REPORT OX PHORONIS BUSKII. 



21 



externally by the aperture just described. This aperture in section is thick and 

 slightly mammillate. The inner half of the outer wall of the organ is more closely 

 grained and streaked than the outer, the fine hypodermic tissue being probably 

 ciliated during life. The other wall of the organ, i.e. that adjoining the second whorl, 

 is uniformly finished with a fine margin — also probably covered with cilia. A little 

 mucus, which stains deeply, occurs in the centre of the chamber. 



In young examples the above-mentioned free flaps are simple, and the central 

 chamber is rudimentarj'. In function this structure is probably connected with sensation. 



In sections immediately behind the nerve-centre, the massive basement-tissue 

 forming the axis of the fan on each side is perforated by the anterior end of the 

 posterior body-cavities, which form narrow spaces — at first directed obliquely outwards 

 and backwards, and finally expanding into the great chambers exterior to the intestinal 

 area. 



Nephridia. 



The external apertures of the nephridia, which perform the function also of 

 generative ducts, occur on each side of the anus, on a lateral elevation a little internal 

 to the skeletogenous elements of the branchial apparatus. These apertures are not 

 readily observed, since they are situated quite at the anterior end of each ridge. They 

 are somewhat spout-like, and carry the ova near the papilla of the hypodermic flap, 

 placed at the base of the second whorl. The exact connections of this aperture with neigh- 

 bouring parts could not be clearly ascertained, and it is possible a functional connection 

 with the hypodermic process may exist. At any rate the ova would issue conveniently 

 for finding their way to the internal whorls. Each aperture leads into a spacious 

 chamber lined by a coat of closely - arranged epithelium, resembling a modification 

 of the hypoderm, and it may be ciliated during life, though Dyster, who examined a 

 British species in life, says that it is not ciliated. A thin basement-layer occurs 

 outside this layer. It is wide at the posterior and outer edge in section in this 

 region, but slit -like anteriorly {i.e. towards the oral region). The channel at 

 this level is more or less free. Just behind the nerve-centre — where the basement- 

 tissue forms a floor or support — the channel forms a firm rounded tube (PI. II. fig. 1, 

 and PI. III. fig. 2, np), its basement-tissue being continuous with that supporting the 

 central nervous system. Externally (posteriorly) is a thick layer of hj'poderm with 

 dark pigment towards the free edge. The channel suddenly widens into a trans- 

 versely elongated chamber, then contracts so as to form a rounded aperture in section 

 in the dense basement-tissue of the region, and, approaching the inner edge of this 

 tissue, debouches into the lateral chambers of the body-cavity outside the division for 

 the intestine, but within the marginal cavity on each side continuous with the vascular 

 lacunae at the base of the branchial system. A considerable amount of basement- 



