12 OLIGOCHAETA 



gatter, however, there are several segments following the first setlgerous 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 Enchytraeus monochaetus and ffesperodriius 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 

 phai-ynx to the parietes. In all Oligochaeta a certain number of anterior segments 

 of the body are without nephridia, 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. Eijidermiis. 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' (Cee- 



fontaine). CLAPAEiDE, 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. Leydig's earlier 



view (6), based upon a study of Phreoryctes as well as Lumh-icus, of the cellular 



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



CLAPAEtDE detected in osmic acid preparations its cellular character. 



* 

 This eiToneous 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 slated by Lankester (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 regardiDg the epidermis as distinctly cellular and built up of the 



two kinds of cells referred to in the quotation from Lankester's memoir. 



Cerfontaine's memoir, published in 1890 (1), contains the most detailed account 

 that has yet appeared of the epidermis of Libtnbvicus. 



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



