262 BULLETIN : MUSEUM OF COMPAEATIVE ZOOLOGY. 



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 

 oesophagus 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 exocoels (directive mesenteries) and 

 those whose longitudinal muscles face the endocoels (non-directive 



mesenteries). 



The directive 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. 1 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. Carlgien, '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 (cf. McMurrich, '91, pp. 135, 137). 

 thou-h two pairs of directives are present, only one siphonoglyph 

 occurs; and in Ptychodactis (cf Appellof, '94, pp. 5 7), though two 

 pairs of directives can be seen, no siphonoglyphs are observable. These 

 instances serve to show that in some actinians directive mesenteries 

 xnay occur without siphonoglyphs, and thus they render more st^-lkmg 

 the correlation between the variations of the directives and of the 

 sinhonoo-lyphs in Metridium marginatum. 



The iLdirective mesenteries vary so much in their number and ar- 

 ranc^ement that thev can best be considered in connection with the par- 

 ticular tvpes with which they occur. In the diglyphic type (53 speci- 

 mens), in addition to the two pairs of directives, there may be from 

 four to ten pairs of non-directives. The frequency of the occurrence 

 of the different numbers of pairs is indicated in the following table : - 



