494 



ANNELIDA. 



which are sometimes of considerable size, even reaching a length 

 of from four to six feet (Microchaeta rappi ; Megascolides australis). 

 The setae are, as a rule, few in number, and are not set in parapodia. 

 The head consists of a prestomium, which is sometimes much elon- 

 gated (Nais lacustris), and is generally marked off from the oral 

 segment (peristomium), or first segment of the body, by a groove. 

 The oral segment is always without setae, and is never reinforced 

 by the fusion with it of posterior body segments. There are no 

 cephalic appendages either on*the pre- or peristomium. The seg- 

 mentation of the body is always well marked externally by circular 

 grooves,* and the coelomic septa are always present. The segments 

 are all alike, and are not divided into regions, though some of the 



FIG. 403. Setae of Oligochaeta (after Vejdovsky, Stole, and Michaelsen, from Beddard). 1, 

 ^ Onychochaeta windlei, posterior seta. 2, ornamented seta of Geoscolecid. 3, Trichochaeta 

 hesperidum; Sb, end of same more highly magnified. 



segments behind the peristomium are occasionally devoid of setae. 

 The gonads are localized thickenings of the coelomic epithelium, 

 and special generative ducts are present to convey their products to 

 the exterior. There is always present a glandular development of 

 the ectoderm, generally having a marked annular form and called 

 the ditellum or girdle : it secretes the cocoon in which the eggs 

 .are laid. 



Considering the great constancy presented by the Oligochaeta 

 in the arrangement and structure of many important organs, e.g. 



* In some species of Lumbricus a curious modification of the metamerism, 

 called spiral metamerism, has been observed as a variation. The grooves between 

 the segments in such cases have a spiral course (Vide Bates on " Materials for 

 the Stitdy of Variation" p. 157, London, 1894). 



