paekee: meteidium maeginatum. 267 



show variation of the kind indicated above, in that one member is 

 larger than the other (Fig. 4), but because of the extreme variabiUty 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. 1 38) 

 in Sagartia, and by Carlgren ('93, 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 E. 

 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 

 exoccel 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 monoglyphic and diglyphic types show 

 rather characteristic differences. In the diglyphic 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 aiTangement of parts 

 like that found in the corresponding half of a diglyphic animal, while 

 the other half contains a more or less crowded 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 

 ('93, 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 



