Historical 21 



This classification is an extension of Savigny's method i.e. the 

 families are grouped around a central type ; they are classified not 

 by definition but by type, not by precept but by example. Although 

 this arrangement presents certain advantages over any previous 

 classification, it does not by any means solve the difficulties which 

 arise when the attempt is made to group the families of Polychaeta ; 

 see, for instance, the assemblage of divergent forms brought together 

 under the group Syllidiformia spionina. The close relationship of 

 the Arenicolidae and Scalibregmidae is well represented in this classi- 

 fication, but the association of these families with the Amphinomidae 

 does not appear to be justified by any community of structure. 



During the ten years following the publication of Dr. Levinsen's 

 memoir no marked changes were introduced into the mode of 

 classification of Polychaeta ; most writers during this period adopted 

 the subdivisions Errantia and Sedentaria, others considered the 

 families separately in sucli order as each author thought best. 



In 1893 Prof. Hatschek and in 1894 (and 1896) Prof. Benham 

 suggested new classifications of the Polychaeta. Prof. Benham's 

 classification, 1 which combines, to some extent, the systems of 

 Quatrefages and Levinsen, divides the Polychaeta into two grades 

 or branches (a) the Eucephala, later 2 named Phanerocephala, in 

 which the prostomium retains its original condition as a lobe over- 

 hanging the mouth and is not overgrown by the peristomium ; the 

 body-segments are similar so that the body is not divided into 

 regions; (b) the Cryptocephala, in which the peristomium grows 

 forwards and fuses with or entirely conceals the greatly reduced 

 prostomium ; the body-segments are differentiated into two regions 

 by a sudden change in the character and arrangement of the chaetae, 

 and also by certain internal differences. The arrangement (1896) of 

 the families in the sub- orders is shown in the following Table : 



A. PHANEROCEPHALA. 

 Nereidiformia. 



Syllidae. Amphinomidae. 



Hesionidae. Eum'cidae. 



Aphroditidae. Glyceridae. 



Phyllodocidae. Sphaerodoridae. 



Tomopteridae. Ariciidae. 



Nereidae. Typhloscolecidae. 

 Nephthydidae. 



1 Eep. Brit. Assoc. (1894), p. 696. 



2 Cambr. Nat. Hist., vol. ii (1896), p. '258. 



