212 NATURAL HISTORY OF BHOMPSTON POND. 



stems of plants, or even swimming freely. The body is 

 segmented (Annelida) and furnished -with small groups of 

 about eight hooked ventral setae and, excepting the first four 

 segments, with pairs of long hair-like setse (a) (Oligochaeta). 

 The muscles attached to the ' setigerous sacs are clearly dis- 

 tinguishable through the thin delicate skin of the living worm. 

 The head bears two black eyes (b), and the praestomium is 

 elongated into a proboscis (c). A large pharynx is situated 

 in the first four segments, while the narrow oesophagus (d), 

 situated in the three following segments, leads into the 

 dilated crop (e). The rest of the alimentary canal is con- 

 tinued as the intestine (f), which is thrown into many folds. 

 The dorsal vessel contains clear and almost colourless blood ; 

 the body cavity septa usually show very distinctly. The 

 nephridia (not present in the first six segments) are easy to 

 detect, but the movement of the cilia within requires a sixth 

 lens. Many of the specimens exhibit a zone of fission (g), 

 marking the point at which the worm divides into two 

 portions. The new head, eyes, and even proboscis (h) 

 (formed from the posterior half of the zone) are visible before 

 the division takes place. The anterior half of the zone gives 

 rise to the tail segments of the other animal. Sexual repro- 

 duction is also present, and the organs can sometimes be seen 

 in the fifth and few following segments. 



Tubifex rivulorum (fig. 40), an allied worm with a long 

 slender reddish body, is almost as common as the last. Several 

 specimens are usually found together, vigorously waving in 

 the water, with the posterior ends of their bodies (a) protrud- 

 ing from the burrows in which they live (b). At the least 

 alarm the worms suddenly withdraw into their tubes and 

 vanish from view. The peristaltic movements of the ciliated 

 alimentary canal, and the movements of the large blood 

 vessel bathed in nutritive fluid, are plainly visible through 

 the transparent body. There are two dorsal and two ventral 

 rows of chaetse. 



Of the leeches, one of the Rhynchobdellse, namely 

 Glossiphonia (Clepsine) (fig. 41) is quite numerous on the 



