Tapeworms an overview

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Tapeworms
Tapeworms (Cestoda) are flat, tape-like worms with the size, depending on the 
species, from only couple of millimeters up to several meters.
From: 
Canine Parasites and Parasitic Diseases, 2019
Related terms:
Genus, Helminths, Intermediate Host, Secernentea, Parasite, Proteins, Larvae, Fe-
ces, Taenia
View all Topics
TAPEWORMS (CESTODES)
JOANN L. COLVILLE DVM, DAVID L. BERRYHILL PHD, in 
Handbook of Zoonoses
, 
2007
ETIOLOGY: PARASITIC
Adult tapeworms are found in the intestinal tracts of their 
definitive
, or 
final
, hosts. 
Each adult tapeworm consists of a head (scolex), which attaches the tapeworm to 
the intestinal wall, neck, and various numbers of segments, developing from the 
neck region. As new segments are formed at the neck, older segments are pushed 
back. Tapeworms are hermaphroditic; each segment has two sets of male and female 
reproductive organs, which will fill the segment with fertile eggs as the segment is 
pushed back from the neck. When the segment is full of eggs, it detaches itself from 
the adult tapeworm and is passed in the feces (Figure 40).
Each genus and species of tapeworm has at least one 
intermediate
host, which ingests 
the tapeworm eggs. After the eggs hatch, the immature tapeworms migrate out 
of the intestine of the intermediate host and travel to various tissues in the body, 
depending on the genus of tapeworm. The immature tapeworm enters tissue in the 
intermediate host and is enclosed in a cyst, in which young tapeworms develop to an 
infective stage. 
Definitive
hosts are infected by eating the cystic tissues of 
intermediate
hosts infected with immature tapeworms.

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There are three tapeworms of zoonotic importance in North America: Dipylidium 
caninum
, Diphyllobothrium latum
, and 
Echinococcus spp
.
> Read full chapter
ANIMALIA
Lynn Margulis University of Massachusetts at Amherst, Michael J Chapman Marine 
Biological Laboratory Woods Hole, Massachusetts, USA, in 
Kingdoms and Domains 
(Fourth Edition)
, 2009
(Flatworms)
Greek 
platys
, flat; 
helmis
, worm
GENERA
Bothrioplana
Convoluta
Dipylidium
Dugesia
Echinococcus
Fasciola
Hymenolepis
Opisthorchis
Planaria
Procotyla
Schistosoma
Taenia
Platyhelminthes are free-living and symbiotrophic flatworms. The soft body of the 
flatworm is bilaterally symmetrical. Structures for capturing and consuming prey 
are localized in the anterior end except in many free-living flatworms, in which the 
mouth is anterior. Flatworm organs are composed of tissues and are organized into 
systems. Flatworms are the simplest metazoans to possess an embryonic intermedi-
ate tissue layer, the mesoderm. The platyhelminth worm, like the cnidarian, lacks an 
anus. The flatworm middle tissue layer, a loose mesoderm called parenchyma, never 
splits into a cavity (coelom) in which internal organs are suspended. Flatworms and 
other animals without a coelom are called acoelomates. Flatworms, having three 
tissue layers, are triploblastic, have spiral cleavage in their eggs, and yet are among 
the least complex of bilaterally symmetrical true metazoans.

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Flatworm classification is constantly being reviewed and revised. The symbiotrophic 
forms, which undergo a transformation of the epidermis to a cuticle-like neodermis 
during development, are generally placed among two or three traditional classes 
within the group Neodermata. Two established classes contain the Trematoda, or 
flukes, and the single to multi-host Cestoda, the tapeworms. Some workers divide 
the Trematoda into the Digenea, or multi-host trematodes, which are internal 
necrotrophs, and Monogenea, which typically have a single host and are largely 
external necrotrophs. The free-living flatworms make up a number clusters or clades 
(using cladistic methodologies), the relationships of each remain unresolved. The 
most distinctly primitive group, the Acoela (Acoelomorpha), may not be a platy-
helminth group at all.
There are about 20,000 species of flatworm altogether. Some species are richly 
colored. Others harbor symbiotic algae called zoochlorellae producing a green 
color. Most necrotrophic forms and those free-living forms that inhabit caves and 
underground water are colorless. Tapeworms are the largest platyhelminths; some 
reach a length of more than 30 m. The smallest are less than 1 mm in length.
Flatworms are masters of adaptation, exploiting an enormous variety of habitats. 
Some live in bat guano, others in the mantle fold of various Mollusca (Phylum 
A-26) where as symbionts they feed on the particles not consumed by the mollusc 
host. Members of many animal phyla, certainly an enormous number of vertebrates, 
play host to ubiquitous flatworm symbiotrophs. In sediments low in molecular 
oxygen, a few flatworms utilize energy by oxidizing hydrogen sulfide. A survey of the 
phylum reveals that flatworms tolerate an immense temperature range from minus 
50 degrees to plus 47 degrees.
Some free-living flatworms are marine inhabitants; most are freshwater forms, and 
several dwell in moist soil. Soil flatworms are mainly tropical whereas aquatic forms 
are more abundant in temperate than in tropical waters. Free-living and non-neo-
dermatid necrotrophic forms have several undulipodia per cell. By simultaneously 
sweeping ventral cilia through secreted mucus and generating muscular waves, 
free-living flatworms glide over surface films on water, plants and soil. On the ventral 
surface of free-living flatworms, duoglands (adhesive and releaser cells) secrete 
either adhesive that attaches the worm to its substrate or a substance that detaches 
it. Most free-living aquatic species are benthic, a few swim with undulations or loop 
along substrate like caterpillars, and some live among the plankton.
Free-living flatworms are detritus feeders, carnivores, and scavengers. They eat 
insects or crustaceans (A-21), tunicates (A-35), bivalve molluscs (A-26), other worms, 
bacteria, mastigotes (Phylum Pr-28), ciliates (Phylum Pr-8), and diatoms (Phylum 
Pr-18). Most free-living flat worms are marine; some inhabit the digestive tract of 
sipunculans (A-23) and echinoderms (A-34); a few are terrestrial in damp habitats 

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or are freshwater species. Digestive systems of free-living flatworms range from a 
straight or branched gut to absence of a gut; food moves from the pharynx of acoel 
(lacking a gut) free-living flatworms into loose digestive cells. Some jab food by using 
a proboscis separate from the mouth. Others “vacuum out” soft parts of their prey 
by using a tubular, muscular pharynx, which may project through the mouth on the 
ventral side. Digestive enzymes secreted into the gut begin digestion; intestinal cells 
continue digestion by engulfing food in food vacuoles.
Necrotrophic flatworms undergo a phenomenon in which the epidermis is replaced 
by a new skin, the neodermis, during maturation. Thus, all cilia are lost and move-
ment within the host is carried out by detaching and reattaching the sucker or 
variations of the sucker.
All flukes (digenean and monogenean trematodes) are internal or external 
necrotrophs, usually of vertebrates. The digenean trematodes have a life cycle that 
includes several types of larvae and sometimes an intermediate host or hosts. 
Trematode larvae include miracidium, sporocyst, redia, cercaria, and metacercaria. 
Schistosomiasis (bilharziasis), caused by several species of the blood fluke 
Schisto-
soma
, is currently the second most prevalent infectious disease worldwide (malaria 
is first). Cercariae, which are distinctive swimming larvae with a tail and sucker, are 
carried by snails that spread schistosomiasis. Snails release cercariae; the cercariae 
swim, attach to and penetrate human skin between the fingers and toes, and then 
mature into adult worms and migrate to take up residence in the liver and other 
organs of the human host. The disease results from the human host's immune 
response to schistosome eggs deposited in host tissues by activation of lymphocytes 
and other immune cells; urinary tract and bowel blockage also can result. Monoge-
nean trematodes typically have only one larval type, the onchomiracidium, which 
is released alive. Larvae move about the host or locate other hosts and attach. The 
larva matures, a neodermis replaces the ciliated epidermis, and the cycle repeats 
itself. Trematodes have one or two suckers; some trematodes feed through their oral 
suckers.
Tapeworms (cestodes) are exclusively internal necrotrophs that usually attach inside 
the gut of vertebrates by means of a specialized structure, the scolex. The scolex 
may contain exclusively suckers or a combination of suckers and other structures 
enabling a firm grasp of the host tissue. Tapeworms lack a gut. Microvilli (minute 
tissue projections) absorb nutrients (amino acids and sugars) from the hosts exploit-
ed by tapeworms. The typical tapeworm body plan consists of the scolex followed 
by repeated segments, each with reproductive organs; these sexually reproducing 
segments bud from the tapeworm's neck. Many tapeworms, however, lack segments. 
Like flukes, most tapeworms have intricate life cycles with several distinctive larval 
types. Others, typically without segments, have simpler life cycles, and may represent 
progenetic forms of more typical segmented tapeworms.

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Figure A. Dorsal view of gliding 
Procotyla fluviatilis
, a live freshwater turbellarian 
flatworm from Great Falls, Virginia. Its protrusible pharynx connects to a branched 
intestine visible through its translucent body. Bar=1 cm. [Photograph courtesy of R. 
Kenk; drawing by L. Meszoly; information from R. Kenk.]
The large surface area of free-living and symbiotrophic flatworms relative to their 
volume has physiological implications. Oxygen, carbon dioxide, and ammonia ex-
change across the body surface. Like cnidarians and ctenophores, flatworms are 
blood-, lung-, and heartless. In symbiotrophic flatworms, gases and nutrients diffuse 
into tissues of the flatworm from the host digestive system or from water. Free-living 
flatworms ingest food. Platyhelminth worms that have a gut discharge solid waste 
through their mouths because they lack an anus. Protonephridia are the organs of 
excretion and osmoregulation in flatworms, except in free-living flatworms that lack 
a gut. Protonephridia are extremely primitive excretory structures that are composed 
of ciliated cells, called “flame cells”, that collect dissolved wastes. Protonephridia 
regulate water and ions by wafting liquid through ducts that exit to the outside 
through pores.
The simplest flatworm nervous system consists of light-sensitive pigment-cup eye-
spots (either single or in groups) connected to a cluster of nerve cells (brain) in the 
head and ventral, longitudinal nerve cords. The nervous system of flatworms ranges 
in complexity from this simple system to the more primitive nerve net of acoel 
free-living flat worms resembling that of cnidarians and ctenophores. Free-living 
flatworms detect chemicals, food, objects, and currents with sensory pits or tentacles 

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on the sides of the head. When flatworms wander away from a scent source, they 
turn from side to side more frequently and so eventually home in on the source.
Triclad free-living flatworms and cestodes have prodigious powers of regeneration 
and reproduce sexually or asexually. Slices of 
Dugesia
, a triclad, regenerate to form 
entire worms. Planarians (freshwater species of triclad free-living flatworms, an order 
of free-living flatworms characterized by a gut having three branches) and taeniid 
cestodes reproduce asexually as well as sexually. Almost all flatworm species are 
simultaneous hermaphrodites. Each individual flatworm bears ovaries and testes. 
Self-fertilization is rare in free-living flat worms and common in cestodes. In mating 
pairs of hermaphrodites a copulatory bursa receives sperm or, in some free-living 
flatworms, a hypodermic-like penis injects sperm through the body wall into the 
body of the mate. Some flatworm sperm have no tails; others have a [9(2)+1] arrange-
ment or a [9(2)+0] arrangement (see Figure I-3). Ribbons of fertilized free-living 
flatworm eggs are laid in cocoons. Freshwater flat worms “glue” eggs to stones. Eggs 
of most free-living flat worms hatch into miniature adults. A few marine free-living 
flatworms develop ciliated larvae known as Muller's larvae. Necrotrophic flatworms 
frequently have complex reproductive cycles, with a succession of larval stages. 
Schistosoma
, a fluke, is dioecious. 
Bothrioplana
, a free-living flat worm, exhibits