THE HABITS AND STRUCTURE OF AREMCOLA MARINA. 107 



addition, certain other structures, such as the setae 1 of the notopodia 

 and some of the buccal papillae, which 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 prostomium, 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 service 

 the former consists of a small median papilla and two larger lateral 

 prominences (PI. IX., 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 " wander- 

 ing 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 stucture 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 prostomial lobes is 

 a depression guarded externally by a fold (just above Nu., PI. IX., 

 Fig. 19, B.). These two pits form the beginning of the nuchal organ 

 and indicate its paired origin. Further back they unite to form a 



1 Retzius has described free nerve-endings on these setxe. 'Biologiska 

 Foreningens Forhandlingar,' Bd. hi, Hefte 4—6, 1891, p. 85. 



