EXTERNAL CHAEACTERS 57 



I. EXTEENAL CHABACTEES. 



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. The general colour of the animal is pinkish, darker in the 



dorsal and anterior part of the body, paler on the 

 ventral surface and in the hinder part of the body ; 

 and with a dark-coloured band down the middle 

 of the back. The dorsal blood-vessel can be seen 

 through the integument, appearing as a dark-coloured 

 wavy line along the dorsal surface. 



3. The amiiili 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 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. 



5. The cuticle is a delicate chitinous membrane investing 



the whole body of the worm. It may usually be 

 detected by its iridescence, and can be readily stripped 

 off a worm which has been killed with chloroform, or 

 one which has been macerated a few hours in water. 

 Examined microscopically it shows a series of- 

 oblique lines, intersecting each other at right angles ; 

 and minute highly refractive thickenings at many of 

 the points where the lines intersect. 



6. The setae are short chitinous spines implanted m the 



body-wall, and arranged in two double rows along 

 each side of the body, each segment except the first 

 and last bearing four pairs. They may be felt on 



