ANATOMY' OF ARENICOLA ASSIMILIS. 771 



end are so exactly similar to those of the Otago specimens 

 that no farther description of them is necessary. 



The specimens above described are interesting from their 

 bearing on the discussion regarding the taxonomic value of 

 the shape of otoliths (see p. 761). The only diiference 

 between the Falkland s specimen and those from New Zealand 

 is that in the former the otoliths are spherical and composed 

 almost entirely of a secreted substance, while in the latter 

 they consist of irregular foreign bodies, such as sand-grains 

 and fragments of spicules. There can be no doubt that the 

 two sets of specimens belong to the same species, or rather 

 to the same variety, so that (as was also proved for A. 

 marina, see p. 762) the shape of the otoliths varies in 

 different specimens of the same species or variety. The 

 closure of the tube of the otocyst along part of its length and 

 the presence of the numerous large gland-cells in its wall are 

 probably the principal factors in determining the shape and 

 nature of the otoliths of the Falklauds specimens. Having 

 proved the presence of spherical otoliths in some examples of 

 A. assimilis, var. affinis, it will be noticed that one of the 

 differences (discussed on pp. 701 — 763) between this new 

 variety and the type of the species disappears ; so that now the 

 only features by which they may be distinguished are (1) the 

 presence of twenty chastigerous segments in the type of the 

 species, whereas the new variet^^ possesses only nineteen, and 

 (2) the slightly different position of the external opening of 

 the otocyst. As the latter is too fine a character for ready 

 application in systematic work, it may be said that the deter- 

 mination rests upon the number of chtetigerous segments. 



Another striking feature about the otocysts is the great 

 difference in their size in specimens of the variety from the 

 two localities. Whereas in the Otago specimens their 

 average diameter is 0'21 mm. (in a specimen 130 mm. long), 

 in one (128 mm. long) from the Falklands their average 

 diameter is 0"34 mm. (cf. figs, l-i, 15), so that the internal 

 volume of the latter is about four times that of the former. 



Adult specimens of the new variety are now recorded from 



