348 



APPLIED BIOLOGY 



(also called metameres). They are much higher than the 

 worms previously mentioned. There is much diversity of 

 form and habit among the members of this group, and we can- 

 not now take time for more than a brief examination of a few 



common examples of 

 the segmented worms. 



301. The Sand- 

 worm. (D) This worm, 

 belonging to the genus 

 Nereis, lives in sand at 

 the sea-shore near low- 

 tide mark, and thousands 

 are dug and used by fish- 

 ermen for bait. In cer- 

 tain seasons they are 

 rapid swimmers in the 

 sea. Specimens may be 

 kept and examined in 

 long test-tubes, or in 

 glass tubing stoppered at 

 each end with a cork and 

 then dipped into hot par- 

 affin. Examine a sand 

 worm according to the 

 following notes. It has 

 a head with jaws, small 

 tentacles, two pairs of 

 bead-like eyes, and a 

 pair of cylindrical palps 

 (feelers). On some speci- 

 mens the proboscis may 

 be found extended from 

 the mouth. There are several pairs of cirri (feelers) back of the 

 head. The body is divided into segments ; count the number in an 

 inch of length and then estimate the total number. Are the seg- 

 ments similar? How are the rowing organs or paddles arranged 

 on the segments ? Perfect specimens have a forked tail-appendage. 

 The main longitudinal blood-vessels appear through the skin on 

 dorsal and ventral surfaces. 



Dissected specimens pinned on pieces of soft wood or on cork may 



FIG. 111. Development of a marine annelid. 

 1, fertilized egg-cell; 2, two-cell stage; 3, 

 four-cell stage; 4, eight-cell stage; 5-8, later 

 stages which end in the free-swimming larva, 

 (9), and this metamorphoses (10) into the 

 adult form with numerous segments (11). 

 m, mouth; a, anus; e, alimentary canal. 

 (From Thomson, after Fraipont.) 



