Kinorhyncha (kin´o-ring´ka) (Gr. kinein
, to move, + rhynchos
are marine worms a little larger than
rotifers and gastrotrichs but usually not
more than 1 mm long. The phylum has
also been called Echinodera, meaning
spiny necked. About 75 species have
Kinorhynchs are cosmopolitan, living
from pole to pole, from intertidal
areas to 6000 m in depth. Most live
in mud or sandy mud, but some
have been found in algal holdfasts,
sponges, or other invertebrates. They
feed mainly on diatoms. About 100
species have been reported. Among
the best-known genera of the Kinorhyncha
are Echinoderes, Pycnophyes
, and Kinorhynchus
Form and Function
The body of the kinorhynch is divided
into 13 segments, which bear spines
but have no cilia (Figure 15-7).
|Figure 15-7 Echinoderes, a kinorhynch,
is a minute marine
is superficial. The head,
with its circle of
spines, is retractile.
retractile head has a circlet of spines
with a small retractile proboscis.
body is flat ventrally and arched dorsally.
The body wall is made up of a
cuticle, a syncytial epidermis, and longitudinal
epidermal cords, much like
those of nematodes. The arrangement
of the muscles is correlated with the
segments, and circular, longitudinal,
and diagonal muscle bands are all
A kinorhynch cannot swim. In the
silt and mud where it commonly lives,
it burrows by extending the head into
the mud and anchoring it with spines.
It then draws its body forward until its
head is retracted into its body. When
disturbed, a kinorhynch draws in its
head and protects it with a closing apparatus of cuticular plates (Figure
The digestive system is complete,
with a mouth at the tip of a proboscis,
a pharynx, an esophagus, a stomachintestine,
and an anus. Kinorhynchs
feed on diatoms or on organic material
in the mud where they burrow.
The pseudocoel is filled with amebocytes
containing fluid. The excretory
system is made up of a multinucleated
solenocyte protonephridium on each
side of the tenth and eleventh segments.
Each solenocyte has one long
and one short flagellum.
The nervous system is in contact
with the epidermis, with a multilobed
brain encircling the pharynx, and with
a ventral ganglionated nerve cord
extending throughout the body. Sense
organs are represented by eyespots in
some and by the sensory bristles.
Sexes are separate, with paired
gonads and gonoducts. There is a
series of about six juvenile stages and a
definitive, nonmolting adult.