LIFE HISTORIES. 205 



lions to a parasitic life, and there is no sharp structural line of demar- 

 cation between free and parasitic forms. Some, like Ascaris, secrete 

 an irritating toxin. Among histological peculiarities, the practically 

 complete absence of cilia paralleled elsewhere only among the 

 Arthropods the nature of the muscle-cells, the condition of the sub- 

 cuticular layer, are to be noticed. Among the grosser structural 

 peculiarities, 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 Sphcerularia> 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 Enoplidae, 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 

 common parasite on cereals ; Strongylus and Dochmius in 

 man. 



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



e.g. Mermis. The fertilised females of Sphcerularia bonibi 

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

 wasp, whence their larvae bore into the intestine and eventually 

 emerge. 



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



which feeds on the first. Thus Ollulanus passes from mouse 

 to cat, Cucullanus from Cyclops to perch. 



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

 most remarkable form is Angiostonium (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 sirongyloides from the intestine of man, and 

 Leptodera appendiculata from the snail. 



There are several quaint reproductive abnormalities, thus -the female 

 Sphcerularia bombi, which gets into the body cavity of the humble-bee, 

 has a prolapsed uterus, linger than itself; the male of Trichodes crassi* 

 cauda passes into the uterus of the female. 



