ANATOMY OF ARE NICOLA A SSI Ml LIS. 763 



sucli as blocks tlie tubes in the specimens with rounded 

 otoliths. It is also worthy of note tliat iu tlie other species 

 of Arenicola (cristata, grubii^ ecaudata) wliicli have 

 rounded or spherical otholiths, formed largely of secreted 

 matter, the otocyst is a closed vesicle. It seems probable, 

 therefore, that the presence or absence of an open passage 

 connecting the otoc^^st to the exterior has considerable 

 influence upon the chaiacter of the otoliths, Avliich varies 

 even in difi'erent specimens of the same species. The fact 

 that the otoliths of A. assimilis are rounded, while those of 

 the New Zealand specimens are irregular, is not of funda- 

 mental importance; it probably indicates that in the former 

 the tube leading from the otocyst to the exterior very soou 

 became blocked, and the otoliths are therefore largely com- 

 posed of material deposited around the small particles which 

 had gained access to the otocyst before the closure of its 

 tube. The otocysts of A. assimilis and of tlie New Zealaud 

 specimens agree in the most important character, namely, 

 that eacli possesses a tube leading to the exterior ; and the 

 modification which takes place in the former, causing a differ- 

 ence in the nature and shape of the otoliths, may be regarded 

 as of secondary importance, since a similar, though not so 

 marked a difference, may be observed within the limits of a 

 single species (A. marina). For further remarks on this 

 subject see p. 771. 



While the number of chastigerous segments in the ecaudate 

 Arenicolida) varies greatly (from about twenty-four to forty in 

 A. grubii, and thirty-five to fifty-six in A. ecaudata), it is 

 peculiarly coustant in three of the caudate species, there being 

 invariably nineteen in A. marina and A. claparedii, and 

 seventeen in A. cristata. Inmost American specimens of the 

 last-named species there is, however, an extension into the 

 tail of structures which are usually associated only with 

 parapodia. Small gills and cirriform processes occur upon 

 the first two or three tail segments of one specimen examined 

 (Gamble and Ashworth, 19U0, p. 412, figs. 81, 32), and similar 

 processes are commonly present on American specimens, but 



