216 Charles Lincoln Edwards 



turns around the tentacle. At the base of the sucker is a supporting 

 ring giving off branches running out at right angles into the wall 

 of the sucker. Guarding the anus are two lateral and one posterior, 

 broadly based, fan-shaped perforated plates (PL I, Fig. 6 ; Pis. 

 II-III, Figs. 9, 10, 12, 14, vv, y) which wave in and out with the 

 contractions of the cloacal muscles. I have interpreted these as 

 structures similar to vestigial anal teeth (1909). 



III. Tentacles. 



In the fourth day, four of the five primary tentacles grow out 

 from the bases of the radial canals, two before and two behind, the 

 peristomial groove. During this day the tentacles extend, pushing- 

 ahead of themselves the overlying ectoderm of the peristome, together 

 with the intervening mesenchyme. The lip of the peristome, divided 

 by the groove into anterior and posterior halves, is moved in and 

 out by the contractions of the developing tentacles within. At this 

 time four of the primary tentacles are well marked (PI. I, Figs. 

 4-5), one arising from the mid-ventral radial canal to the right 

 (MVrl), and one dorsad from the right (RVdl) and left ventral 

 (LVdl) and the left dorsal (LDdl) radial canals respectively. In 

 addition, the hud of the fifth primary tentacle appears, arising from 

 the mid-ventral radial canal to the left. Thus the four-day embryo 

 presents a w^ell marked tentacle in each interradius except the right 

 ventral, which, however, contains the bud of the fifth primary 

 tentacle. Since MVrl and LDdl are larger than RVdl and LVdl 

 (PI. I, Fig. 4), it is probable that the two former arise first. The 

 ventral radial canal has grown much faster than the others and ends 

 posteriorly in the bud of the first pedicel (PI. I, Figs. 4-5, pvp). 



During the fifth day, while the embryo is still within the vitel- 

 line membrane, the tentacles grow but slightly. At this time the 

 tentacles contract comparatively rapidly. They push out against 

 the vitelline membrane now covered on the outside by slime, which 

 contains diatoms and other protists and their debris. Once in a 

 while a tentacle adheres to the unbroken egg-shell, pulling it in. 

 By the sixth day the embryo has succeeded, by constant manipula- 

 lation with the tentacles, in breaking the vitelline membrane. The 



