Inter-relationships of the Species of Arenicola 157 



sixty segments, and that the definitive number of segments in 

 A. cristata is also about sixty (seventeen chaetiferous and forty 

 caudal), suggests that this may have been approximately the 

 number present in the common ancestor of the genus. 



A. ecaudata possesses the largest number of nephridia, namely, 

 thirteen pairs, and probably, therefore, exhibits the nearest approach 

 to the primitive condition ; the other species do not possess normally 

 more than five or six pairs, but that the series of nephridia in these 

 species formerly extended further back is indicated by the occasional 

 occurrence of nephridia in one to three segments behind that 

 containing the last normal nephridium (see p. 82). 



Well-developed statocysts are so characteristic a possession of 

 the genus Arenicola, there being only one species pusilla in which 

 these organs are absent, that it may be assumed they were present in 

 the ancestral form. The statocysts have retained their primitive 

 condition, as epidermal invaginations opening to the exterior, in 

 A. marina, assimilis and glacialis ; in the other species they have 

 become transformed into closed vesicles, either with numerous stato- 

 liths as in the ecaudate species, or with a single endogenous statolith 

 as in A. loveni and cristata, which exhibit the most specialised con- 

 dition of the organ. The absence of statocysts in A. pusilla appears 

 to be a secondary, and not a primary condition. 



The prostomium, which in all species of Arenicola is reduced to 

 small dimensions (a condition no doubt associated with the semi- 

 sedentary mode of life), has the simplest form in the ecaudate species, 

 in which it has retained its primitive relations, being situated 

 anterior to the peristomium and partially overhanging the mouth. 

 In the caudate species the prostomium is retractile into the nuchal 

 groove and then no longer exhibits the typical relations. 



The primitive characters are not all concentrated in one species 

 of Arenicola, but it seems clear that a greater number of them have 

 been preserved in the ecaudate species, and especially in A. ecaudata, 

 the statocysts of which have, however, advanced some distance on the 

 way of specialisation. In A. branchialis the segments and nephridia 

 have both been diminished in number, probably correlatively, and 

 the gonads are much smaller, but otherwise this species is similar to 

 A. ecaudata. 



The caudate species are divisible into two divergent series, one- 

 including A. marina, glacialis, loveni and cristata possessing 

 elongate neuropodia, septal pouches and a single pair of oesophageal 

 glands, the other comprising A. assimilis and pusilla in which the 



