12 OLIGOCHAETA 



gutter, however, there are several segments following the first setigerous segment, 

 which have no setae ; in the other Naids a variable number of segments, varying with 

 the genus, are without dorsal setae ; for the details the reader is referred to the 

 special description of the family. The only other examples among the lower Oligo- 

 chaeta are afforded by Enchytracue monochaetus and ffesperodrilus albus. 



The formation of a head or cephalisation is not however confined to the absence, 

 or reduction in number, of the setae. Other organs show analogous modifications. 

 It is common, for instance, to find the intersegmental septa not clearly definable in 

 the first two or three segments of the body ; their place is taken or their existence 

 is concealed by masses of muscular fibres which pass from the buccal cavity and 

 pharynx to the parietes. In all Oligochaeta a certain number of anterior segments 

 of the body are without nephrLdia, or the nephridia if present are modified. There 

 are various other organs which show peculiarities at the anterior end of the body 

 and contribute to the formation of a ' head.' 



6. Epidermis. The epidermis of all Oligochaeta consists of cells which are 

 separated from the subjacent muscles by ' a condensation of the connective tissue of 

 the latter layer, which presents in parts the appearance of a veritable lamella' (CfiR- 

 FONTAINE). CLAPAREDE, adopting WEISMANN'S term hypodermis, described this layer 

 in Lumbricus as consisting of a nucleated meshwork in which no cell-outlines could 

 be distinguished, enclosing spaces filled with a colourless substance ; these bodies were 

 regarded as of a glandular nature, but no nucleus was discovered. LEYUIG'S earlier 

 view (6), based upon a study of Phreoryctes as well as Lumbricus, of the cellular 

 nature of the entire epidermis was discarded, except for the prostomium, where 

 CLAP A HE DE detected in osmic acid preparations its cellular character. 



This erroneous view appears to have been first rectified for Lumbricus by PERKIER 

 (9) in a preliminary dissertation upon the structure of that worm, which precedes 

 his account of the anatomy of Urochaeta. A few years later the cellular nature 

 of the epidermis was stated by LASKESTER (12) ; he speaks of it as consisting of 

 ' varied forms of goblet cells and, excessively delicate, elongate, interstitial, or 

 "packing" cells, instead of the altogether improbable syncytium of CLAPAREDE.' 

 These results are mentioned as being confirmatory of those of HORST and v. 

 MOSJISOVICS, which had been published previously. Since that date all observers 

 have agreed in regarding the epidermis as distinctly cellular and built up of the 

 two kinds of cells referred to in the quotation from LANKKSTER'S memoir. 



CEKFONTAINE'S memoir, published in 1890 (1), contains the most detailed account 

 that has yet appeared of the epidermis of Lumbricue, 



The cells are disposed in two rows, the cells of the innermost row being small. 



