160 A renicolidae 



Arenicolidae, Cirratulidae, Capitellidae and Maldanidae in the suIj- 

 order Diilomorpha. On passing in review the characters of these 

 families it is at once seen that the Sternaspidae are far removed from 

 the Arenicolidae by the peculiar arrangement of their gills, chaetae 

 and gonad, by the presence of ventral shields, genital ducts, a coiled 

 alimentary canal without oesophageal glands, and only a single pair 

 of nephridia, and by the absence of septa. The anatomy of the 

 Chlorhaemidae, which is, however, only imperfectly known, affords 

 no evidence of relationship between this family and the Arenicolidae ; 

 there are so many pronounced structural differences — for instance, in 

 the nature of the prostomium, gills, chaetae, nerve-cord, the arrange- 

 ment of the septa and the nephridia — that it may be concluded these 

 are not allied families. The Cirratulidae are also separated from the 

 Arenicolidae by a series of striking differences, for instance, in their 

 chaetae, gills, nephridia and prostomium. The Capitellidae, thougli 

 having some points of external resemblance to the Arenicolidae, 

 exhibit such differences that it seems clear these families are not 

 nearly related ; consider, for example, the nephridia and genital 

 funnels of Capitellids, the different arrangement of their parapodia, 

 the presence of genital chaetae and hooded crotchets, the form 

 of the gills (when present), the regularly septate coelom, the 

 ganglionated nerve-cord, and the absence of blood-vessels and 

 oesophageal glands. 



There are several characters common to the families Arenicolidae, 

 Scalibregmidae and Opheliidae, which are probably due to their 

 similar mode of life, the members of these families being limivorous. 

 They have a spacious coelom, subdivided anteriorly by septa but 

 non-septate in the middle portion of the animal, the alimentary canal 

 consists of an eversible pharynx, followed by an oesophagus (bearing 

 one or more pairs of glandular caeca), a dilated " stomach " and a 

 straight intestine. In addition to these features the Scaliliregmidae ^ 

 agree with the Arenicolidae also in the subdivision of their segments 

 into annuli, in the sculpturing of the skin, and in the presence in 

 Sccdibrcgma and Eumcnia of branched gills of a type similar to those 

 of Arenicola. The brain and the non-ganglionated cord of Scalihregma 

 are similar to those of the caudate species of Arenicola. But there 

 are several characters in which the Scalibregmidae differ sharply 

 from the Arenicolidae ; for instance, the two rami of the parapodia 

 of the former are practically identical in form, there are no crotchets, 



' See J. H. Ashworth, " The Anatomy of Scalibregvm inflatum Ratlikc," iu 

 Q. J. Micr. Sci., xlv (1901), p. 237. 



