76 University of California Publications in Zoology [VOL. 13 



or it may be continuous with the oral disk as in figure 2, plate 3, or it 

 may end abruptly as in figures 6 and 7, plate 5, all depending upon 

 the state of contraction and retraction or expansion and extrusion of 

 the oral region. 



The adoral membranelles and the oral tufts of cilia are very 

 different, both in their composition and in their arrangement. 

 Starting from a point (x, fig. A) slightly anterior. and ventral to the 

 left extremity of the dorsal row of membranelles, the row of adoral 

 membranelles (ador. m., figs. A, B) circles at first ventrally and 

 slightly posteriorly, then to the right and slightly anteriorly. Still 

 ascending, it next curves dorsally and then to the left, reaching its 

 highest level as it crosses the sagittal plane. Still curving to the left 

 and ventrally, it gradually descends to a more posterior level, where it 

 occupies a position internal and ventral to the starting-point and on 

 a plane about midway between its highest and lowest levels. At this 

 point (y, fig. A) the adoral zone proper may be said to terminate, 

 but the row of membranelles turns suddenly inward, again ascends to 

 a higher level, then recurving upon itself begins the secondary inner 

 circlet, which runs parallel with, but in a direction exactly opposite 

 to, that of the outer row of adoral membranelles. At the sudden turn 

 (y, fig. A) the membranelles become much shorter and finer, lose their 

 brushlike construction, become somewhat flattened in appearance, and 

 may now be described as a row of less distinct groups of large cilia 

 surrounding the mouth or oral opening, viz., oral cilia (or. cil., figs. A, 

 B, C; pi. 3, figs. 1, 2; pi. 4, fig. 3). The membranelles of the adoral 

 zone resemble those of the dorsal zone with the exception that they are 

 neither so long nor do they individually contain so many ciliary 

 processes. There are from thirty to thirty-six of these membranelles, 

 each consisting of from forty to fifty separate cilia. Their bases are 

 situated in the ectoplasm immediately posterior to the inner adoral 

 furrow. Here also, as in the case of the dorsal membranelles, each 

 membrarielle seem to be composed of two sets of roots, which, however, 

 in this case must be designated as internal and external roots. The 

 internal roots take their origin from the ectoplasm in the region of 

 the boundary layer, and the external roots take their origin from the 

 ectoplasm which lies close to the outer wall of the body. The oral 

 cilia are too short and too fine and are located too centrally to be of 

 much service as organs of locomotion, but since they are a direct con- 

 tinuation of the adoral row of membranelles they will be described in 

 this place. 



