in connection with the par- 
ticular types with which they occur. In the diglyphie type (53 speci- 
in addition to the two pairs of directives, there may be from 
mens), 
The frequency of the occurrence 
four to ten pairs of non-direocti ves. 
of the different numbers of pairs is indicated in the following table: — 
