138 McMURRICH. [Vol. IV. 



second grade, and there are consequently twenty-three of them. 

 These third-grade mesenteries extend only a short distance below 

 the internal opening of the stomatodaeum, not more than a centi- 

 metre, and usually less than that. Like the longer ones, they 

 are perfect in their upper part. 



Figure 2, PI. VII., is a semi-diagrammatic representation of the 

 mesenteries of the various grades, and it will be seen from this 

 that there is still a fourth grade of mesenteries, shorter than 

 any that have hitherto been described, and alternating with the 

 mesenteries of the other three grades. There are, therefore, 

 forty-six of them, and altogether in all the grades, accordingly, 

 there are ninety-two mesenteries, one more than the number of 

 tentacles, marginal or oral, of which I counted ninety-one. These 

 fourth-grade mesenteries hardly reach below the lower opening of 

 the stomatodaeum, and are not readily seen in a preserved speci- 

 men, being usually overlapped by the adjoining mesenteries, and 

 further concealed by the tangled mass of acontia which arise 

 from the edges of the mesenteries just below the stomatodaeum. 

 The ventral directive mesenteries, as already mentioned, belong 

 to the fourth grade. 



Figures 3, 4, and 5 show some interesting features in the rela- 

 tions of the mesenteries of the four grades. They are transverse 

 sections of the ventral region of the column in its upper part. 

 Figure 3 passes through the stomatodaeum shortly above its 

 lower extremity, cutting its reflected portion. The ventral 

 siphonoglyphe {si) is readily made out, the ectoderm lining it 

 not being thrown into folds as it is elsewhere. The ventral 

 directives {D) are still in connection with the stomatodaeum, 

 but the other mesenteries have separated from it. Sections a 

 little higher up show that all the mesenteries are perfect ; but 

 the ventral directives retain their connection with the sto- 

 matodaeum throughout a greater portion of its length than do the 

 others. The mesentery (i) immediately adjoining the direc- 

 tives is one of the ventral continuous mesenteries, which at 

 this level are narrow, as are all the mesenteries immediately 

 below the point where they lose connection with the sto- 

 matodaeum. Succeeding it comes a mesentery of the fourth 

 grade (4), and following this one of the third grade {3), both of 

 the same width as the ventral continuous mesentery. The mes- 

 enterial filaments of these three have the same structure, being 



