LIFE HISTORIES. 205. 
histological peculiarities, the absence of cilia—paralleled elsewhere: 
only among the Arthropods—the nature of: the muscle cells, the con- 
dition 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 Spherularia, be degenerate. Of the relationships 
nothing is known. 
Lire HIsToRIES 
I. The embryo grows directly into the adult, and both live in fresh. 
or salt water, damp earth, and rotting plants—Enoplide, e.g. 
Enoplus. 
2. The larvee are free in the earth, the sexual adults are parasitic im 
plants, or in Vertebrate animals, e.g. Zylenchus scandens, «a 
common parasite on cereals; Strongylus and Dochmius im 
man. 
3. The sexual adults are free, the larve are parasitic in insects, 
e.g. Mermis. The fertilised females of Spherularia bombt 
pass from the earth into the body-cavity of humble-bee and 
wasp, whence their larvze bore into the intestine and eventually- 
emerge. 
. The larvee are parasitic in one animal, the sexual adults in another 
which feeds on the first. Thus O//zdanus passes from mouse: 
to cat, Czcullanus from Cyclops to perch. 
as 
There are other life histories, and many degrees of parasitism. The 
most remarkable form is Axgiostomum (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- 
Spherularia bombz, which gets into the body cavity of the humble-bee, 
has a prolapsed uterus, larger than itself; the male of 7rechodes crassi-~ 
cauda passes into the uterus of the female. 
[TaBLE. 
