II. NEMATHELMINTHES: NEMATODA 265 



number of nerves, those in the mid-dorsal and ventral lines being strongest. 

 At points on these nerves are collections of ganglion cells, but a formation 

 of ganglia, as in the annelids, does not occur (fig. 245). The only sense 

 organs are tactile papillae near mouth and genital opening, and eyespots in 

 a few free living forms. 



The sexual organs of these rarely hermaphroditic forms are very 

 simple. !Males and females are easily distinguished, not only by the 

 copulatory organs, but by the openings of the genital ducts. These, in 

 the male (fig. 246), are in the end of the alimentary canal, which hence is a 

 cloaca. In the female (fig. 244) there is a special genital opening on the 

 ventral surface between mouth and anus, the position varying with the 

 species. In general the structure of the reproductive organs is alike in 

 both sexes. These are long tubes coiled forward and back and ending in 

 fine threads which produce eggs or sperm (ovaries, testes), while the rest 

 serves as seminal vesicle, or rcceptaculum seminis, and ducts. In the male 

 the genital tube is always single; in the female it is usually double, the 

 right and left halves uniting a little before the external opening (fig. 244, 

 va). Most common of copulatory organs in the male are spicula, bent 

 spines, which lie in a sheath behind the vent and can be protruded 

 through the cloacal opening. Besides there may be valves to right and 

 left to clasp the female, or, as in Trichina, the whole cloaca is protrusible. 



Since there is copulation, the eggs are fertilized in the uterus, after which 

 they are either laid or retained for more or less of their development, many, like 

 Trichina, being viviparous. The postembryonic development depends largely 

 upon the mode of life. Free-living species grow by repeated molts without 

 much change of form. In many Anguillulida:, which show how free life can be 

 transformed into parasitic, there is an alternation of generations (heterogony) 

 from a protandric hermaphroditic entoparasitic to a free dicecious generation. 

 The occasional suppression of the free generation which occurs in many Anguil- 

 lulids leads to the Strongylida;, where the offspring of the parasitic generation 

 can live free for a time (rhabditis larvae), but must return to parasitism to undergo 

 a metamorphosis and become sexually mature. The free life is shortened again 

 in the Ascaridas, where the eggs must pass to the exterior for a longer or shorter 

 time, but the embryos only escape when the eggs are taken into another host. 

 Lastly, there are species like Trichina where the free life is entirely suppressed 

 and transportation from host to host takes place in the encysted condition pas- 

 sively by food. This purely parasitic condition leads to species in which the 

 rhabditis larva developed in water, enter a second host for encystment as the 

 larvK of Filaria medinensis in the Cyclopidas. 



Family i. Anguillulid.e; small thread-like nematodes which live in mud, 

 organic fluids or plants, rarely in animals; male with two spicula. Anguilltda 

 aceti* vinegar eel, in vinegar and stale paste. Rhabditis {Rhabdonema) nigro- 

 venosa, lives in mud and stands in heterogony with a second form which lives in 

 the lung of frogs. Strongyloides intestinalis , which has recently appeared in 

 southern Europe, has a somewhat similar history, the adult stage being reached 

 in the human intestine. In the tropics one stage of this is passed in moist earth, 

 but in colder climates the free-living generation drops out. Here belong 



