REPORT ON PHORONIS BUSKII. 13 



has a rounded opaque mass of the circuhiting fluid, surmounted in many by a granuUir 

 band, as if from a fine wall, it is possible that it represents the vessel (or at least a 

 division or septum) with its contents. Proceeding distally, both sets of channels (in 

 the double rows) become complete, and the hypoderm covers both the outer and inner 

 edges of each — the somewhat spongy vascular tissue having disappeared. Then eacli 

 row becomes more individualised, having a thin layer of hypoderm on the inner or 

 concave edge, and a prominent pennate mass (in section) — several times thicker — on 

 the outer. The rings are bound together only by a little connective-tissue in the 

 middle, and they soon become free (PI. I. figs. 7 and 8 — in transverse section), the 

 outer layer of hypoderm still remaining thicker, while that on the sides (where the 

 densest portion of the chitinous ring exists) is less developed. When viewed longitu 

 dinally, this peculiar wall of basement-tissue has a series of very bold and rather regulai 

 transverse folds or wrinkles ; and an included vessel and its contents are apparent. 

 The presence of but a single well-marked vessel is of moment in connection with 

 Professor AUman's remark that the blood-stream returned by the same vessel in the 

 tentacle, though it entered the vein at the bottom. 



A special arrangement of the branchial apparatus occurs at the mouth (PI. III. fig. 

 3). The oral surface of the region being formed of a continuous web at the base of the 

 branchiae presents a great contrast to the anal, which has a large median hiatus between 

 the two fans. The second whorl on the oral side, however, is formed of two halves, 

 with a median slit, each half springing from a free edge with short tentacles. If the 

 animal be placed with the anal hiatus between the fans uppermost, then the left whorl, 

 with its short tentacles and sense-organ, is in front, the right free edge with its 

 tentacles being behind the former. 



The skeletal elements in the branchial apparatus of this form thus show a further 

 development of the homologous tissue in Cejjhcdodiscus and Rhahdople-ura. The most 

 elementary type is observed in the latter, in which no vascular spaces occur in 

 connection with the bases of the lophophoral plumes. Cephalodisctis again is further 

 differentiated, very considerable vascular channels being present at the Ijase of the 

 apparatus, and the central region of each plume having apparently reticulate tissue 

 capable of transmitting fluid. The degree of specialisation, however, attained by 

 Phoronis in this respect much exceeds that in Cephalodiscus, for special vessels pass 

 along the whole length of the tentacular processes, and the contained fluid is richly 

 corpusculated. 



The bases of the whorls show a thickened plate of hypoderm, which probably 

 performs special functions in connection with the movement of the water (by ciliary 

 action), and perhaps also with sensation. 



The arrangement of the tentacles in the forms described by Wright, AUman, and 

 Dyster seems to difl'er considerably from that in the present species. In the former the 



