| Polyclads
are hermaphrodites with male and female reproductive systems
in one body. Self fertilization is possible but outcrossing
is the rule. The reproductive structures of polyclads are
not well-defined organs as may be found in many other animals.
Instead, they are scattered throughout the parenchyma.
When sperm
are produced, they often collect in an enlarged reservoir
called the seminal vesicle before being moved into the copulatory
organ. Prostatic secretions that nourish the sperm and extend
their life span, are added to the mixture. The arrangement
of the seminal vesicle, prostatic vesicle, copulatory organ
and various ducts is extremely varied and complex.
Reproductive
system of Armatoplana colombiana. The anterior end
of the animal is to the left.
It
is precisely this arrangment that can be used by taxonomists
to determine many of the polyclad species. The copulatory
organ can be armed with hard, sclerotinized structures such
as stylets or spines.
 |
Armed
stylet extruded from the acotylean Styloplanocera
fasciata.
Photo
credit: Bolanos & Quiroga
|
Eggs are
produced in ovaries and moved to uteri that usually lie alongside
the pharynx. Egg-filled uteri can sometimes been seen through
a transparent body wall or when a worm is turned upside down.
Fertilized eggs are surrounded by a hardened shell or capsule.
Egg capsules are released to the outside where they are being
glued down by additional secretions from the female pore.
Melloplana
ferruginea
showing egg-filled uteri along either side of pharynx.
Fertilization
of the eggs is internal and sperm must be delivered to the
interior of the mate. It is possible that fertilization is
mutal because polycalds are hermaphroditic, thus, able to
function both as male and female. However, it is more likely
that one worm acts as the male and the other as the female.
A few
species of flatworms (especially the colorful pseudocerotids
that live on coral reefs) have evolved other mechanisms to
assure fertilization. They use a method known as hypodermic
insemination. It means that they jab their penis through the
skin of a partner and and then release their sperm into the
other worm's body. The sperm then finds its way to the eggs
for fertilization.
Which
worm acts as the male injecting its sperm and which worm acts
as the female bearing the burden of egg laying? Their roles
are determined during a behavior known as penis fencing. Penis
fencing was first described and documented on film by
Dr. Leslie Newman, one of the flatworm wranglers (the link
will take you to the PBS site "The Shape of Life"
where you can see a short video clip of penis fencing flatworms;
you will need the RealPlayer plug-in to view the video).
Penis
fencing begins, when two pseudocerotid worms approach each
other and start to rear up with their anterior ends. They
begin to trust out their penis (in some species two of them),
armed with a fine, hard stylet that acts like a hypodermic
needle.
Each worm
then tries to jab its penis into any part of the partner's
body. The worm to first penetrate its partner's body will
be able to release its sperm. Penis fencing may go on repeatedly,
and often the wounds filled with whitish sperm can be seen
on the surface of the partner (arrow).
Once fertilized,
the eggs
are attached to the substrate, develop, and after a few days,
young worms hatch and crawl away. Most acotylean polyclads
develop this way. In others though, the eggs hatch into tiny
larvae covered with 4 or 8 ciliated arm-like lobes. These
larvae, called Müller’s or Götte’s larvae
depending on the number of lobes, will swim about in the plankton
and will have to undergo metamorphosis before settling to
the bottom.
|
The
larva of a polyclad flatworm. This image is from BIODIDAC
|
|
