2 OLIGOCHAETA 



Home, Leo, Moreen published their researches, but our present accurate knowledge of that 

 animal may be said to date from the memoir of d'Udekem (4), who first discovered the ovaries. 

 Subsequently to that Lankester (9), CLAPAEiiDE (1), Hoest (1), v. Mosjisovics (1, 2), 

 Cebpontaine, and others have dealt with the structure of Lumbricus. The aquatic Oligochaeta of 

 Europe were first investigated by 0. F. MtIller ; but d'Udekem's memoir upon Tubifex, and those 

 of Claparede upon that and other forms, are the memoirs from which our modern knowledge dates. 

 Since the publication of Clapaeede's two memoirs, the aquatic Oligochaeta have been principally 

 studied by Lankester, Benham, and myself in this country; on the continent by Leydig, Dieffenbach, 

 and others ; in Ailierica by Eisen. 



I. The Body- Wall and External Chaeactees. 



§ I. External fcn^m, and segmentation. The Oligochaeta are segmented worms of very 

 variable size ; at the one extreme we have the minute species of Aeolosoma and certain 

 Naids, I mm. or so in length ; at the other the gigantic Microchaeta rappi and Megascolides 

 australis, which measure from four to six feet. 



In all the Oligochaeta, with the exception of Aeolosoma, where it is at any rate less 

 marked, the external segmentation corresponding to the internal metamerism is very 

 obvious. The grooves which separate the segments from each other are clearly defined. 

 It often happens however, particularly among the larger earthworms, that there is 

 a ' secondary ' annulation of the segments. The term ' secondary ' is used, because this 

 division of the segment by transverse furrows into two or more parts appears to have no 

 relation to other organs, whether internal or external ; at the same time this annulation 

 of the segments and the number of annuli appears to be fairly constant for the species. 

 The number of segments in the body of the Oligochaeta is very small (6-20) in Aeolosoma 

 and some others among the lower forms. As many as 500 or 600 segments have been 

 counted in some of the larger earthworms. There are at present no exact data as to the 

 constancy of the number of segments among earthworms. ' In all probability the number 

 is not absolutely fixed, but there appears to be a mean for each species round which 

 there is a certain amount of variation. There is but little specialization among the 

 segments of a worm's body ; it is an invariable rule that the first segment of the body, 

 and it occasionally happens that a few of the following segments also, are devoid of 

 setae ; this ' cephalisation ' is dealt with more at length on a subsequent page. As a rule 

 the anterior segments of the body in earthworms are wider and marked by more 

 numerous secondary annulations than those which lie behind the clitellum ; the clitellum 

 itself is formed by a specialized set of segments. 



§ 2. Prostomium. In the majority of Oligochaeta a process of the first segment of the 

 body overhangs the mouth on the dorsal side. In a few forms it is of considerable length, 



