NEMATODES. 1S3 



ually into an oviduct, a uterus, and a siiort 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 — Enoplidoe, e.g., 

 Enophts. 



2. The larva: are free in the earth, the se.xual adults are parasitic in 



plants, or in Vertebrate animals, e.g., Tylench^ts scandens, a com- 

 mon parasite on cereals ; Strojigyhts and Dochmuis in man. 



3. The se.xual adults are free, the larva; are parasitic in insects, e.g., 



Mermis. The fertilised females of Sphierjtlaria bonibi pass from 

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

 their larva bore into the intestine and eventually emerge. 



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



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

 cat, Citcidlaiius from Cyclops to perch. 



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

 most remarkable form is Aiigiostontin]! {or Ascaris, or Lepiodera) 

 nigrovenositni. 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, tiere there 

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

 told of lihabdonema strongyloides from the intestine of man and 

 Leptodera appeiidiciilata from the snail. 



There are several quaint reproductive abnormalities, thus — the female 

 Sphicrula7-ia boinhi, which gets into the body cavity of the humble- 

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

 trassicauda passes into the uterus of the female. 



