pakker: metridium marginatum. 269 



Sagartia, in which, as Haddon ('89, p. 300) remarksj 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 (of. 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 se(\uence of the mesenteries is discovered 

 for the two types of Metridium, the determination of dorsal and ventral 

 in this actinian will be made with as much certainty as in any other, 

 and we shall probably then know whether in the monoglyphic type the 

 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 monoglyphic and 

 diglyphic types. AVhen 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 monoglyphic specimens, 

 five were females and five were males ; in twent^'-seven diglj-phic 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, the 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 monoglyphic 

 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 waiTant the assumption that the two types are really 

 varieties, and the 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 types 

 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, the types could be shown to breed true, they might with justice 

 be described as varieties. 



