- Also called:
- platyhelminth
- Related Topics:
- fluke
- tapeworm
- planarian
- Monogenea
- turbellarian
Both free-living and parasitic platyhelminths utilize oxygen when it is available. Most of the parasitic platyhelminths studied have a predominantly anaerobic metabolism (i.e., not dependent upon oxygen). This is true even in species found in habitats—such as the bloodstream—where oxygen is normally available.
Parasitic platyhelminths are made up of the usual tissue constituents—protein, carbohydrates, and lipids—but, compared to other invertebrates, the proportions differ somewhat; i.e., the carbohydrate content tends to be relatively high and the protein content relatively low. In larval and adult cestodes, carbohydrate occurs chiefly as animal starch, or glycogen, which acts as the main source of energy for species in low oxygen habitats. The level of glycogen, like other chemical constituents, can fluctuate considerably, depending on the diet or feeding habits of the host. In some species, more than 40 percent of the worm’s dried weight is glycogen.
Because carbohydrate metabolism is important in parasitic flatworms, a substantial amount of carbohydrate must be present in the host diet to assure normal growth of the parasite. Hence the growth rate of the rat tapeworm (Hymenolepis diminuta) is a good indicator of the quantity of carbohydrate ingested by the rat. Experiments have shown that most parasitic worms have the capability of utilizing only certain types of carbohydrate. All tapeworms that have been studied thus far utilize the sugar glucose. Many tapeworms can also utilize galactose, but only a few can utilize maltose or sucrose.
An unusual constituent of both trematodes and cestodes is a round or oval structure called a calcareous corpuscle; large numbers of them occur in the tissues of both adults and larvae. Their function has not yet been established, but it is believed that they may act as reserves for such substances as calcium, magnesium, and phosphorus.
The chief proteins in cestodes and trematodes are keratin and sclerotin. Keratin forms the hooks and part of the protective layers of the cestode egg and the cyst wall of certain immature stages of trematodes. Sclerotin occurs in both cestode and trematode eggshells, especially in those that have larval stages associated with aquatic environments.
Platyhelminth eggs hatch in response to a variety of different stimuli in different hosts. Most trematode eggs require oxygen in order to form the first larval stages and light in order to hatch. Light is thought to stimulate the release of an enzyme that attacks a cement holding the lid (operculum) of the egg in place. A similar mechanism probably operates in cestodes (largely of the order Pseudophyllidea) whose life cycles involve aquatic intermediate hosts or definitive hosts, such as birds or fish.
In many cestodes, especially those belonging to the order Cyclophyllidea, the eggs hatch only when they are ingested by the host. When the host is an insect, hatching sometimes is apparently purely a mechanical process, the shell being broken by the insect’s mouthparts. In vertebrate intermediate hosts, destruction of the shell depends largely on the action of the host’s enzymes. Activation of the embryo within the shell and its subsequent release depend on other factors, including the amount of carbon dioxide present, in addition to the host’s enzymes. Factors involving a vertebrate host are also important in establishing trematode or cestode infections after encysted or encapsulated larval stages have been ingested. Under the influence of the same factors, tapeworm larvae are stimulated to evaginate their heads (i.e., turn them inside out, so to speak), a process that makes possible their attachment to the gut lining.
Evolution
The origin of the platyhelminths and the evolution of the various classes remain unclear. There are, however, two main lines of thought. According to the more widely accepted view, the Turbellaria represent the ancestors of all other animals with three tissue layers. Other authorities have agreed, however, that flatworms may be secondarily simplified; that is, they may have degenerated from more-complex animals by an evolutionary loss or reduction of complexity.
It is generally believed that the parasitic groups are derived from the Turbellaria, many of which form close associations with other animals. These associations often show great host specificity, a characteristic of truly parasitic forms. There are a number of views regarding the evolutionary relationships among the various parasitic groups. One school of thought proposes that rhabdocoel turbellarians gave rise to monogeneans; these, in turn, gave rise to digeneans, from which the cestodes were derived. Another view is that the rhabdocoel ancestor gave rise to two lines; one gave rise to monogeneans, who gave rise to digeneans, and the other line gave rise to cestodes. A further modification of the latter view, based largely on the study of the larval forms, proposes that cestodes were derived from monogeneans.
In considering the evolution of the parasitic groups, the digeneans should be mentioned in particular. With very few exceptions, mollusks act as intermediate hosts in digenean life cycles. This condition has led to the widely accepted view that digeneans were originally commensals of mollusks that subsequently turned parasitic. Digeneans later formed an association with vertebrates; the vertebrates, in turn, became incorporated into the life cycle as definitive hosts.
Classification
Distinguishing taxonomic features
The flatworms are acoelomate triploblasts—i.e., they lack a body cavity and have three embryonic tissue layers: endoderm, mesoderm, and ectoderm. Also, they are unsegmented, and the bulk of their body is occupied by mesenchyme.
In the traditional classification of platyhelminths the principal criteria are: habitat of organism (i.e., free-living or parasitic); the characteristics of the body covering; the form and position of organs for attachment to host (when present); the presence or absence of segmentation; the form of the reproductive system, especially with respect to vitellaria (yolk glands) and the number of testes; the presence or absence of an alimentary canal; the characteristics of the pharynx (when present); and the nature of protective egg membranes. Molecular and ultrastructural studies are rapidly altering traditional views of evolutionary relationships in this phylum.
Annotated classification
There is no unanimity concerning the classification of platyhelminths. The following classification should be considered provisional.
- Phylum Platyhelminthes (flatworms)
- Flat, unsegmented worms; gastrovascular cavity and respiratory, skeletal, and circulatory systems absent; excretion by means of flame-bulb protonephridia; mesenchyme fills all spaces between organ systems; a variable number of longitudinal nerve cords with transverse connectives; body structure triploblastic (i.e., 3 embryonic layers); reproductive system hermaphroditic and complex.
- Class Turbellaria
- Epidermis usually ciliated at least in part, provided with rhabdoids (minute rodlike structures); body unsegmented; gut present except in order Acoela; life cycle simple; mostly free-living, some ectocommensal, endocommensal (i.e., living, respectively, outside or inside another organism without harming it), or parasitic; about 3,000 species.
- Order Acoela
- Exclusively marine; mouth present; pharynx simple or lacking; no intestine; without protonephridia, oviducts, yolk glands, or definitely delimited gonads; about 200 species.
- Order Neorhabdocoela
- Saclike linear intestine; protonephridia and oviducts usually present; gonads few, mostly compact; nervous system generally with 2 longitudinal trunks; about 600 species.
- Order Catenulida
- Mostly freshwater; about 70 species.
- Order Macrostomida
- Mostly inhabiting the areas between grains of sand; about 200 species.
- Order Polycladida
- Pharynx simple, bulbose, or plicate (many ridges); intestine may have short diverticula, or pockets; protonephridia paired; testes usually numerous; penis papilla generally present; nervous system with 3–4 trunks; nearly 800 species.
- Class Monogenea
- Oral sucker lacking or weakly developed; posterior end with large posterior adhesive disk (opisthaptor) usually provided with hooks; excretory pores paired, anterior and dorsal; parasites of the skin and other superficial locations, especially on the gills of fish; life cycle simple, no alternation of hosts; about 1,100 species.
- Class Cestoda (tapeworms)
- Elongated endoparasites with alimentary canal lacking; epidermis modified for absorption and secretion; usually divided into segments (proglottids); adhesive organs limited to anterior end; except in Cestodaria, adult stages almost entirely parasites of vertebrates; life cycles complicated with 1 or more intermediate hosts; about 3,500 species.
- Subclass Cestodaria
- Unsegmented tapeworms containing 1 set of genitalia; parasites of the body cavity or intestine of annelid worms or fish; about 105 species.
- Order Amphilinidea
- Uterus long and N-shaped; genital pores at or near posterior extremity; intestinal parasites of teleosts (bony fish); 105 species.
- Order Caryophyllidea
- Uterus a coiled tube; genital pore well separated from posterior extremity; intestinal parasites of teleosts, occasionally in annelids; about 85 species.
- Order Gyrocotylidea
- Testes confined to anterior region; genital pores near anterior end; parasitic in intestine of fish of the genus Chimaera; 105 species.
- Subclass Eucestoda
- Polyzoic tapeworms with scolex (head) of varying structure; body usually with distinct external segmentation; parasitic in intestine of vertebrates. Known commonly as the “true” tapeworms; well more than 3,000 species.
- Order Tetraphyllidea
- Scolex with 4 bothridia (leaflike muscular structure); vitellaria located in lateral margins of proglottids; genital pores lateral; parasites of elasmobranchs; about 200 species.
- Order Lecanicephalidea
- Reproductive system similar to Tetraphyllidea, but scolex divided into an upper disklike or globular part and a lower collarlike part bearing 4 suckers; mainly parasites of elasmobranchs; 5 species.
- Order Proteocephalidea
- Scolex with 4 suckers, sometimes a 5th terminal one; vitellaria located in lateral margins; genital pores lateral; mainly parasites of cold-blooded vertebrates; about 185 species.
- Order Diphyllidea
- Two bothridia, each sometimes bisected by a median longitudinal ridge; large rostellum (cone-shaped or cylindrical projection) armed with dorsal and ventral groups of large hooks; cephalic peduncle (fleshy stalk on head) with longitudinal rows of T-shaped hooks; genital pore median, parasitic in elasmobranchs; 1 genus, Echinobothrium, with 2 species.
- Order Trypanorhyncha
- Scolex with 2 or 4 bothridia; vitellaria in continuous sleevelike distribution; parasites of elasmobranchs; about 115 species.
- Order Pseudophyllidea
- Scolex with 2 elongated, shallow bothria, 1 dorsal and 1 ventral; genital pore lateral or median. Vitellaria lateral or extending across proglottid and encircling other organs; parasites of teleosts and land vertebrates. Order includes the largest of all known tapeworms, Polygonoporus giganticus, which reaches lengths of 30 metres (100 feet) in sperm whales. About 315 species.
- Order Nippotaeniidea
- Scolex bears 1 apical sucker; parasites of freshwater fish; 1 genus, Nippotaenia; 3 species.
- Order Cyclophyllidea (Taenoidea)
- Scolex with 4 suckers; no uterine pores; 1 compact vitellarium behind ovary; mainly parasites of birds and mammals; probably more than 2,000 species.
- Order Aporidea
- No sex ducts or genital openings; parasites of swans, ducks, and geese; 4 species.
- Order Spathebothriidea
- Scolex without true bothria or suckers; strobila with internal segmentation but no external segmentation; parasites of marine teleosts; 10 species.
- Class Trematoda (flukes)
- Ectoparasites or endoparasites; no ciliated epidermis; body undivided; adhesive organs well-developed; life cycles generally complex with 2 or more hosts; about 11,000 species.
- Subclass Aspidogastrea
- Oral sucker absent; main adhesive organ occupying almost the entire ventral surface, consists of suckerlets arranged in rows; excretory pore single and posterior; endoparasites of vertebrates, mollusks, and crustaceans; about 35 species.
- Subclass Digenea
- Oral and ventral suckers generally well-developed; development involves at least 1 intermediate host; usually endoparasites of vertebrates; about 9,000 species.
- Order Strigeidida
- Cercaria (immature form) fork-tailed; penetration glands present; 1–2 pairs of protonephridia; about 1,350 species.
- Order Echinostomida
- Cercaria with simple tail and many cyst-producing glands; life cycle with 3 hosts; about 1,360 species.
- Order Plagiorchida
- Cercaria typically armed with a stylet; encystment in invertebrates, rarely vertebrates; excretory vessels not open to the exterior. Most representatives require 3 hosts to complete one life cycle. Many hundreds of species.
- Order Opisthorchiida
- Cercaria never armed; excretory pores open on margins of tail; about 700 species.
Critical appraisal
There is disagreement on many aspects of the taxonomy of Platyhelminthes, especially regarding class divisions. For example, some authorities consider Monogenea to be a subclass within the class Trematoda.
James Desmond Smyth