SCALISETOSUS ASSIMILIS. 377 



developing ova. He could not say that this membrane represented the wall of the 

 segmental organ ; indeed the absence of cilia inclined him to think it did not. He also 

 drew attention to certain cell-masses, with yellowish concretions in the centre, in the 

 intestinal diverticula — which he thought excrementitious. Prof. Giard agrees with me in 

 considering the distinctions of this form and 8. communis insignificant. 



Marenzeller (1876) found 8. communis — in the Bay of Muggia at Trieste at a depth 

 of 18 metres, on Ophiothrix alopecurus — with thirty-three segments, and measuring 10 mm. 

 His example had fourteen pairs of elytra. He also mentions the advantages of the species 

 for the study of the nerves of the elytra, and alludes to the characteristic structure of the 

 bristles. 



Langerhans (1879) records large examples (2 cm.) from Madeira. In connection 

 with the scales he discourses on tactile hairs and rudimentary organs of various animals. 



Hornell (1891) found all his specimens as commensals on Echinoderms — one in the 

 ambulacral groove of Astropecten irregularis, another on Orossaster papposa, and the third 

 on Ophiothria? rosula. In one the pellucid scales had posteriorly a crescent of orange, 

 while the first pair had an orange belt all round. 



2. Scalisetosus assimilis, Mcintosh, 1875. 



Specific Characters. — Head similar to that of 8. communis, the eyes occupying the 

 same position, and the larger anterior pair having lenses. Median tentacle long, smooth, 

 slightly dilated below the filiform tip ; lateral tentacles short, also slightly dilated below 

 the tip. Palpi smooth. Tentacular cirri similar to the median tentacle. Body narrow 

 and elongated, with a brownish-black median band from the nuchal collar to the posterior 

 end. It is widest at the posterior part of the proboscidian region, though even there 

 less than a third of the breadth of the dorsum. Segmental eminence prominent, but a 

 special papilla could not be made out. Scales even more delicate and transparent than 

 in S. communis, minutely punctate on the under surface. The outer and posterior borders 

 and neighbouring surface have short clavate cilia — less numerous than in the former 

 species. The finely branched nerves and nerve-endings are similar. From the dorsum 

 the tips of the feet are blunt, and viewed laterally the dorsal eminence for the bristles 

 is large, while the spine has a broader border of granular epiderm. The dorsal bristles 

 are smaller than in 8. communis, slightly curved, the spinous rows being less prominent 

 and covering a shorter region of the bristle. The tip is bluntly rounded, bearing a minute 

 terminal claw and a small secondary process with a notch between. The ventral division 

 shows a broad terminal lobe with a bluntly rounded margin in front of the tuft of 

 bristles. The latter, as in S. communis, has the distal end of the shaft expanded, with a 

 small spinous collar, and from this the spinous region, with a slight bend to the dorsum, 

 tapers to the tip, which turns bluntly round to the spiked side and ends in a small hook ; 

 then, after an edge directed obliquely backward, a secondary process, lateral in position, 

 occurs. The rows of spines are extremely fine. The dorsal cirri are apparently shorter 

 than in the previous species, but of a similar shape— smooth, with a long filiform process 

 at the tip. Only one was observed. 



