NO. 1772. ANNELIDS OF THE ARENICOLID^—ASHWORTH. 



known signs, tha sand-rope-like castings and the mouth of the burrow, 

 which indicate the presence of A. marina on a sandy beach, have 

 no good counterparts in the case of the ecaudate species, in which 

 both the castings and the mouth of the burrow are inconspicuous 

 among their surroundings. Whether these species are present in any- 

 given area is therefore not obvious from a superficial examination, 

 as is often the case where A. marina is concerned; their presence 

 can only be ascertained after careful, and sometimes prolonged, search 

 in likely places, such as those above suggested. 



Four caudate species of Arenicola and a variety of one of them are 

 now moderately well known, namely, A. marina (Linnaeus), A.clapa- 

 redii Levinsen, A. assimilis Ehlers, A. assimilis, var. affinis Ash- 

 worth, and A. cristata Stimpson, all of which have been recorded from 

 the shores of North or South America. A. glacialis Murdoch, A, 

 fusilla Quatrefages, and A. natalis Girard, species concerning which 

 comparatively little is known, have been found near Point Barrow 

 (Alaska), Coquimbo (Chile), and Chelsea, Massachusetts, respectively, 

 A. natalis is not a valid species; it is now merged with A. marina 

 (see p. 6). I have recently shown that A. pusilla does not exhibit 

 any characters which entitle it to retain individuality as a species 

 and that it should be merged with A. claparedii (see p. 14). A. 

 glacialis is shown in this memoir to be a distinct and valid species, 

 although the characters given in the original diagnosis are insufficient 

 to firmly establish it as such. I have investigated the original speci- 

 mens on which Murdoch founded the species, and give below (see 

 p. 24) a description of them in as much detail as is possible, 

 having regard to their condition and their value as unique examples. 



The external characters upon which reliance is placed in differen- 

 tiating the caudate species of Arenicola are : 



1. The number of chsetigerous segments and the number of seg- 

 ments provided with gills. 



2. The mode of branching of the gill axes. There are two principal 

 modes of branching; in the first type, each of the axes of which the gill is 

 composed bears lateral branches closely set or irregularly placed so 

 that the gill has a bushy appearance: in the second type the lateral 

 branches on each gill axis are more numerous and are placed at almost 

 regular intervals, producing a pinnate appearance. 



3. The relative size of the median and lateral lobes of the pros- 

 tomium." 



4. The presence or absence of apertures of statocysts.*^ 



5. The segments on which the apertures of the nephridia are 

 situated." 



o For the examination of the apertures of the statocysts and nephridia, the shape of 

 the prostomial lobes and other minute features, a binocular dissecting miscroscope 

 is almost indispensable. 



