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



6 mm. in width owing to distension by ova, while in another 

 specimen the vesicles of the nephi'idia were filled with sperms, 

 and were 5 mm. in length and 4 mm. broad. In mature 

 females of A. ecaudata ripe ova accumulate in large num- 

 bers in the vesicles of the nephridia, producing on the inner 

 side of the vesicles thin sac-like outgrowths which may attain 

 a length of 4 mm. to 5 mm. (fig. 47). 



15. The Specific Characters and Classification 

 of the Arenicolidas. 



The characters which chsetopodists have relied upon when 

 separating the different species of Arenicola, and which 

 are in general use throughout the Polychgetes, are furnished 

 by the external organs. The general divisions of the body, 

 the arrangement and structure of the gills, parapodia, and 

 setae, are thought sufiicient to differentiate one species from 

 another. The internal anatomy has not been so carefully 

 studied from this point of view, with the exception of the pro- 

 stomium and otocysts, which have been carefully examined by 

 Ehlers. The result of his work was, amongst other things, 

 to show that, with reference to these organs, the species exhi- 

 bited a series of developmental stages. The otocysts, for 

 instance, proved to be very different in A. Grubii from those 

 of A. marina, while they are absent (or, as Ehlers concluded, 

 merely indicated) in A. Claparedii. With the exception 

 of this paper, however, there has been no attempt to show 

 how far the internal organs furnish evidence for specific 

 characters. It has been thought that external features were 

 sufficient. 



The result of this idea of the sufficiency of obvious marks 

 is well shown by the confusion of different species under one 

 name, and by the same form at different stages of growth 

 receiving different names. As an example of the first kind 

 of mistake we may refer to the doubt which has so long 

 hung over the specific identity of A. ecaudata. Named 

 originally by Johnson in 1835, it has been confused with A. 



