NEMATODES 239 



which passes gradually into an oviduct and a uterus at 

 each side, and a short unpaired vagina. The genital 

 aperture is ventral and anterior. 



The ova meet the spermatozoa at the junction of uterus 

 and oviduct. Segmentation is total, and resuhs in the 

 formation first of a blastula and then of a gastrula. The 

 germ-cells are distinguishable very early from the body- 

 cells. Blastopore and archenteron are obliterated, the 

 mid-gut arising as a secondary splitting between two rows 

 of endoderm cells. The eggs pass out of the gut of the 

 host and probably hatch in water, and are thus re-intro- 

 duced. No intermediate host is known. There is evidence 

 that the larvae of Ascaris in some hosts exhibit an extensive 

 migration within their host before settling down to mature 

 in the intestine. The same may be true in man. 



Though parasitism is exceedingly common among Nematodes many 

 are free-Hving for at least a part of the life-cycle, and feed on putre- 

 fying organic matter. Although the number of individuals which may 

 infest one host shows how successful the parasitism is, yet Nematodes 

 exhibit few of the ordinary adaptations to a parasitic Ufe, and there is 

 no sharp structural line of demarcation between free and parasitic 

 forms. Some, like Ascaris, secrete an irritating toxin. Among histo- 

 logical pecuharities, the practically complete absence of cilia — 

 paralleled elsewhere only among the Arthropods — the nature of Hhe 

 muscle-cells, the condition of the subcuticular layer, are to be noticed. 

 Among the grosser structural pecuharities, the nature of the excretory 

 system, of the cavity of the body, and of the nervous system, are 

 worthy of special note. Sense organs are never well developed, but 

 in the free-living forms simple eyes may occur. The alimentary canal 

 is usually completely developed, but may, as in Sphcsrularia, be 

 degenerate. Of the relationships nothing is known. 



Life-Histories 



1. The embryo grows directly into the adult, and both live in fresh 



or salt water, damp earth, and rotting plants— Enoplida, e.g. 

 Enoplus. 



2. The larvce are free in the earth, the sexual adults are parasitic in 



plants, or in Vertebrate animals, e.g. Tylenchus scandens, a 

 common parasite on cereals ; Strongyhis and Dochmius in 

 man. 



3. The sexual adults are free, the larvae are parasitic in insects, 



e.g. Merniis. The fertilised females of Sphcerulana bombi 

 pass from the earth into the body cavity of humble-bee and 



