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Multilayered
animals. Worms.
Worms
Although all worms have more than two layers of cells and
most have long, slender bodies, the various groups of worms
are different from each other in other respects.
The
simplest worms are the flatworms (phylum Platyhelminthes),
most of which have flattened shapes like leaves or ribbons.
Although musculoepithelial cells have been found in some
flatworms, the muscle cells in most are distinct from the
epithelial cells. There is a layer of circular muscle fibres
immediately under the epidermis, a layer of diagonal fibres,
and a still deeper longitudinal layer. There are also dorsoventral
muscle fibres running from the upper to the lower epidermis
of the flattened body. These sets of muscle fibres act in
various combinations to make the body long and thin, short
and fat, or bent to one side or the other. These muscles
are also used by some of the larger flatworms to pass waves
of muscular contraction along the body, enabling the worm
to crawl in a snaillike fashion.
Many
flatworms have a mouth opening connected to the pharynx,
a muscular tube that conveys food from the mouth to the
intestine. In some flatworms the pharynx is protruded and
inserted into invertebrate prey, to digest and suck out
the contents. The sucking is done by peristalsis, waves
of muscular contraction that move along the tube from the
mouth toward the gut. Although the muscle cells of flatworms
are generally not musculoepithelial, their nuclei are found
in large cell bodies. The muscle fibres of vertebrates and
higher invertebrates, on the other hand, have no projecting
cell body.
Roundworms
(phylum Nematoda) also have large cell bodies on their muscle
cells, but these muscle cells are unique in that nerve fibres
do not travel to them as they do in the muscles of other
animals. Instead, narrow projections of the muscle cell
bodies extend to the principal nerves and contact nerve
cells there.
Roundworms
have obliquely striated, longitudinal muscle but no circular
muscle. They are enclosed in a thick cuticle that allows
bending but prevents swelling. Therefore, contraction of
the longitudinal muscle can only bend the body. Roundworms
do not bend from side to side likeeels or snakes, but up
or down (dorsal or ventral). By preventing swelling, the
cuticle ensures that shortening of one muscle group stretches
the other; thus, it makes the dorsal and ventral longitudinal
muscles antagonistic to one another. Most crawl between
soil particles or among the villi of a host's gut by undulating
waves of muscular contraction. Similar movements also enable
some roundworms to swim.
The
segmented worms (phylum Annelida) include the earthworms
and many marine worms. Inside the body, between body wall
and gut, is a fluid-filled cavity, the coelom, which in
some annelids, including earthworms, is divided into successive
segments. The body wall has an outer layer of circular muscle
and an inner layer of longitudinal muscle.
Earthworms
crawl by peristaltic contractions of the body wall. Each
segment is alternately elongated (by contraction of its
circular muscles) and shortened (by contraction of its longitudinal
muscles). The muscles of each segment contract just after
those of the segment in front, so that waves of contraction
pass backward along the body, enabling the worm to moveslowly
forward. The same movements also serve for burrowing. While
shortened, the segments are pushed against the burrow wall,
and when they elongate again the worm moves forward.
