ON PHYMOSOMA VARIANS. 75 



brain ; while a sense-pit opens on to it on each side' (figs. 1 

 and 7). 



The introvert is dividable into several regions. Immediately 

 behind the head follows a narrow, perfectly smooth region, 

 extending for about 2 mm. At the posterior edge of this 

 region is attached a small but very extensile collar, its 

 anterior margin jjeing free (figs. 1 and 4). Behind the 

 attachment of the collar the introvert swells slightly, and 

 there follows a region about 6 mm. in length, which bears 

 about twenty rows of hooks. Then follows a region of V9,riable 

 length, bearing papillse; and lastly a second region of hooks, 

 which in our specimens bore from forty to between fifty and 

 sixty rings. Among the hooks of the posterior region are 

 many papillse; and these in passing backwards get more and 

 more conspicuous, at the expense of the rings of hooks. 

 These papillae also exhibit traces of a tendency to form rings 

 round the base of the proboscis. The characters of the hooks 

 have been well described by Selenka and by Keferstein'' : it 

 will be sufificient here to refer to the description given by these 

 authors, and to the diawing (fig. 21). 



The papillse on the introvert have the fornfx shown in fig. 15; 

 they are hemispherical or hemielliptical, being often higher 

 than broad, each having a central opening surrounded by 

 three or four plates of chitin, which often fuse into a single 

 piece ; and surrounding this central piece are numerous small 

 rounded plates covering at least the upper half of each papilla. 



The papillse on the trunk (figs. 11, 14, and 16) have a some- 

 what different appearance, being larger and flatter, and having 

 no marked central plate. They are also surrounded by a much 

 pigmented ring. These trunk papillae agree with the de- 

 scription given by Selenka, who, however, seems to have over- 

 looked the difference between the papillse in the two regions of 

 the body. The papillae are large and conspicuous at the two 

 extremities of the trunk, where they are present on all sides; 



' Cf. Spengel, "Die Sipunculiden," 'Eeisen im Archipel der Philippinen,' 

 Bd. iv. 1883. 



- Selenka, loc. cit., Kefersteiu, 'Zeit. fur Wiss. Zool.' Bd. xv. 1865. 



9—2 



