26o 



THE INVERTEBRATA 



It lives in the mud at the bottom of ponds and lakes with its head 

 buried and its tail waving in the water; the latter movements are 

 respiratory. They draw water from upper layers which contain more 

 oxygen : when the oxygen content of the water in general falls a greater 

 length of the worm is protruded and its movements become more 

 vigorous. A great deal of detritus passes through its alimentary canal 

 so that Tuhifex plays the same sort of part in fresh water that the 

 earthworms play on land. 



Lumbriculus resembles Tubifex superficially but has only eight 



d.v. pi 



Fig. 192. Blood circulation in Lwnbricidus variegatus. After Haffner. A, 

 Head and anterior region showing dorsal and ventral vessels joined by a 

 network of vessels round the gut. B, Single segment of the middle region 

 with a much closer plexus. C, Posterior end with a continuous sinus round 

 the gut connected at intervals with the dorsal and ventral vessels. An. anus; 

 bl. blind contractile sac of the dorsal vessel (d.v.); M. mouth; pi. plexus; 

 sin. sinus ; v.v. ventral vessel. 



chaetae in a segment, placed as in Lumbricus ; chaetae double pointed ; 

 not often met with in sexual state but reproduces habitually by 

 breaking up into pieces each of which regenerates the missing 

 segments. 



In this worm the primitive nature of the blood system is well seen 

 (Fig. 192). At the posterior end there is a continuous sinus round the 

 gut, in the middle region this becomes resolved into a dense plexus 

 of capillaries and at the anterior end there is the beginning of a seg- 

 mental arrangement. 



