130 BULLETIN OF THE 



of the tentacles also, are in free communication with the fluids of the 

 common body cavity. As Figure 51, Plate V., shows, the posterior ends 

 of the ring canals open into a pair of cavities which are the bases of the 

 lophophoric pockets, and by a comparison of Figures 61-63, can. crc, can. 

 crc.', Plate VII., it will become apparent that they each become confluent 

 with a furrow which passes up the lophophore arm, and from which the 

 outer lophophoric row of tentacles is developed. Further, by a com- 

 parison of can. crc", can. crc.'", in Figures 61-63, Plate VII. (dextro- 

 sinistral vertical sections), and Figure 50, Plate V. (horizontal section, 

 compare also Fig. 52, a sagittal section), it will be seen that from 

 the tip of the lophophoric arm a groove (can. crc") passes down upon 

 the side opposite to the ascending groove (can. crc'), and, reaching the 

 base, turns abruptly anteriorly (can. crc.'", Fig. 50), and finally, in 

 a later stage, becomes confluent with its fellow of the opposite side 

 in the median plane just behind the epistome and above the brain. 

 It would be quite unnecessary for me to give figures showing the 

 course of this supraganglionic canal (cf. Fig. 52, Plate V.). It has 

 long been recognized, and is shown in Kraepelin's ('87) Figure 66, 

 Taf. II. This is probably what Verworn ('87, pp. 114, 115, Figs. 20 a, 

 20b, Taf. XII.) has described as a "segmental organ." Braem ('89 b , 

 p. 679) has given to it the name "Gabelkanal." The " Ringkanal " of 

 Nitsche is, then, to my mind, merely the circumoral portion of a groove 

 which is elsewhere unclosed to form a proper canal and which lies at 

 the base of all tentacles. My reason for avoiding another term for the 

 unenclosed portion of the " canal " is, that I regard the whole as mor- 

 phologically equivalent to the ring canal of Gymnolsemata, which is 

 said to be closed throughout. 



2. Development of the Lophophore. — The early stages in the forma- 

 tion of this organ are well known, both from the descriptions of Nitsche 

 ('75, pp. 357, 358) and the earlier ones of Allman and others. 



I have already (page 114) shown how the cavities of the lophophoric 

 pockets become confluent between the rectum and ganglion, and how 

 their opposed walls, formerly passing over into each other through the 

 floor of the brain, are now anteriorly continuous by means of the new 

 floor of the atrium, and posteriorly are fused together. 



The union of the inner layers of the two opposed walls of the lopho- 

 phore arms (Plate V. Fig. 44, loph.') continues, however, for some dis- 

 tance above the floor of the atrium, up to within a short distance of the 

 tips of the young arms (Plate VII. Figs. 61, 62, loph.'). As the arms 

 grow longer, the relative extent of their free and fused portions remains 



