REPORT ON PHORONIS BUSKII. 25 



known AspidopLorous or other Polyzoon, except as regards the lophophore, the 

 structure of which approaches in certain respects that of Cephalodiscus and Rhahdo- 

 pleura. The only trace of the buccal shield is the epistome, which in this species 

 appears to be represented by the two peculiar hypodermic organs with the external 

 apertures, which probably subserve a sensory function. 



Branchial and Circulatory Systems. — In the structure of the branchial tentacles, 

 as just mentioned, a common plan pervades Cephalodisciis with its allies and this form, 

 since the basement-tissue and the hypodermic investment are similar, though it must 

 be borne in mind that other types present a close resemblance. These branchial 

 tentacles appear to arise from a region corresponding to the collar-region of Cephalc- 

 discus. Moreover, the blood-vessel contained in each filament, and the great trunks in 

 the vascular space at the base in Phoronis, are diagnostic. Further, the blood-vessels 

 have distinct and highly contractile walls in every case, and the contained red 

 nucleated corpuscles are remarkable. This system therefore differs from the lacunar 

 arrangement at the base of the plumes in Cephalodiscus. 



Digestive System. — This closely corresponds in arrangement with that in the 

 Aspidophorous group of the Polyzoa, as well as with the latter in general. The various 

 parts of the apparatus, its minute structure, the approximation of mouth and anus, are 

 all features common to the group. Moreover, it is not easy to see on what grounds Mr. 

 E. B. Wilson ^ has assumed that the flexure of the gut in Phoronis renders it probable 

 that a primitively straight form by the force of a tubicolous habit has become bent, and 

 the anus by and by conveniently fixed near the mouth. The illustration he takes, viz. 

 Sahellaria, does not seem to have much weight, for there are many other tubicolar 

 annelids in which a very diff'erent condition exists, yet the curvature of the tail, 

 according to ]\Ir. Wilson's hypothesis, would be equally necessary. This author further 

 states that the resemblance of Phoronis to the Polyzoa is an entirely secondary one, 

 and " a result of strictly tubicolous life." The habit, therefore, seems to account for 

 various features. The nature of the food and the mode by which it is obtained agree 

 v,rith the condition in the Aspidophora. No gill-slits, however, have yet been recognised 

 in Phoronis. 



Nervous System. — This occurs as a hypodermic development bounded internally 

 by the basement-tissue, very much as in Cephalodiscus, though it is double in 

 Phoronis, apparently in connection with the two great branchial whorls, and the 

 intrusion of the ncphridia and median anus. In minute structure it is similar to that in 

 Cephalodiscus; but in addition to the central masses, there are two posterior cords 

 which proceed for a short distance along the body-wall, and then, spreading out under 

 the lateral hypoderm, disappear. The absence of a large anterior region (as in Cephalo- 

 discus), and the fusion of the parts at the tentacular base, are probably in connection 



• Quart. Journ. Micr. Sci, vol. xxi. p. 210, 1881. 

 (ZOOL. CHALL. EXP. — PART LXXV. — 1888.) Ffff 4: ■ 



