EXTERNAL CHARACTERS. 67 



Lumbricus terrestris, the largest of the commoner species, 

 will, however, apply with but slight alterations to any of the 

 others. 



The worm is most conveniently killed by chloroform, or 

 by warm corrosive sublimate, and kept in 2% Formalin. 



I. EXTERNAL CHARACTERS. 



1. Form. The body is cylindrical along the greater part of 



its length, flattened in its hinder part. It is pointed 

 in front, blunt behind, and is thickest about one- 

 third of its length from the anterior end. 



2. Colour. The earthworm is of a pale pinkish or yellowish 



colour, replaced in front by a dark purplish brown, 

 which extends a considerable distance backwards as 

 a mid-dorsal stripe. The hinder end is also tinged 

 with brown. The dorsal blood-vessel can be seen 

 through the integument, appearing as a dark- 

 coloured wavy line along the dorsal surface. 



3. The annuli, into which the body is divided externally 



by transverse grooves, are larger and more obvious 

 in front than behind. Each annulus corresponds 

 to a segment, and is subdivided by smaller grooves, 

 the number of which varies in different regions of the 

 body, and also on the dorsal and ventral surfaces. 



4. The prostomium, or cephalic lobe, is the rounded lobe 



forming the extreme anterior end of the worm. In 

 Lumbricus it extends backwards on the dorsal sur- 

 face so as completely to intersect the first annulus 

 and meet the second. In Allolobophora it extends 

 about halfway through the first annulus. 



5. The clitellum, or cingulum, is the thickened integument 



of the dorsal and lateral surfaces of the thirty-second 

 and five following segments, but varies slightly both 

 in position and extent in different specimens and at 

 different times of the year. In other species of earth- 

 worm it occupies other segments. 



