Evolution of Fishes from Invertebrates 73 



be rapidly protruded, within a more external, eversible 

 and muscular half (Fig, 5), that functions as a sensitive, 

 tactile and offensive organ. The specially thick and mus- 



FiG. 5. Diagram of raetanemertean proboscis as retracted 

 within (a) or extruded from (b) the proboscis sheath, p.s. The 

 rim of attachment, r, of the proboscis to its sheath seems to corres- 

 pond with the velum of vertebrates, ap. anterior protrusible and 

 muscular part of proboscis; m.p, median muscular stylet area; s, 

 stylet; g.p, inner glandular part; p.c, proboscis muscle. 



cular area, that unites the inner and outer parts of the 

 tube in metanemerteans, develops the characteristic stylets 

 already treated of, and that can be steadily renewed as 

 old ones are worn away. The surface of the external coat 

 is often abundantly beset with fine glandular papillae. The 

 entire organ is innervated by an abundant set of circularly 

 disposed nerve-fibres, that have been minutely described 

 and figured by von Graff (5.^:430) and his successors. 



In contrast to the eversible, muscular, and often 

 papillose outer tube, the inner is thin-walled, and its surface- 

 layer consists of richly glandular cells that stain deeply. 

 So it may shortly be said that the proboscis in metane- 

 merteans consists of three portions, ( i ) an outer eversible 

 tube portion, that in Geonemertes and others has highly 

 muscular walls and papillose surface; (2) a median some- 

 what swollen and highly fibro-muscular zone in which 

 develop and are embedded the semicalcareous semicorneous 

 stylets; (3) the internal thin-walled tube that in the re- 

 tracted proboscis is in line with the outer, and is con- 



