NEMATODES. 183 



ually into an oviduct, a uterus, and a short vagina. 

 The genital aperture is ventral, usually about the middle 

 of the body, but it is occasionally far forward or far 

 back. 



Development. 



The ova meet the spermatozoa at the junction of uterus 

 and oviduct. Segmentation is total and may take place 

 before or after laying. Indeed the embyro may be hatched 

 within the uterus. Before the embryo exhibits adult char- 

 acteristics, several, e.g., three, moultings of the cuticle usually 

 occur. 



LIFE HISTORIES. 



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



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

 Enoplus. 



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



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

 mon parasite on cereals ; Strongylus and Dochmius in man. 



3. The sexual adults are free, the larvse are parasitic in insects, e.g., 



Mermis. The fertilised females of Spharularia bombi pass from 

 the earth into the body cavity of humble-bee and wasp, whence 

 their larvse bore into the intestine and eventually emerge. 



4. The larvee are parasitic in one animal, the sexual adults in another 



which feeds on the first. Thus Ollulanus passes from mouse to 

 cat, Cucullamis from Cyclops to perch. 



There are other life histories, and many degrees of parasitism. The 

 most remarkable form is Angiostoinum (or Ascaris, or Leptodera] 

 nigrovenosum. In damp earth males and females occur, the progeny of 

 which pass into the lungs of frogs and toads. There they mature into 

 hermaphrodite animals (the only example among Nematodes), which 

 produce first spermatozoa and then ova. They are self-impregnating, 

 and the young pass out into the earth as males or females. Here there 

 is alternation of generations, and a somewhat similar story might be 

 told of Rhabdonema strongyloides from the intestine of man and 

 Leptodera appendiculata from the snail. 



There are several quaint reproductive abnormalities, thus the female 

 Spharularia bombi, which gets into the body cavity of the humble- 

 bee, has a prolapsed uterus, larger than itself; the male of Trichodes 

 crassicauda passes into the uterus of the female. 



