PAEKER : 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 mcom23lete 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 exocoel 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 ('93, p. 106) states that in Metridium 

 dianthus, in addition to a single pair of directives, six, seven, or even 

 nine pairs of non-directives may occur, and F. Dixon ('88) has shown 

 that in several species of Sagartia the number of non-directives may 

 reach twelve or even sixteen pairs. Further, in four specimens of 

 Bunodes thallia, G. Y. and 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. 



