REPORT ON PHORONIS BUSKII. 19 



this ring are situated a pair of sense-organs, which I shall speak of as ' ciliated pits.' 

 They lie in the concavity of the lophophore, on either side of the anus. They have 

 the characteristic structure of sensory epithelium, consisting of sense-cell, ganglion, and 

 nerve-fibres. Sars has figured in lihabdopleicra a pair of ciliated protuberances in 

 what I hope to show is a homologous position. 



" A further concentration takes place in the form of a cord, which runs from the 

 median dorsal part of the nerve-ring two-thirds of the length of the foot along its left 

 side. It is therefore asymmetrical, and lies in the epidermis outside the basement- 

 membrane. Inside this nerve-cord lies an apparently hollow tube. This tube recalls 

 the so-called large fibres of the Chsetopoda." 



Moreover, during development, Caldwell found the ectoderm became thickened in 

 two regions, viz. in the prseoral lobe, and in the form of a postoral ring round the mouth. 

 " The former becomes the future nerve-ganglion ; the latter indicates the position of 

 the line of future tentacles and the circumoesophageal nerve-ring of the older animal." 

 From the former, in some species, Caldwell states that a number of nerve-fibres pass 

 forwards to a sense-organ. In one species four eye-spots are present. Further, 

 " along a line at the base of the rudiments of the adult tentacles, the nervous prolonga- 

 tions of the ectoderm have formed a definite ring." A series of remarkable events, 

 however, occurs during the metamorphosis from a free to a fixed life, " the whole 

 prseoral lobe with ganglion and sense-organs passes into the stomach," and is there 

 digested. Thus no anterior dorsal sensory part of the central nervous system persists 

 in the adult, the postoral circumoesophageal ring alone remaining, and being in con- 

 nection with the sense-organs. Similar features were observed by Kieinenberg ' in 

 Lopadorhynchus, the circular nerve disappearing completely with the vibratile 

 organ. 



The previous information on this subject has been given somewhat fully because 

 the condition of the specimens procured by the Challenger ofi"ers various features both 

 interesting and novel. The central region of the system lies over, i.e. on the anal 

 side of, the mouth in connection with the firm basement-tissue, forming the support of 

 the branchial apparatus, and thus it occupies a similar situation to that in Cephalo- 

 discus. 



On making sections of the region just indicated, and at the level of the hypoderm 

 of the nephridial eminences, but before reaching the lumen of the canal, it is found 

 that the branchial whorls have considerably diminished, though three volutions are 

 more or less present on each side. The oral funnel is still wide, and the second or 

 inner line of the series has on the anal (or neural) aspect the two free fiaps formerly 

 mentioned. Each inner edge of the latter shows a well-defined though narrow border 

 of hypoderm with blackish pigment — beneath which is a firm and somewhat thick layer 



' Atti (1. R. AccaJ. d. Lined, t. vi p. 15, 1881 ; and Ann. and Mag. Nat. Hht., ser. 5, vol. ix. p. 07. 



