300 CCELHELMINTHES. 



ture of the reproductive organs is alike in both sexes. In both, 

 on account of the great fertility, these are long tubes coiled for- 

 ward and back and ending in fine threads which produce eggs or 

 sperm (ovaries, testes), while the rest serves as seminal vesicle, or 

 receptaculum 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. 263, 

 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, appropriate muscles caus- 

 ing them to retract. Besides there may be valves to right and left 

 to clasp the male, or, as in Trichina, the whole cloaca is pro- 

 trusible. 



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 post- 

 embryonic development depends largely upon the mode of life. 

 Free-living species grow by repeated molts without much change 

 of form. In many Anguillulidae, which show how free life can be 

 transformed into parasitic, there is an alternation of generations 

 (heterogony) from an hermaphroditic entoparasitic to a free 

 dioecious generation. The occasional suppression of the free gen- 

 eration which occurs in many Anguillulids leads to the Strongylidse, 

 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 Ascaridae, 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 con- 

 dition passively by food. 



Family 1. ANGUILLULIDS ; small thread-like nematodes with double 

 pharyngeal swelling which live in mud, organic fluids or plants, rarely in 

 animals ; male with two spicula. Anguillula aceti, vinegar eel, 2 mm. 

 long, in vinegar and stale paste. Rhabditis (Rhabdonema) nigrovenosa r 

 not 1 mm. long, lives in mud and stands in heterogony with a second 

 form which lives in the lung of frogs. Strongyloides intestinalis of the 

 tropics, but which has recently appeared in southern Europe, has a some- 

 what similar history, the adult stage being reached in the human intes- 

 tine. Here also belong numerous plant parasites of which Tylemhus 

 tritici and Heterodera schachti demand notice, the first doing great dam- 

 age to wheat, the second to turnips in Europe. Tylenchus devastatrix 

 attacks rye and hyacinths. 



