EXTERNAL CHARACTERS 57 



Each seta is sigmoidally curved : it is thickest at 

 its middle, and tapers towards each end. 



C. 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 bodies at many of the 

 points where the lines intersect. 



7. External apertures. 



a. The mouth is a small crescentic aperture on the 



ventral surface of the first segment. It is over- 

 hung by the prostomium or cephalic lobe. 



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 placed 

 on the ventral surface of the fifteenth segment, 

 one on each side, slightly to the outer side of 

 the inner double row of setae. They have 

 tumid lips. 



ii. The apertures of the oviducts are similarly 

 placed in the fourteenth segment. They are 

 minute transverse slits not seen so easily as 

 the male apertures, their lips not being tumid. 



iii. The apertures of the spermathecae or receptacula 

 seminis, of which there are two pairs, are 

 situated in the grooves between the segments 

 and in the line of the outer row of setae, the 

 first pair between segments ix. and x., the 

 posterior pair between segments x. and xi. 



d. The excretory apertures are difficult to see. Each 



segment, except the first three and the last, has 

 one on each side, on the ventral surface a little 



