PARKER: METRIDIUM MARGINATUM. 269 
Sagartia, in which, as Haddon (789, p. 300) remarks, it seems impossible, 
in our present state of knowledge, to determine dorsal and ventral rela- 
tions. It is probable that this determination can be made only after 
the sequence of development of these mesenteries has been discovered. 
In the four types of sequence thus far known (cf. Fowler, 94, p. 470), 
the ventral directives are always the third pair of mesenteries to form, 
and the dorsal directives either the second or fourth. It is probable 
that, when the developmental sequence of the mesenteries is discovered 
for the two types of Metridium, the determination of dorsal and ventral 
in this actinian will he made with as much certainty as in any other, 
and we shall probably then know whether in the monoglyphie type tho 
single siphonoglyph is a dorsal one, a ventral one, or in some specimens 
one and in others the other. 
Before concluding this account of the mesenteries in Metridium, I 
wish to consider briefly some other aspects of the monoglyphio and 
diglyphie types. When I first perceived that there were two structural 
types in Metridium, I suspected that they might be correlated with 
sexual differences. To test this question, I determined the sexes of a 
number of individuals of each type. In ten monoglyphie specimens, 
five were females and five were males; in twenty-seven diglyphie speci- 
mens, fifteen were females and twelve were males. Evidently the two 
types are not correlated with difference in sex. 
The fact that the two sexes occur in about equal numbers under both 
types suggests that these two types may in reality be two varieties of 
the species Metridium marginatum. In support of this opinion, it may 
be mentioned that the two types show a difference in the degree of their 
variability, tho diglyphic type having only one subtype, the monoglyphic 
three; and, further, that, while the diglyphic type presents usually a 
rather typical Hexactinian arrangement of mesenteries, the monoglyphie 
type shows a general tendency to crowd the non-directive mesenteries to 
the region opposite the one siphonoglyph. 
These differences, however, fairly marked as they are, are insufficient 
in my opinion to warrant the assumption that the two types are really 
varieties, and tho determination of this question must wait, I believe, 
till more is known of the breeding habits of Metridium. If it can be 
shown that in the offspring of one animal representatives of both typos 
occur, the idea that we are dealing with varieties could not be main- 
tained, and the species could at most be said to be dimorphic. If, how- 
ever, tho types could be shown to breed true, they might with justice 
be described as varieties. 
