522 EVOLUTION OF ANIMALS Part V 



Nematodes have no special circulatory or respiratory systems. The fluid 

 contained in the body cavity distributes digested food and collects metabolic 

 waste. The microscopic nematodes of the soil evidently exchange respiratory 

 gases through the outer cuticle just as minute insect larvae exchange gas 

 through their extremely thin chitinous covering. Ascaris is mainly anaerobic, 

 obtaining oxygen from the body fluids of its host and energy from the break- 

 down of its own stored glycogen. Ascaris has a definite excretory system. Two 

 canals, one running along each side of the body, come together at the anterior 

 end and open to the outside through a ventral pore. The nervous system is a 

 delicate ring of nervous tissue about the esophagus. Two large nerves con- 

 nected with the ring extend the length of the body, one on the dorsal and one 

 on the ventral side with connecting branches. The higher invertebrates have 

 a main ventral nerve chain and the vertebrates a dorsal nerve cord. Ascaris is 

 not committed to either plan. 



The male and female reproductive systems are in separate individuals and 

 in either one the organs occupy a large part of the body cavity. The eggs are 

 fertilized in the uterus. Each one is later surrounded by a hardy chitinous shell. 

 The egg shells are so resistant to chemicals that they will develop while im- 

 mersed in a weak formalin solution. 



Free Living Soil Nematodes. Myriads of little animals find pasture on the 

 plants in the shallows of fresh waters. These millions feed on one another, on 

 the algae that cloak the living plants, and on the soft tissues of decaying ones. 

 Among them in untold numbers are the nematode worms recognizable under 

 the microscope by their glassy smoothness and translucence. Among other 

 wigglers of different kin, bristle worms and gnat larvae, the sweeping curves of 

 the nematodes are distinctive. 



Numerous as parasitic nematodes may be, those that live independently in 

 fresh and salt water and soil probably far outnumber them. Their home niches 

 are astonishingly various, on lake bottoms, in hot springs, and in polar seas, 

 in soils, even in deserts. 



Vinegar Eels. Who has seen live vinegar eels? Probably nobody who has 

 used only "store vinegar," pasteurized and bottled. Vinegar eels are the 

 nematode worms (Turbatrix aceti) of raw cider vinegar. They are about one- 

 sixteenth of an inch long and their characteristic nematode thrashing move- 

 ments are recognizable when the vinegar containing them is held up against 

 strong light (Fig. 26.1). They are distributed on the fruit mainly by fruit flies, 

 Drosophila melanogaster, the famous fly of genetics. It is also the fly of rotting 

 apples. 



Plant Parasites 



Minute nematodes bore into the roots of a great variety of plants. Some of 

 them, such as the sugar beet worm, Heterodera schactii, live in only a few 



