Platyhelminthes (Flatworms) - Tapeworms
Flatworms are a very diverse and remarkable group of organisms. The body is typically dorsoventrally flattened (it is much
wider than tall, like tape). This increases the surface area to volume ratio for gas exchange, which occurs across the body
surface (and also increases surface area in contact with the substrate in those forms that move by ciliary crawling). The
model above is of a parasitic variety and is a tapeworm. Tapeworms or cestodes are intestinal parasites and in contrast to
most other flatworms they exhibit metameric segmentation (i.e. the repetition of organ systems in each segment). Many
(especially earlier) texts deny that flatworms of any variety exhibit metamerism, but in the tapeworm we have a prime
example of metamerism in many body systems, especially the reproductive systems, which repeat in each segment.
However, in tapeworms the segmentation is less evolved than in annelids and suggests evolution from a mode of asexual
reproduction in which new individuals form by budding at the rear but fail to detach straight-away, resulting in a chain of
segments or proglottids, which can be seen as incomplete individuals resulting from aborted asexual reproduction. As it
happens, the segments do eventually detach, when they are ripe and full of eggs, and then they pass out of the intestine in
the faeces to complete the life-cycle.
The segments of the tapeworm, called
proglottids bud sequentially from the neck
behind the
scolex, such that the younger
segments are nearer the front of the worm
and the hindmost segments are the oldest
and most mature. (A few tapeworms have
unsegmented bodies of moderate length
and are refrred to as monozoic, segmented
tapeworms being polyzoic). The body is
flattened (the name cestode derives from
the Latin, cestus, a girdle, as in a ribbon or
belt). The adult cestodes are (usually
intestinal) parasites of vertebrates and they
have complex life-cycles with one or two
intermediate hosts. The body varies from 1
mm to 12 metres in length and, in polyzoic
forms, contains 4 to 4000 proglottids.

The scolex is a solid, muscular mass of cells
and bears hooks and/or adhesive suckers
(acetabula). Glandular adhesive regions,
called bothria or more muscular bothridia,
may also be present on the scolex. The
bothria may extend as leaf-like phyllidia in
some species. If present, then the
bothria/bothridia and suckers are four in
number and the suckers may be armed with
hooks or spines.

The proglottids increase in size towards the
rear, where they are broadest or longest.
The exact shape of mature proglottids
depends on species, in some they are
broader than long, in some they are longer
than broad, and in others they may be
squarish. The chain of proglottids is called
the
strobila.
Above: the anterior end of a tapeworm is narrow, small and often
overlooked, but is their most distinctive feature. It ends in a rounded
structure, the scolex (or head). The scolex helps attach the worm to
the surface of the intestinal lumen, with the aid of special
attachment structures. Typically 4 suckers are present and this
specimen has two rings of hooks (on a protruding rostellum).
Reproductive System

Most species are hermaphroditic, that is possess both male and female reproductive organs (in
Dioecocestus, the sexes are separate and dimorphic, with the female longer and broader). Most are
protandrous, meaning that the male organs develop first in the maturing proglottid. Rudimentary
reproductive organs appear shortly after proglottid formation, when it is not far behind the neck. In older
proglottids, both male and female systems are developed and at the posterior of the strobilus the most
mature proglottids are ripe (gravid) being filled with a branching uterus and capsules/embryos and the
male organs, and other female parts, are then degenerate. In some species, the standard pattern of
proglottids maturing gradually from front to rear is not followed but instead the development of the strobila
is diffuse and then there is no gradation of development from front to rear and instead the proglottids
develop in synchrony.

The
gonoducts (oviduct/vagina and sperm duct) usually open via a common opening, with the ducts
leading into a hemispherical chamber called the
genital atrium which opens to the outside via the
gonopore. The gonopore is at one side of the proglottid, either central or toward one end. A sphincter
muscle can open or close the gonopore. The gonopore of each proglottid may all occur on the same side
of the worm, as in our piece of worm drawn above, or they may be on both sides in an irregular and
random fashion. (Though each proglottid only has one gonopore on either side). In some species, there is
a double system with each proglottid having two gonopores, one on each side.

The testes are usually small and numerous rounded bodies, numbering up to 100 to 1000 per proglottid.
They connect to sperm ductules which may form a fine branching and anastomosing network that joins to
form a single sperm duct, a large highly coiled tube terminating in the cirrus sac. The cirrus sac is an
elongated muscular body (the wall of which usually has an inner circular muscle layer and an outer
longitudinal muscle layer) and opens into the antrum. Inside the cirrus sac the sperm duct becomes the
ejaculatory duct (noneversible) which ends in the distal cirrus (behind which the duct may swell to form a
seminal vesicle). The cirrus is usually lined by spines, bristles or hooks and is eversible and acts as an
intromittant or copulatory organ. Prostatic glands are present in some species, and these open into the
sperm duct or the cirrus sac.

The female system is quite variable and complex, and the following account has been simplified. The
ovary is single or double and often comprises two lobes joined by a bridge, forming a H or X shaped
structure. From this extends a short oviduct, which meets the various other female ducts (in some species,
though not
Taenia, the oviduct ends in a muscular bulb, called the ovicapt which contracts and expands to
suck eggs from the ovary into the oviduct).

Yolk glands (vitelline glands) may occur in two bands, one on either side of the proglottid, in a superficial
layer enclosing the other reproductive organs, or it may, as in taenioids like
Taenia, be a small body
behind the ovary or absent. A short vitelline duct connects the yolk glands to the uterine duct, oviduct and
vagina.

The vagina is a straightish tube that links the antrum to the oviduct. Near its base, the vagina forms a
swelling, called the seminal receptacle, which temporarily stores sperm received through the gonopore.
The uterus begins to develop after the gonads mature, and is usually highly lobed or coiled. It ends in the
uterine duct. In some species the uterus opens either ventrally or dorsally by a slit, pore or row of pores. In
forms like
Taenia, the uterus has no opening to the outside and the embryos are released by proglottid
disintegration. The uterus may fragment into many small uterine capsules, each containing one or more
embryos. In some species, the uterus may connect to one to many fibrous sacs (parauterine organs)
which receive and retain the embryos.

The ripe proglottid is filled with the uterus, which is full of embryos. In apolytic tapeworms, the gravid
proglottids are shed into the host intestine, passing out with the host faeces to the outside where they
disintegrate to free the embryos. In anapolytic forms, the uterus opens by one or more uterine pores which
shed encapsulated embryos whilst proglottids are retained. In most tapeworms, yolk cells pass yolk into
the cytoplasm of the egg cell and the shed proglottids contain fully developed embryos (in some the egg is
shed along with egg cells, all enclosed in a thick capsule, and development to the embryo occurs outside
the proglottid).

Copulation. The cirrus is inserted into the antrum to inject the sperm. Proglottids usually fertilise
themselves or other proglottids on the same strobilus, though when two or more strobila occur in the same
host then cross-fertilisation may occur. Some forms lack a vagina and in these species impregnation is
hypodermal - the cirrus, armed with hooks, acts like a syringe and injects sperm beneath the body wall.

Embryonic and Larval Development and Life Cycle

In the taenioid tapeworms (including Taenia) a thin shell encloses one egg and one yolk cell. Cleavage
(division) of the egg cell is total but unequal, producing larger yolk-containing cells called macromeres and
smaller micromeres. The macromeres fuse into a syncitium surrounding the other cells as a nutritive outer
embryonic membrane. In addition, some inner cells form an inner embryonic membrane which produces a
thick cuticularised shell. The inner mass of micromeres form the
oncosphere, with 3 pairs of hooks at the
posterior end. In these tapeworms, there is usually one intermediate host, which may be an arthropod,
annelid, mollusc or vertebrate, though some have no intermediate host.

In the pork tapeworm,
Taenia solium, the embryos (not strictly eggs) and/or ripe proglottids (detached
from the end of the strobilus) pass out from the primary human host in the faeces. The oncosphere is
enclosed in a thick striated shell (the inner embryonic membrane). Pigs or humans may act as the
intermediate host if they ingest these embryos/proglottids. In this case the oncospheres hatch from their
encasing membranes (shells) and travel to the muscles (or other organs like the liver or the coelom)
where they develop into a
cysticercus larva or bladder worm (plural, cysticerci) which can grow quite
large. The oncosphere develops a central fluid-filled cavity, or bladder, and a hollow knob develops as a
thickening on the inner wall and invaginates into the fluid-filled cavity and becomes the scolex (which at
this point is inside-out, like a glove with the finger inside). When pork meat containing these cysticerci is
eaten by the final human host, the scolex evaginates and attaches to the intestinal wall of the host and
sheds the bladder and other larval parts and then proliferates, budding off proglottids at the rear. In some
other species, eversion and strobilation begins whilst inside the intermediate host (e.g. in
Cysticercus
taeniaeformis
which infects mice/rats as intermediate host and cats as primary host). These larval strobili
retain the bladder at their rear and the whole is known as a strobilocercus.

Pseudophyllid tapeworms infect mammals as primary hosts and either crustaceans or fish as
intermediate (secondary) hosts. Usually there are two intermediate hosts, first a copepod crustacean and
then a fish (which eats the crustacean and is in turn eaten by the primary host). The egg is enclosed by a
thick capsule or shell which encloses the egg and several yolk cells. The egg undergoes total and equal
cleavage (the whole egg cell and its derivatives each divide into two new cells of the same size by mitosis).
An oval or spherical embryo develops, called the
oncosphere. The oncosphere has three pairs (six)
hooks at the posterior pole and may possess a pair of penetration glands. It possesses some muscle
fibres and one pair of flame cells (see excretion and osmoregulation) and no epithelium. It is enclosed in a
membrane and in these aquatic forms it is enclosed by a further outer membrane coated in long cilia,
allowing it to swim in the water, and is then called a coracidium larva. The outer membrane disappears and
the capsule (inner membrane plus oncosphere) escape into the water and is eaten by the intermediate
host. Inside the host gut, the inner embryonic membrane is shed and the oncosphere freed to infect the
host tissues. The oncosphere then elongates into a
procercoid larva.
These diagrams and the computer model at the top of the page are based on the pork tapeworm, Taenia
solium
, the adults of which infect the human small intestine.
Body Cavity

All platyhelminths are acoelomates, that is they lack a coelom (a fluid-filled cavity lined by body cells).
Coelomates (animals with a coelom) are generally considered more advanced as they include annelids,
echinoderms and vertebrates (including humans). Instead the body of the flatworm is packed with
mesenchyme cells (packing tissue) filling the regions between organs. The exact nature of these cells is
controversial, but they appear to be somewhat star-shaped cells that put out slender processes which
connect to those of other mesenchym cells to form a web of connected cells (probably a syncitium, in
which the cytoplasm of the connected cells forms a continuous unit) with microscopic fluid-filled
in-between. Such an arrangement is an excellent way to stiffen or form-up the tissue since the fluid-filled
spaces resist compression, whilst the material is quite light.

Nutrition

The tapeworm has no digestive tract at all and no mouth or anus. Instead the strobilus exhibits contact
digestion
. It sits inside the host intestine absorbing nutrients through its body wall, utilising both host
enzymes and its own enzymes in its own surface membrane, or host enzymes that attach to the worm's
surface. The microtriches increase surface area for this purpose - increasing the area of enzyme-rich
membrane in contact with the food in the host intestine and the are for nutrient absorption. One tapeworm
generally causes little harm to a healthy host, but 2 or 3 occurring within the same host causes a
significant drain on host reserves, leading to fatigue. Tapeworms are parasitic, deriving all their
nourishment from the host by stealing food from the host's intestine.

Tapeworms secrete signal molecules, such as cGMP (cyclic guanosine monophosphate), which cause the
smooth muscle in the intestine wall of the host to contract, restricting and decreasing the flow of material
through the intestine. This may reduce the forces tending to flush the tapeworm out of the gut and/or may
increase the time the tapeworm is in contact with the food, increasing its nutrient absorption, before the
food passes to other regions of the intestine. Tapeworms may also migrate up and down within the
intestine, in a diurnal cycle, moving up to feed during the day and down at night (when proglottids may
detach and be eliminated through the host's anus in some cases).

Tapeworms have been shown to inhibit the action of trypsin (presumably mainly in the region of the
intestine in which they reside) a small intestine enzyme which digests proteins. They acidify intestinal
contents to a pH of about 5, at which pH trypsin ceases to work effectively. They possibly also secrete
specific trypsin inhibitors. This action prevents the host from digesting the tapeworm.

Excretory and Osmoregulatory Systems

Tapeworms possess protonephridia, which certainly have an osmoregulatory function and may have some
excretory function (though their main function is thought to be osmoregulation). Protonephridia with
terminal flame bulbs are strewn throughout the mesenchyme (bulk body cells that pack between
specialised organs). These protonephridia drain, via capillaries and secondary canals, into main
longitudinal canals. Usually there are two canals on each side (a dorsal pair and a ventral pair) though the
ventral pair is usually larger and may be equipped with valves. In some taenioids there may be as many
as 20 canals. The ventral canals (and sometimes also the dorsal canals) may be connected by a
transverse canal in the rear of each proglottid. The ventral canals run the entire length of the strobila, but
the dorsal canals usually terminate in a ripe proglottid. there may be an excretory pore in each proglottid,
or two to many pores in certain parts of the strobila. These longitudinal canals reach the scolex, where
they may be cross-connected by transverse canals, or terminate in a plexus of canals or simply join
together at their ends to form a single canal that curves back on itself. The canals are lined by a cuticle
with underlying epithelium. Each flame bulb is a single cell (flame cell) with a conical 'flame' (which is really
a cluster of beating cilia) and is derived from the epithelium lining each protonephridium. In this process, a
single epithelial cell sinks into the mesenchyme surrounding the protonephridium and divides to produce
4-5 daughter cells, all but one of which differentiate into flame cells. The remaining cell forms a capillary
connecting the cluster of 3-4 flame cells with the protonephridial canal.

Nervous System

One pair of lateral longitudinal nerves run near to the protonephridial canals throughout the strobila. An
additional pair of accessory lateral nerves may be present as maybe a pair of dorsal longitudinal nerves
and sometimes a pair of ventral longitudinal nerves, resulting in up to 10 longitudinal nerves or nerve
trunks. One or more ring nerve (a connecting nerve or commissure) may connect the nerve trunks in each
proglottid and a ganglion nerve centre swelling may occur at each junction between a commissure and a
longitudinal nerve. The lomngitudinal nerves themselves are rarely ganglionated nerve cords.  The
longitudinal nerves and commissures give off branches to innervate the various organs of the proglottid,
especially the reproductive organs. A netwrok of finer nerves may also connect the longitudinal nerves.

The longitudinal nerves reach the scolex and the two main lateral nerves may each end in a ganglion
swelling. This pair of ganglia  may be cross-connected by a transverse nerve, which may be ganglionated.
More often, each of the longitudinal nerves also ends in a ganglion and these ganglia are then connected
by a circular or polgonal commissure nerve. This ring nerve may be further connected to the transverse
nerve by additional commissure nerves. The whole then constitutes a protobrain (though the ganglia are
not as tightly connected and packed together as in true brains and the ganglia also contain few cells,
most cells residing in the commissures). Nerves from this protobrain innervate the scolex.

Sensory Systems

Their are no obvious sensory organs, but free nerve endings innervate the body surface and the organs
of the scolex. Chemoreceptors (taste receptors or tangoreceptors) may also be present.
The surface membrane of the cestode also extends over the microtriches and this may consist of one
phospholipid bilayer membrane or two such membranes, depending on species. The outer membrane is
often described as porous, especially at the tip (with pores of about 10 nm diameter). These structures
develop from cilia. They consist of a base and a cap. The cap is filled with microtubules (and appears
electron dense or 'dark' under the transmission electron microscope). A baseplate separates the cap from
the base. The base consists of an electron-dense periphery (the tunic) and a lighter core of protoplasm.
The base contains microfilaments. This model is based on various sources.

Beneath the protoplasmic layer is a basement membrane and then a sheath of subcuticular muscle fibres:
an outer circular muscle layer (forming a cylinder of fibres that run around the circumference of the
proglottid) and an inner longitudinal muscle layer (forming a cylinder whose fibres run along the length of
the proglottid). These layers are not massively developed, since the tapeworm is a sedentary parasite it
has little need to move forcefully. Beneath these muscle layers is the mesenchyme, the outermost layer of
which are tegumental cells which put-out protoplasmic processes (i.e. cytoplasmic processes lined by
cell-surface membrane) which cross the muscle sheaths through pores and then merge into the
protoplasmic syncitium of the pseudo-epidermis.
Above: structure of the tapeworm body wall (integument) - click image to view full-size.
The outer pseudo-epidermis, rich in vesicles, mitochondria and rhabdiform organelles
(RO) (an organelle characteristic of flatworms in general) forms a continuous cylinder
of protoplasm, covered on both surfaces by cell-surface or plasma membrane (the
external plasma membrane, EPM and the inner plasma membrane, IPM). Its outer
surface extends into processes called microtriches (sing. microthrix or microtrix). This
layer rests on an extracellular basement membrane (BM). SER, smooth endoplasmic
reticulum; RER, rough endoplasmic reticulum.
Body Wall

The literature on the structure of the cestode body wall seems confused to me, and attempts to unify
terminology I do not find completely helpful. The outermost layer is the cuticle, secreted by underlying
cells. Sometimes the cuticle is referred to as the glycocalyx, however, it can be very well-developed in the
manner of a cuticle and is usually described as comprising three layers: an outer fringe which may be
scaly, hair-like or spiny, a middle homogeneous layer and a basement membrane. However, only the
outermost of these layers is the cuticle proper, or glycocalyx fringe. This outermost layer is reported to be
visibly porous in some electron microscope studies, though one can not rule out the effects of chemical
fixation. (Most classical studies used chemical preservatives prior to electron microscopic examination and
this can change the appearance of certain structures). In life, the glycocalyx is likely to appear as a slimy
or gelatinous covering, porous to microscopic materials.

Some others describe the underlying cell layer as an epidermis, others as a syncitium (in which cells are
fused into a single protoplasm) as the middle homogeneous layer. It is on the basis of electron
microscopy, however, more accurately described as a sheet of protoplasm, syncitial in nature, produced
by underlying body cells (mesenchyme cells) as these cells send out protoplasmic projections (which
cross the body wall muscle layers) that fuse with the syncitium. (In this case it appears not to be a true
epidermis at all). The outer plasma membrane of this protoplasmic sheet underlies the glycocalyx which it
secretes. Another, inner plasma membrane, underlies the basement membrane matrix (which is
extracellular and supports the protoplasmic layer (which can be described as a pseudo-epidermis).

The protoplasmic layer is coated with cytoplasmic spine or cilia-like processes called microtriches (singular
microtrix) that are about 1 to 2 micrometres in length and vary in shape.
Right: gravid proglottids of Taenia solium (the
pork tapeworm) - the male system has largely
degenerated and the female system now
consists of the uterus, which has a central axis
giving off branches. The uterus is now filled
with developing fertilised eggs and embryos.
Adaptations to Parasitism

Tapeworms exhibit a number of classical adaptations to a parasitic mode of life. Some of these are:

1)
Massive reproductive capacity. The strobila is essentially an oncosphere producing machine. This form
of parasitism relies on chance - most oncospheres would never be consumed by a suitable intermediate
host, therefore to eliminate chance, the tapeworm produces vast numbers of oncospheres.

2)
Attachment to the host. The intestine is a harsh environment. Tapeworms have to resist the
mechanical mnovements of the intestine which sweep most matter along it. The scolex has remarkable
powers of adhesion with its various suckers, glands and hooks. The michrotriches may also play a role in
adhesion. They were once hypothesised to function like velcro, inserting between the microvilli lining the
host intestine. This may be an exaggeration, but microtriches come in a very specific range of shapes
and I would not rule out an adhesive function.

3)
Resistance to host defences. The tapeworm not only has to resist passage through the acidic stomach
(as an encased oncosphere) but has to resist digestion by the enzymes of the host intestine and also
destruction by the host's immune system. Much of this resilience is thought to lie in the glycocalyx, which
forms a protective slime layer that is constantly secreted and is resistant to host enzymes.

4)
Transmission from host to host. It is not easy to rely on passive transfer from one host to another. To
facilitate this tapeworms often incorporate one or more intermediary hosts, working their way along the
food chain.

5) Degeneration of non-essential systems. Sensory systems, muscles and locomotory systems are
reduced and the gut is absent. These systems are no longer needed by the tapeworm as the host
protects it and the worm relies on the host to detect and escape from danger and to begin the breakdown
of food.
Above: a sensory receptor in the integument of the tapeworm Echinococcus granulosus. The
extensive tubules in and beneath the sensory process suggest at least a mechanoreceptive role,
perhaps this receptor is sesnitive to touch, though it could be sensitive to other modalities.[See D.J.
Morseth, 1967. Observations on the fine structure of the nervous system of
Echinococcus
granulosus
. J. Parasitol. 53: 492-500.]
proglottid labelled