The popular name for Nematomorpha
(nem´a-to-mor´fa) (Gr. nema
thread, + morphe
, form) is “horsehair
worms,” based on an old superstition
that the worms arise from horsehairs
that happen to fall into water, and they
look something like hairs from a
horse’s tail. They were long included
with nematodes, with which they share
the structure of the cuticle, presence of
epidermal cords, longitudinal muscles
only, and pattern of nervous system.
However, since the early larval form of
some species has a striking resemblance
to the Priapulida, it is impossible
to say to what group the nematomorphs
are most closely related.
About 250 species of horsehair
worms have been named. Worldwide
in distribution, they are free living as
adults and parasitic in arthropods as
juveniles. Adults do not feed but will
live almost anywhere in wet to moist
surroundings if oxygen is adequate.
Some juveniles, such as Gordius
(named for an ancient king who tied
an intricate knot), a cosmopolitan
genus, are believed to encyst on vegetation
that may later serve as food for a
grasshopper or other arthropod. In the
marine form Nectonema
swimming, + nema
, thread), juveniles
occur in hermit crabs and other crabs.
Form and Function
|Figure 15-18 Structure of Paragordius, a nematomorph. A, Longitudinal section
through the anterior end. B, Transverse section. C, Posterior end of male and
female worms. Nematomorphs, or “horsehair
worms,” are very long and very
thin. Their pharynx is usually a solid cord of cells and is nonfunctional.
Paragordius, whose pharynx opens through to the intestine, is unusual in this
respect and also in the
possession of a photosensory organ (“eye”).
Horsehair worms are extremely long
and slender, with a cylindrical body.
Their length ranges from 10 to 70 cm,
but their diameter is only 0.3 to
2.5 mm. Their anterior end is usually rounded, and their posterior end is
rounded or has two or three caudal
lobes (Figure 15-18).
The body wall is much like that of
nematodes: a secreted cuticle, a hypodermis,
and musculature of longitudinal muscles
only. Ventral, or dorsal
and ventral, but not lateral, hypodermal
cords are present. In most nematomorphs
the ventral nerve cord is connected
to the ventral hypodermal cord
by a nervous lamella
The digestive system is vestigial.
The pharynx is a solid cord of cells,
and the intestine does not open to the
cloaca. Larval forms absorb food from
their arthropod hosts through their
body wall, and adults apparently live
on stored nutrients.
Circulatory, respiratory, and excretory
systems are lacking. There are a
nerve ring around the pharynx and a
midventral nerve cord.
Juveniles do not emerge from their
arthropod host unless water is nearby.
Adults are often seen wriggling slowly
about in ponds or streams, with males
being more active than females. Each
sex has a pair of gonads and a pair of
gonoducts that empty into the cloaca.
Females discharge eggs into the water in
long strings. Juveniles hatch from the
eggs and somehow gain entry into the arthropod host. After several months in
the hemocoel of the host, the matured
worm emerges into the water. Curiously,
if the host is a terrestrial insect,
the parasite stimulates the insect by an
unknown mechanism to seek water.