24 F. W. GAMBLE AND J. H. ASHWORTH. 



on account of their position, movements, and the nerves ending 

 in them, may be considered as probably belonging to this 

 category. 



Professor Ehlers' (1892) account of the nuchal organ and 

 otocysts is an almost exhaustive description of these organs in 

 Arenicola. We have worked over the whole subject again, 

 however, and are able to add a few points to this important 

 paper. 



The nuchal organ belongs to the prostomiura, whereas the 

 otocysts belong to the metastomium. The prostomium and the 

 nuchal organ are found, in varying degrees of complexity, in 

 nearly all Polychsets ; the otocysts, however, occur in few and 

 widely separated families. 



The general appearance of the prostomial lobes and the 

 opening of the nuchal organs have already been described. 

 Seen from the dorsal surface the former consists of a small 

 median papilla and two larger lateral prominences (PI. 4, 

 fig. 19), which together correspond with the single prostomial 

 papilla of allied forms (cf. Racovitza's figure of Leiocephalus, 

 1896, pi. V, fig. 5). In young Arenicola these lobes are 

 transparent, and therefore red from the underlying blood- 

 vessels. In old specimens they become dark-coloured and 

 opaque from the deposition of pigment in them. In no species 

 of Arenicola have eyes been discovered, although they are 

 known to occur on these lobes in many related genera. 



The prostomial epithelium is a complex of several distinct 

 kinds of cells, — unaltered columnar elements, fusiform sense- 

 cells, each ending in a conical prominence, glandular cells, and 

 apparently also " wandering cells " from the body-cavity. 

 Underneath the epithelium is a connective tissue continuous 

 with the supporting tissue, the neuroglia of the brain, which 

 binds together the large ganglion-cells of the cornua of the 

 brain. The prostomial sensory structure thus formed is very 

 sensitive to light, but what function it subserves has not been 

 determined with accuracy. 



Nuchal Organ. — To the outer side of the lateral prosto- 

 mial lobes is a depression guarded externally by a fold (just 



