19 



Besides the digestive, sexual, and excretory systems, nematodes 

 also have a sort of nervous system, consisting principally of a so 

 called nerve ring, which surrounds the oesophagus just behind its 

 median bulb. This, however, is usually very indistinct and not highly 

 developed. A circulatory system is entirely wanting in nematodes. 



Description of the Parasitic, Gall Forming Nematode, Heterodera 



radicola. 



(a) EARLY LIFE. 



Turning now from this typical species of a nematode in its sim- 

 plest form, to the gall forming species which causes the injury to 

 plants, we shall find some similarities in structure and development 

 and also some striking differences. The egg (PI. IV., figs. 1-16), as 

 in the other species is an elliptical or rather bean shaped body .imm. 

 (o-i^f of an inch) in length, composed of a chitinous membrane inclo- 

 sing a mass of granular protoplasm and fat globules. 



The covering, although very thin, is extremely tough and very 

 resistant to heat, cold, chemical substances, etc., affording to the 

 egg contents a protection which is well nigh absolute against the 

 ordinary influences of nature. In its earliest stage the mature &^^ 

 consists inside the membrane of a loose, undivided mass with a nu- 

 cleus in the centre. After fertilization the nucleus divides and two 

 cells are formed (Fig. 3). These divide again and again passing 

 through various embryological changes and developing into a young 

 worm as shown at fig. 16. The worm moves about freely in the 

 shell and finally ruptures it and escapes. In its earliest life it resem- 

 bles the free living species having a similar form and structure. It is 

 a minute worm-shaped creature about .33mm. {.K of an inch) in length, 

 quite invisible to the naked eye. Plate VII., figs. 5 and 6, are intended 

 to give an idea of the size of the worm at this stage. Fig. 5 show^s 

 it among the particles of a fine loam 'soil, while Fig. 6 shows an 

 enlarged portion of an angle worm with two black lines upon it near 

 the centre, the shorter of which represents a young nematode in its 

 proportionate size to the angle worm. The longer black line repre- 

 sents the length of the mature male nematode, at the greatest length 

 it attains at any time or in any form. Imagining the angle worm 

 reduced to its normal size, some idea will be obtained of the minute- 

 ness of the nematode when similarly reduced. It is in this young 

 stage and in the egg that the worm exists in the soil. Its structure 



