MUSEUM OF COMPARATIVE ZOOLOGY. 133 



behind the anus. The condition of the adult Ectoproct, on the other 

 hand, was reached by the curving oralwards, and the meeting of the 

 free ends of the rows of tentacles between the mouth and anus, thus 

 shutting the anus outside of their circle. In evidence of this latter 

 assertion, I submit the following comparative statement. 



As Nitsche has shown for Gymnolaemata, the tentacles on the ring 

 canal are first arranged in two rows, placed bilaterally, and meeting 

 in front, but not behind. Later the hindermost of the tentacles move 

 forward and toward the median plane, thus completing the circlet of 

 tentacles at a point behind the mouth, but in front of the anus. I be- 

 lieve the circumoral ring canal plus the early invaginations of the lopho- 

 phoric arms in Phylactolsemata to be homologous with the ring canal 

 of Gymnolaemata in its early stage ; like the latter, it is closed in front, 

 but has two free ends behind. The difference lies in the greater devel- 

 opment of the posterior ends of the canal, which latter have become 

 thrown into a vertical fold to afford space for more tentacles. At this 

 stage of development it would be difficult to say whether the anus 

 opened within or without the corona of tentacles. As in Gymnolaemata 

 the circle is completed by a movement inward of the posterior tentacles, 

 so in Phylactolsemata the corona of tentacles is completed in front of the 

 anus by the two anterior processes, can. crc.'", Figure 50 (cf. Fig. 44), 

 of the lophophore arm, which come to unite just behind the epistome, 

 Figures 52, 81, can. ere'" The lumen of this process of the lophophore 

 arm thus forms that portion of the ring canal which, as I shall show 

 directly, is the morphological equivalent of the most posterior portion 

 of the ring canal in Gymnolaemata. The tentacles which arise from 

 this portion of the ring canal are ontogenetically, and therefore phylo- 

 genetically, the youngest. As in Gymnolsemata, so here the moving for- 

 ward of the most posterior tentacles obliterates the basin-like floor of 

 the atrium, such as we see in Endoprocta, and leaves the anal opening 

 far outside the circlet of tentacles. 



The answer to the question, How may the horseshoe-shaped tentac- 

 ular corona of Phylactolaemata be homologized with circular ones'? is 

 involved in the answer to the preceding query. Nitsche ('75, p. 357) 

 believed the lophophoric arms to be "primary tentacles," and the 

 tentacles borne on them to be secondary tentacles, " Gar nicht ohne 

 Weiteres mit den Tentakeln der Infundibulata von Gervais zu vercrlei- 

 chen." The only evidence which he offers in support of his theory is 

 the fact that the tentacles on the lophophore arm arise later than the 

 arm itself. 



