HIGH SCHOOL ZOOLOGY. 221 



The order to which the earthworm belongs, however, 

 (Oligochseta) chiefly includes fresh-water (limicolous) or teres- 

 trial (terricolous) worms, where the bristles are few in number, 

 and lodged in setigerous follicles, there being no parapodia, nor 

 appendages of the natux'O of feelers or gills. The Limicolse are 

 small forms living for the most part in the mud at the bottom 

 of ponds or streams. Son\e of them, the NaididcB, are parti- 

 cularly remarkable on account of their reproducing themselves 

 by budding, so that they are often found in chains, still 

 attached to each other. They live chiefly on decaying vegetable 



Fi^'. 149.— Marine lob-worm. {Arenicola piscatorum). 

 The bunches of setae are more apparent in front of the gills than behind. 



matter, but one species of Chcetogaster lives a parasitic life in 

 the lungs of various pond-snails. The earthworm {^Lumhricws 

 terrestris) is the most familiar of the Terricolse ; its setse are not 

 conspicuous, but each segment carries eight, disposed in four 

 groups. One region of the body is often swollen and noticeable* 

 bearing the clitellum ; it furnishes a cocoon in which the eggs 

 are developed. The researches of Darwin proved the earth- 

 worm to be of the first importance in the loosening of the soil 

 and the formation of mould. This is effected in the course of 

 its burrowing, which it does partly by separating the particles 

 of earth, partly by swallowing them. Although no special respir- 

 atory organs are present, yet the skin is traversed throughout 

 by capillary vessels, which bring the blood close to the surface. 

 The fluid portion of the blood (not the corpuscles, I, 60) eon- 

 tains haemoglobin ^ and its circulation through the skin, as well 



