Affinities . 161 



but characteristic furcate chaetae are present, along with capillary 

 chaetae, in both rami of the parapodium ; in some of the Scalibregniidae 

 the parapodia form laminate appendages bearing dorsal and ventral 

 cirri; the gills do not extend, in the Scalibregniidae, backwards 

 beyond the fifth or sixth segment, the heart is a median dilatation 

 on the dorsal vessel, the nephridia are numerous and minute, and 

 complex lateral sense organs are generally present, but statocysts 

 and eyes are absent. 



Besides the features mentioned above (p. 160) as common to the 

 three limivorous families, the Opheliidae * agree with the Arenicolidae 

 in possessing nephridia of a similar type, but differ in several 

 important respects, for instance, in the form of the prostomium, in 

 having a ganglionated nerve-cord, in the nature of their gills which, 

 when present, are cirriform, in the presence of anal cirri and a dorsal 

 heart, in the absence of septa in the posterior region, and in the 

 absence of crotchets and statocysts. 



The Arenicolidae present clear affinities with the Maldanidae. 2 

 The head of some of the Maldanids, e.g. Praxillura, is similar in 

 form to that of the ecaudate species of Arenicola, but the cephalic 

 plate present in many Maldanids has no counterpart in Arenicola, 

 while, conversely, a trilobate prostomium like that of the caudate 

 species of Arenicola does not occur in the Maldanids. The two 

 families agree in the form of their parapodia and chaetae, capillary 

 chaetae and crotchets being present in both. The notopodial chaetae 

 of Maldanids closely resemble those of Arenicolidae, especially those 

 of young examples, and the crotchets of some Maldanids, e.g. Petalo- 

 proctus, might easily be mistaken for those of Arenicola. The brain 

 and non-ganglionated nerve-cord, the nephridia and gonads of 

 Maldanids are of a type similar to those of the Arenicolidae. The 

 Maldanids differ from the Arenicolidae in having paired nuchal 

 organs, and elongate and non-annulate segments, in lacking gills, 3 

 oesophageal caeca, hearts and statocysts, 4 and in the specialisation 

 of the anal segment. Brancliiomaldane has been regarded, on 



1 See W. Kiikenthal, Jenaische Zeits., xx (1887), p. 511 ; andM. Philippson, 

 Zool. Anz., xxii (1899), p. 417. 



2 See I. Arwidsson, Zool. Jahrb. Abt. Syst., Suppl. ix (1907), p. 1. 



3 The Maldanid Jch?istonia, has, on the six pre-anal segments, numerous, 

 short, simple elevations, which serve as respiratory organs. 



4 The Arenicolidae are regarded as having been derived from an ancestor 

 having statocysts. These organs are absent in Arenicola pusilla and Brancliio- 

 maldane vincenti, which, however, exhibit marked specialisation in several 

 other respects. 



M 



