EXTEENAL CHARACTEES. 61 



6. The cuticle is a delicate cLitirtous 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 by 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. 



7. The setae are short chitinous spines implanted in 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 

 drawing the worm through the fingers from head to 

 tail. The setsB are to be regarded as special localised 

 thickenings of the cuticle. 



The seta are easily isolated by boiling a bit of a luorm in 

 a solutio7i of caustic potash : they should then be ivashed and 

 mounted in glycerine, or dried and mounted in balsam. 



Each seta is sigmoidally curved : it is thickest at 

 its middle, and tapers towards each end. 



8. External apertures. 



a. The mouth is a small creseentic aperture on the 



ventral surface of the first segment. It is over- 

 hung by the prostomium. 



b. The anus is an oval aperture at the hinder end of 



the body. 



c. The genital apertures are four pairs. 



i. The apertures of the vasa deferentia are situated 

 on the ventral surface of the fifteenth segment, 

 one on each side, slightly to the outer side of 

 the inner double row of sets. They are trans- 

 verse slits, with tumid lips. 



ii. The apertures of the oviducts are similarly 

 situated in the fourteenth segment. They are 



