ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 543 



of nephridia opening on segments 5 — 17. Gonads large, 

 twelve pairs ; in females each ovary bears as many as thirty 

 digitiform vascular processes bearing ova ; in males each 

 testis is produced into one or more large thin reniform lobes. 

 Nineteen to forty pairs of gills. 



For diagram illustrating this classification see woodcut on 

 p. 642. 



16. Affinities of the Arenicolidae. 



On account of the very incomplete condition of knowledge 

 concerning the anatomy and development of allied families 

 (Maldanidse, Oplieliida3, Scalibregmidse, Chlorhsemidse) it is 

 impossible to speak with any confidence of the affinities of 

 Arenicola.^ In its external characters (with the exception 

 of the gills) this genus perhaps resembles the Maldanidee 

 more closely than the other families. The peculiarities 

 of the gills, however, cannot be overlooked. They form 

 the distinctive mark of the genus Arenicola, and it is 

 probable that they are special vascular processes rather than 

 modified dorsal cirri. The branched gills on the first few 

 segments of Scalibregma and Eumenia are somewhat 

 similar to those of Arenicola in form, but different in 

 arrangement, since they are present both above the noto- 

 podium and below it, and they are confined to the anterior 

 segments, on which they never occur in Arenicola. If then 

 the gills in two families have been independently acquired, 

 we have to look for other indications of affinity. 



The general agreement in the subdivision of the coelom 

 by septa, and the segmental blood-vessels in Arenicolidse, 

 Scalibregmidse, and Opheliidse has been pointed out by 

 Wiren (1887). In these limnivorous families the ccelom is 

 septate in its anterior and posterior regions, but in the 

 middle portion, apparently to allow the gastric loop room for 



1 In this summary we purposely exclude Branchiomaldane, since its 

 anatomy is not suflSciently investigated. We have discussed its position on 

 p. 536. 



