MUSF.UM OF COMPAKATIVE ZOOLOGY. 133 



behind the aims. The condition of the advdt Ectoproct, on the other 

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

 free ends of the rows of tentacles between the moutli and anus, thns 

 shuttiniT the anus outside of their circle. In evidence of tliis latter 

 assertion, 1 submit the following comparative stiitement. 



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

 canal are first arranged in two rows, placed l)i]aterally, and meeting 

 in front, but not behind. Later tlie hindennost uf the tentacles move 

 forwaid and toward tlie median plane, thus comiileting the circlet of 

 tentacles at a point behind tlie inoutli, but in front of tlie anus. I l)c- 

 lieve tlie circnnioral ring canal 5)lus tlie early invaginations of the lopho- 

 jihoric arms in Phylactolremata to be homologous with the ring canal 

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

 l)ut has two free ends behind. The difference lies in the greater devel- 

 opment of the posterior ends of tlie canal, which lattei- have become 

 thrown into a vertical fold to alford space for more tentacles. At tliis 

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

 opened within or without the corona of tentacles. As in Gymnola-niata 

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

 so in Phylactolccmata the corona of tentacles is cmiijileted in front of the 

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

 of the lo[)liophore arm, which come to imite just behind the epistome, 

 Figui'es o2, 81, can. err.'" The lumen of this pi'ocess of the lophopliore 

 arm thus forms tliat portion of the ring canal which, as I sh.ill show 

 directly, is the morphological equivalent of the most ])osterior portion 

 of tlie rin<>' canal in OvninoliL'iuata. The tentacles which arise from 

 tiiis portion of tlie ring canal are ontogcnetically, and thei'cfore phylo- 

 "eneticallv, the voimuest. As in (Ivmnohvmata, so liere the moviuu- foi-- 

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

 the atrium, siicli as we see in Kndoprocta, 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 riiylactolaDuiata l)e homologized with circular ones? is 

 involved in the answer to the jireceding query. Nitsche C"-"), p. 3.')7) 

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

 tentacles borne on tliem to l)e secondary tentacl(>s. '• ( lar iiicht ohne 

 Weiteres niit den Tentakelu der Infundibulata von (Imunais v.w vcrglei- 

 chen." The only evidence which he offers in suppoi-t of his theoi-y is 

 tlie fict that the tentacles on the lophopliore arm arise latfi- than the 

 arm itself 



