LIFE-HISTORIES. 177 



and parasitic forms. Among histological peculiarities, the 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 body cavity, 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 Sphaiidaria, be degen- 

 erate. 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 — Enoplida?, e.g. 

 Enoplus. 



2. The larva; 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 larva; are parasitic in insects, 



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

 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 Olhdanus 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 Angiostomum (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 

 Sphcerularia 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. 



Table. 



