28 A renicolidae 



between the A renicolidae and Maldanidae is obvious and admitted, 

 it is not sufficiently close to justify the fusion suggested by Prof. 

 Mesnil. Moreover, the affinities of the Arenicolidae are not entirely 

 with the Maldanidae, but also with the Scalibregrnidae. The degree 

 of relationship in which these families stand to each other cannot be 

 determined fully until further investigations have been carried out, 

 but it seems to the present writer highly improbable that the results 

 of such investigations will lead to the fusion of any two of them. 

 These three families present characters which appear to indicate 

 their evolution along independent lines ; a discussion of their 

 affinities will be found on pp. 160, 161. 



The account of Branchiomaldane (p. 147) shows that this genus 

 presents several resemblances to Arenicola, and that it must be 

 included in the family Arenicolidae, but the writer is not prepared 

 to follow Prof. Fauvel in merging the two genera, as they present 

 important differences, especially in regard to the gills, nephridia, 

 gonads and early stages of development. 



The family Arenicolidae is therefore regarded as containing two 

 genera, adult examples of which may be distinguished by the 

 characters given in the following key : 



Branchial segments subdivided into five annuli ; gills 



much branched, borne on the chaetiferous 



annulus ; prostomium small and may be sunk in 



the deep nuchal invagination ; dioecious, gonads 



borne on the nephridia only ; an achaetous tail 



in some species.... Arenicola, p. 29. 



Branchial segments for the most part subdivided 



into two rings, one chaetiferous, the other 



branchiferous ; gills composed of one to four 



finger-shaped filaments ; prostomium large, nu- 

 chal groove shallow ; monoecious, gonads borne 



on the septa and oblique muscles ; ecaudate ; 



small, not more than 2 cm. long Branchiomaldane, p. 147. 



Young post-larval stages of the ecaudate species of Arenicola 

 exhibit a general external resemblance to BrancMomaldane, but the 

 presence in the latter of gonads, and of bi-annulate branchial 

 segments in which the chaetae and gills are borne on consecutive 

 annuli, and the absence of statocysts and pigment, are useful 

 differential characters. 



