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Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): university of Texas clinical Branch in ~ Galveston; 1996.


General Concepts

The helminths room worm-like parasites. The clinically relevant groups are separated according to their general exterior shape and the organize organ they inhabit. There room both hermaphroditic and bisexual species. The definitive category is based upon the external and also internal morphology of egg, larval, and also adult stages.

Flukes (Trematodes)

Adult flukes space leaf-shaped flatworms. Significant oral and also ventral suckers aid maintain position in situ. Flukes space hermaphroditic except for blood flukes, which space bisexual. The life-cycle consists of a snail intermediary host.

Tapeworms (Cestodes)

Adult tapeworms space elongated, segmented, hermaphroditic flatworms the inhabit the intestinal lumen. Larval forms, which space cystic or solid, inhabit extraintestinal tissues.

Roundworms (Nematodes)

Adult and larval roundworms room bisexual, cylindrical worms. They inhabit intestinal and also extraintestinal sites.


Helminth is a basic term an interpretation worm. The helminths are invertebrates defined by elongated, flat or ring bodies. In medically oriented schemes the flatworms or platyhelminths (platy indigenous the Greek root meaning “flat”) include flukes and also tapeworms. Roundworms space nematodes (nemato from the Greek root meaning “thread”). These teams are subdivided because that convenience follow to the host organ in which lock reside, e.g., lung flukes, extraintestinal tapeworms, and also intestinal roundworms. This chapter faces the structure and breakthrough of the three significant groups the helminths.

Helminths construct through egg, larval (juvenile), and adult stages. Table 86-1 provides the names used to assorted larval helminths. Expertise of the various stages in relation to their expansion and development is the basis for knowledge the epidemiology and pathogenesis that helminth diseases, and for the diagnosis and also treatment of patient harboring this parasites. The contribute of assorted stages to condition are listed in Table 86-2.


Table 86-1

Common Larval forms of Helminths uncovered in Humans.


Platyhelminths and also nematodes the infect people have similar anatomic attributes that reflect usual physiologic requirements and also functions. The external covering of helminths is the cuticle or tegument. Prominent exterior structures that flukes and cestodes are acetabula (suckers) or bothria (false suckers). Male nematodes the several types possess accessory sex organs that room external modifications of the cuticle. Internally, the alimentary, excretory, and also reproductive systems have the right to be figured out by an skilled observer. Tapeworms are distinct in doing not have an alimentary canal. This lack method that nutrients need to be soaked up through the tegument. The blood flukes and nematodes room bisexual. All other flukes and also tapeworm types that infect human beings are hermaphroditic.

With few exceptions, adult flukes, cestodes, and also nematodes develop eggs that space passed in excretions or secretions that the host. The various stages and also their unique qualities will it is in reviewed in more detail as each major group of helminths is considered.

Flukes (Trematodes)

The structure of flukes is summarized in numbers 86-1 and also 86-2. A dorsoventrally planarization body, bilateral symmetry, and also a identify anterior end are functions of platyhelminths in general and of trematodes specifically. Flukes room leaf-shaped, ranging in size from a couple of millimeters come 7 come 8 cm. The tegument is morphologically and also physiologically complex. Flukes own an dental sucker around the mouth and a ventral sucker or acetabulum that have the right to be provided to adhere to hold tissues. A human body cavity is lacking. Organs are installed in specialized connective tissue or parenchyma. Great of somatic muscle penetrate the parenchyma and attach come the tegument.


Figure 86-1

Structure of flukes. (A) Hermaphroditic fluke. (B) Bisexual fluke. (Modified from Hunter GW, Swartzwelder JC, Clyde DF: A manual of tropical Medicine. Fifth Ed. WB Saunders, Philadelphia, 1976, through permission.)


Flukes have actually a well-developed alimentary canal with a muscular pharynx and esophagus. The intestine is typically a branched tube (secondary and tertiary branches may be present) consists of a solitary layer that epithelial cells. The key branches might end thoughtlessly or open into an excretory vesicle. The excretory vesicle likewise accepts the two key lateral collecting ducts of the excretory system, which is the a protonephridial type with flame cells. A flame cell is a hollow, terminal excretory cell that consists of a beating (flamelike) group of cilia. These cells, anchored in the parenchyma, direct tissue filtrate through canals right into the two key collecting ducts.

Except because that the blood flukes, trematodes room hermaphroditic, having actually both male and also female reproductive guts in the very same individual. The masculine organ consists usually of two testes with accessory glands and also ducts leading to a cirrus, or penis equivalent, that extends right into the typical genital atrium. The mrs gonad consists of a solitary ovary through a seminal receptacle and also vitellaria, or yolk glands, that attach with the oviduct as it increases into one ootype. The tubular uterus extends indigenous the ootype and also opens right into the genital atrium. Both self- and cross-fertilization occur. The materials of the egg room assembled in the ootype. Egg pass with the uterus right into the genital atrium and also exit ventrally v the genital pore. Fluke eggs, other than for those that schistosomes, room operculated (have a lid).

The blood flukes or schistosomes space the just bisexual flukes the infect human beings (Fig. 86-1). Return the sexes are separate, the basic body framework is the very same as the of hermaphroditic flukes. In ~ the definitive host, the male and female worms inhabit the lumen of blood vessels and also are found in close physical association. The mrs lies within a tegumental fold, the gynecophoral canal, top top the ventral surface ar of the male. The medically necessary flukes belong come the taxonomic classification Digenea. This group of flukes has a developmental bike requiring at the very least two hosts, one gift a snail intermediary host. Relying on the species, other intermediate hosts might be involved to perpetuate the larval kind that infects the definitive person host.

Flukes walk through number of larval stages, each v a details name, before reaching adulthood. Taking into account variations among varieties (see Fig. 86-2), a generalised life cycle of digenetic flukes runs the following course. Eggs space passed in the feces, urine, or sputum of humans and also reach one aquatic environment. The egg hatch, publication ciliated larvae, or miracidia, i m sorry either permeate or are consumed by a snail intermediate host. In rare instances land snails may serve together intermediate hosts. A saclike sporocyst or redia stage creates from a miracidium within the tissues of the snail.

The sporocyst provides rise one of two people to rediae or to a daughter sporocyst stage. In turn, from the redia or daughter sporocyst, cercariae develop asexually and also migrate out of the snail organization to the exterior environment, i m sorry is generally aquatic.

The cercariae, which might possess a tail for swimming, build further in among three ways. Lock either pass through the definitive host and also transform straight into adults, or penetrate a 2nd intermediate host and develop together encysted metacercariae, or castle encyst ~ above a substrate, such as vegetation, and also develop there as metacercariae. When a metacercarial cyst is ingested, cradle of the cyst liberates an immature fluke the migrates to a details organ site and develops into an adult worm.

Tapeworms (Cestodes)

As members of the platyhelminths, the cestodes, or tapeworms, possess many basic structural features of flukes, but additionally show to mark differences. Number 86-3 mirrors the general features of the structure and advance of tapeworms.

Figure 86-3

Structure of tapeworms. (Modified native Jeffery HC, Leach RM: Atlas of clinical Helminthology and also Protozoology. Churchill Livingstone, Edinburgh, 1968, v permission.)

Whereas flukes room flattened and also generally leaf-shaped, adult tapeworms are flattened, elongated, and also consist of segments dubbed proglottids. Tapeworms differ in size from 2 to 3 mm come 10 m, and may have actually three to number of thousand segments.

Anatomically, cestodes are split into a scolex, or head, which bears the offal of attachment, a neck that is the an ar of segment proliferation, and also a chain the proglottids called the strobila. The strobila elongates as new proglottids kind in the neck region. The segment nearest the neck are immature (sex organs not totally developed) and those much more posterior space mature. The terminal segments room gravid, with the egg-filled uterus together the most prominent feature.

The scolex has the cephalic ganglion, or “brain,” the the tapeworm worried system. Externally, the scolex is defined by holdfast organs. Depending on the species, this organs consists a rostellum, bothria, or acetabula. A rostellum is a retractable, conelike framework that is located on the anterior end of the scolex, and also in some species is equipped with hooks. Bothria space long, narrow, weakly muscular grooves that room characteristic that the pseudophyllidean tapeworms. Acetabula (suckers like those that digenetic trematodes) are characteristic of cyclophyllidean tapeworms. Differential functions of pseudophyllidean and also cyclophyllidean tapeworms are detailed in Table 86-3. Most human being tapeworms room cyclophyllideans.

Table 86-3

Differences in between Pseudophyllidean and also Cyclophyllidean Tapeworms.

A characteristic attribute of adult tapeworm is the absence of one alimentary canal, which is attractive since all of these adult worms inhabit the little intestine. The lack of an alimentary tract means that substances enter the tapeworm throughout the tegument. This framework is well adjusted for deliver functions, because it is covered with numerous microvilli the same, similar thing those lining the lumen that the mammalian intestine. The excretory system is of the flame cell type.

Cestodes are hermaphroditic, every proglottid own male and female reproductive systems comparable to those of digenetic flukes. However, tapeworms differ from flukes in the mechanism of egg deposition. Egg of pseudophyllidean tapeworms exit through a uterine sharp in the facility of the ventral surface quite than v a genital atrium, together in flukes. In cyclophyllidean tapeworms, the woman system contains a uterus without a uterine pore (Fig. 86-3). Thus, the cyclophyllidean eggs are released only as soon as the tapeworms melted gravid proglottids into the intestine. Some proglottids disintegrate, releasing eggs that are voided in the feces, whereas other proglottids space passed intact.

The eggs of pseudophyllidean tapeworms are operculated, but those the cyclophyllidean types are not. Eggs of all tapeworms, however, contain in ~ some stage of advancement an embryo or oncosphere. The oncosphere the pseudophyllidean tapeworms is ciliated externally and also is dubbed a coracidium. The coracidium establishes into a procercoid stage in that micro-crustacean an initial immediate host and also then right into a plerocercoid larva in its next intermediate organize which is a vertebrate. The plerocercoid larva creates into one adult worm in the definitive (final) host. The oncosphere of cyclophyllidean tapeworms, depending upon the species, develops into a cysticercus larva, cysticercoid larva, coenurus larva, or hydatid larva (cyst) in particular intermediate hosts. These larvae, in turn, end up being adults in the critical host. Number 86-4 illustrates these larval forms and representative life cycles.

Figure 86-4

Generalized life cycle of tapeworms. Hymenolepsis nana, Hdiminuta, Taenia saginata, T solium, Diphyllobothrium latum, Dipylidium craninum. Keep in mind hexacanth embryos. Cysticercus larva in cow and also pig; procercoid larva in copepod, plerocercoid (sparganum) (more...)

Roundworms (Nematodes)

Figure 86-5 mirrors the framework of nematodes. In contrast to platyhelminths, nematodes room cylindrical fairly than flattened; hence the common name roundworm. The body wall surface is composed of an outer cuticle that has a noncellular, chemically facility structure, a thin hypodermis, and also musculature. The cuticle in some varieties has longitudinal ridges referred to as alae. The bursa, a flaplike extension of the cuticle ~ above the posterior end of some types of masculine nematodes, is offered to grasp the female during copulation.

Figure 86-5

Structure the nematodes. (A) Female. (B) Male. Transverse sections through the midregion of the mrs worm (C) and through the esophageal region (D). (Modified from Lee DL: The Physiology of Nematodes. Oliver and Boyd, Edinburgh, 1965, through permission.) (more...)

The to move hypodermis bulges into the human body cavity or pseudocoelom to form four longitudinal cords—a dorsal, a ventral, and two lateral cords—which might be checked out on the surface ar as lateral lines. Nuclei that the hypodermis are situated in the an ar of the cords. The somatic musculature lying in ~ the hypodermis is a solitary layer that smooth muscle cells. Once viewed in cross-section, this layer can be viewed to be separated into 4 zones by the hypodermal cords. The musculature is innervated by expansions of muscle cell to nerve trunks running anteriorly and posteriorly indigenous ganglion cells the ring the midportion of the esophagus.

The space between the muscle layer and also viscera is the pseudocoelom, i m sorry lacks a mesothelium lining. This cavity contains fluid and also two to 6 fixed cell (celomocytes) which room usually linked with the longitudinal cords. The role of these cells is unknown.

The alimentary canal of roundworms is complete, with both mouth and anus. The mouth is surrounded by lips bearing sensory papillae (bristles). The esophagus, a conspicuous feature of nematodes, is a muscular structure that pumps food into the intestine; it different in form in various species.

The intestine is a tubular structure composed of a single layer the columnar cell possessing influential microvilli on your luminal surface.

The excretory device of some nematodes consists of one excretory gland and a pore located ventrally in the mid-esophageal region. In various other nematodes this structure is drawn into expansions that give rise to the more facility tubular excretory system, i beg your pardon is generally H-shaped, with two anterior limbs and two posterior limbs situated in the lateral cords. The gland cells and also tubes are thought to serve as absorptive bodies, collecting wastes native the pseudocoelom, and also to function in osmoregulation.

Nematodes room usually bisexual. Males are usually smaller than females, have a bent posterior end, and also possess (in some species) copulatory structures, such as spicules (usually two), a bursa, or both. The males have one or (in a few cases) 2 testes, which lie in ~ the free end the a convoluted or recurved tube leading right into a seminal vesicle and eventually right into the cloaca.

The female device is tubular also, and also usually is consisted of of reflexed ovaries. Each ovary is continuous, v an oviduct and also tubular uterus. The uteri join to form the vagina, which in turn opens come the exterior through the vulva.

Copulation between a female and also a masculine nematode is necessary for fertilization except in the genus Strongyloides, in which parthenogenetic development occurs (i.e., the advance of an unfertilized egg right into a brand-new individual). Some proof indicates the sex attractants (pheromones) beat a role in heterosexual mating. Throughout copulation, sperm is transferred into the vulva that the female. The sperm beginning the ovum and also a fertilization membrane is secreted by the zygote. This membrane progressively thickens to form the chitinous shell. A 2nd membrane, below the shell, provides the egg impervious to basically all substances except carbon dioxide and also oxygen. In some species, a third proteinaceous membrane is secreted together the egg passes down the uterus through the uterine wall surface and is deposited exterior the shell. Many nematodes that are parasitic in humans lay egg that, when voided, contain one of two people an uncleaved zygote, a team of blastomeres, or a fully formed larva. Part nematodes, such as the filariae and Trichinella spiralis, develop larvae that are deposited in host tissues.

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The developmental procedure in nematodes entails egg, larval, and adult stages. Every of four larval stages is followed by a molt in which the cuticle is shed. The larvae are dubbed second-stage larvae after the first molt, and so on (Fig. 86-6). The nematode formed at the fifth stage is the adult. Number 86-7 summarizes the life cycles of number of intestinal nematodes.

Figure 86-6

Stages in the advance of nematodes. (Adapted native Lee DL: The Physiology the Nemotodes. Oliver and also Boyd, Edinburgh, 1965, v permission.)