How the way of life shown by the four classes of Platyhelminthes is reflected in their structure

How the way of life shown by the four classes of

Platyhelminthes is reflected in their structure

The platyhelminthes show a wide range of adaptations to their lifestyle. The essay considers the main morphological adaptations shown by the four classes, beginning by looking at the predominantly free-living class, the turbellaria, and working progressively towards the class that have adapted to a parasitic existence in the most extreme way, the cestoda. The intermediate monogenians and digeneans are featured in between. Regarding larval stages, attention has only been given to free-swimming larvae as their structure clearly illustrates key aspects of their lifestyle.

The majority of turbellaria are free-living active predators so have undergone a certain degree of cephalisation accompanied by the development of an array of sense organs. Their bilaterally symmetrical body plan with a dorsal and ventral surface, the ventral surface being specialised for locomotion, enables directed forward movement essential for their active, predatory lifestyle (Hickman et al. 1988).

The predatory feeding habits of the turbellaria have also influenced the development of sense organs (Meglitsch 1972). The numerous chemoreceptors found in the skin at the anterior and in the pharynx (Grzimek 1974) are sensitive to meat juices and some species also possess rheoreceptors which are sensitive to water disturbances made by nearby prey (Meglitsch 1972). Touch receptive sense cells are located at the edges of the body and the light-sensitive eyespots (ocelli) possessed by many turbellarians, can determine the direction of a light source. At least two ocelli but often several more will be present. Turbellarians that lack ocelli respond to light via light-sensitive ectodermal cells (Grzimek 1974). Statocysts for monitoring gravity and low-frequency vibrations are found in acoels, a number of catenulids and many more highly evolved turbellarians (Grzimek 1974). They give the animal a sense of equilibrium and enable it to determine its orientation.

Locomotion is essential to these animals and the outer surface of the integument consists of a single layer of epithelial cells which are heavily ciliated, particularly on the ventral surface. Turbellarians secrete a trail of mucus from gland cells within the integument which the cilia work against to achieve movement. Many species have eosinophilous glands which produce adhesive substances that together with mucous help to immobilise prey. The adhesive secretions also help the turbellarians cling to the substrate during the struggle with prey (http://faculty.purduenc.edu/jcamp/parasit/flatworm.html). The penis armature may be used to stab and further subdue the prey (Meglitsch 1972). Other glands produce releasing secretions to break the adhesive bond.

Nearly all turbellarians have rod-like rhabdoids that arise from the epidermal cells and rhabdites from the gland cells in the parenchyma. When ejected they rapidly form a strong protective covering of mucous around the turbellarian’s body (Barnes et al. 1993). Rhabdites can also produce toxic substances that can paralyse and kill a range of prey (http://www.uel.ac.uk/biosciences/res/year2/BS257/Platyhelminthes/Platyhelminthes.htm). Predators avoid the rhabdite-laden mucus which is produced when turbellarians are disturbed (Barnes et al. 1993). Nematocysts are present is some species. Mucus also protects the body of turbellarians against bacterial and fungal infection whilst preventing terrestrial species from desiccation (Grzimek 1974).

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Parasitic organisms have to protect themselves from many hazards. The outer surface of the parasite must be able to withstand attack from the host’s immune system. The turbellarian families Umagillidae, Graffillidae, Pterastericolidae, Fecampidae and Acholadidae have several parasitic members (Whittington 1997). The distal cytoplasm of the outer layer of some of these species is continuous and undivided by cell membranes (syncytial) (Hickman et al. 1988). Cytoplasmic connections run between the synctium and the nuclei which are situated in sub-tegumental cell bodies. This is an adaptation for parasitic life as the nuclei are protected deep within the sunken epidermal cell bodies. In combination with a synctium, this is a much safer and more reliable system in that it allows the outer surface to be renewed by the nuclei and mitochondria without the risk of a breach in the parasites defences caused by damaged nuclei. In addition to cilia parasitic turbellarians have small projections arising from the epidermal surface (microvilli) that serve to increase the surface area for absorption of nutrients across the synctial layer (http://www.path.cam.ac.uk/~schisto/OtherFlukes/Platy_Gen/Platy_Teg.html).

The vast majority of monogenians are ectoparasites, mostly on fish. They are found in positions where ordinarily it would be easy to become dislodged (particularly on highly mobile fish hosts) such as on the gills and external surfaces. Effective apparatus for attachment in these positions is of primary importance. Monogenians are equipped with various arrangements of hooks, suckers and clamps for this purpose. These occur on both the posterior and anterior ends of the body but the more complex posterior attachment organs are especially prominent and form the opisthaptor (Fig. 1.) (Barnes et al. 1993).

Fig.1. From left to right, body forms of typical monoisthocotylean monogenean and

polyopisthocotylean monogenian showing the opisthaptor (Barnes et al. 1993).

Many monogenians have a single host and are highly host-specific. The structure of certain host surfaces are more compatible with particular kinds of monogenian attachment systems (Buchmann and Lindenstrøm 2001). For example, in species that parasitize the gills of fish, the opisthaptor is often adapted for adhesion to the gill lamellae, being divided into several sucker-like organs which are reinforced with spicules of calcium carbonate (sclerites) and clamp around the gill filaments (http://www.aber.ac.uk/parasitology/Edu/MonoAcan/MoAcaTxt.html).

To complete the life cycle the ciliated larva (oncomiracidia) that hatches from the monogenian egg needs to sense and find a new host (Hickman et al. 1988). The oncomiracidia possess a nervous system and hairlike sensilla (chemo-, photo- and mechanoreceptors) to facilitate this process. Some of the sensilla with which adult monogenians are supplied may also be used in host finding (Buchmann and Lindenstrøm 2001). Light-sensitive eyespots are present in many oncomiracidia and some adult monogenians (Parker and Haswell 1972). The oncomiracidia possess an opisthaptor, although this becomes modified in the adult.

Monogeneans have to protect themselves from many of the same hazards as the parasitic turbellarians, and are similarly equipped with a syncytial tegument and nuclei protected deep within sunken epidermal cell bodies. Cilia are absent from the outer surface but microvilli are featured to increase the surface area for absorption of nutrients across the tegument. The endoparasitic digeneans have a similar but not exactly homologous tegument to the other parasitic platyhelminthes. Electron dense spines and tubercles to facilitate attachment to the host are present on the outer surface but microovilli are absent (except in the aspidogastria) (http://www.path.cam.ac.uk/~schisto/OtherFlukes/Platy_Gen/Platy_Teg.html).

Attachment organs are probably not as important for the digenea as the monogenea since the location of their attachment sites means that they do not experience the same degree of physical buffeting that say a monogenian clamped on to the gill lamellae of a fish would experience. However, attachment is still important and most digeneans have a large ventral sucker (acetabulum) and a small oral sucker surrounding the mouth for this purpose (Meglitsch 1972).

Most digenians feed on blood (Haematophagy) and in several polyopisthocotyleans this has necessitated modification of the digestive system in order to deal with extracellular accumulations of haematin pigment. Some monopisthocotyleans are skin browsers, so they possess a protrusible pharynx and feed in a similar way to predatory turbellarians (Halton 1997).

The miracidium is the free-swimming larva that hatches from the digenean egg. It is in many ways similar in structure to the monogenian oncomiracidium as it too needs to sense and find a new host. Locomotion is provided by ciliated epidermal plates. At the anterior end of the larvae is a nipplelike projection (terebratorium) containing sensory organs and the apical and penetration gland openings. The terebratorium acts as a sucker whilst secretions from the penetration glands help the miracidium penetrate the outer surface of the first intermediate host (a mollusc). Eye spots are also present on the dorsal surface. (http://www.path.cam.ac.uk/~schisto/SchistoLife/Miracidium.html).

The cercaria is the other free-swimming digenean larva. It emerges from the mollusc intermediate host and needs to locate the next host so has a tail for swimming and various sensory organs. The body of the cercaria contains penetration gland cells to facilitate entry into the next host and cystogenous gland cells used to secrete a cyst wall for the metacercarial stage. Oral and ventral suckers similar to those of the adult aid initial attachment of the cercaria to the host before penetration proceeds (http://www.path.cam.ac.uk/~schisto/OtherFlukes/Platy_Gen/Platy_Teg.html).

The cestodes have become adapted for parasitic existence in the vertebrate gut. Here, among many other hazards, they have to withstand attack by digestive enzymes and alkaline secretions (http://www.uel.ac.uk/biosciences/res/year2/BS257/Platyhelminthes/Platyhelminthes.htm). However, one significant advantage is that they are surrounded by pre-digested food and can draw nourishment directly from it through the tegument. Therefore, they have no need of a gut, and indeed do not have one, relying instead entirely on the host for their nutrition. Cestodes have a syncytial tegument and nuclei within sunken epidermal cell bodies similar to that seen in the other parasitic platyhelminthes but with some additional unique features. The eucestode tegument bears modified microvilli with electron dense tips (microtriches). These greatly increase the surface area available for nutrient absorption, essentially allowing the tegument to function in a similar way to the mucosa of the vertebrate gut. They also help in attachment to the intestinal wall of the host. The densely packed microvilli on the outer surface of cestodarians resemble mitrotriches but lack electron dense tips. (http://www.path.cam.ac.uk/~schisto/Tapes/Tapes_Gen/Tapes_Teg.html).

A surface layer of polysaccharide (glycocalyx) coats the cestode tegument and may help avoid possible damage from the phagocytic and immune response of the host and act as a reservoir for nutrients by binding certain molecules before passage through the cestode tegument. The glycocalyx may also facilitate attachment (http://www.bact.wisc.edu/Bact330/lecturead). The tegument of cestodes also contains glands that emit compounds that hinder the digestive enzymes of the host by reducing the pH of the environment to that which suits the parasite and not the enzymes (http://www.uel.ac.uk/biosciences/res/year2/BS257/Platyhelminthes/Platyhelminthes.htm).

The elongated string of repeated segments (proglottides) and intricate attachment organ, the scolex, is an efficient design for life spent in the vertebrate intestinal tract (Barnes et al. 1993). This permanently parasitic existence has resulted in the loss of not only the gut but also special sense and locomotor organs. However, this does not mean that cestodes are completely without senses. Many of the free nerve endings that occur on their bodies are probably sensory and it has been shown that the club-shaped neurosensory endings are tactile (Meglitsch 1972). The cestodes are not totally sedentary but are able to move by muscular contraction. (Laverack and Dando 1987).

There are four different kinds of scolices: with no attachment organs, with elongated, weakly muscular grooves (Bothria) (Fig. 2.), with broad leaflike muscular structures (Bothridia) (Fig. 3.) or with four acetabulate suckers which may additionally have various arrangements of hooks, suckers, glandular areas and a rostellum armed with hooks (Fig. 4.) (http://Path.cam.ac.uk/~scisto/Tapes/Tapes_Gen/Tapes_Teg.html). Intricate attachment organs are necessary so that the cestode avoids being dislodged by the incessant buffeting it must experience as peristaltic waves and food pass down the gut (Barnes et al. 1993).

Fig. 2. Cestode bothria (http://Path.cam.ac.uk/~scisto/Tapes/Tapes_Gen/Tapes_Larvae.html).

Fig. 3. Various cestode bothridia (http://Path.cam.ac.uk/~scisto/Tapes/Tapes_Gen/Tapes_Larvae.html).

Fig. 4. Scolex of adult Taenia hydatigena showing c- circle of hooks; r- the rostellum;

s-suckers (Lyons 1978).

The string of proglottids are produced by budding, usually in the germintive zone just behind the scolex, and form the main body (strobilla). As each new proglottid is produced, the other proglottids move down the chain. Individual proglottids contain both male and female reproductive organs and one proglottid may self-fertilise another by folding of the body or alternatively may be cross-fertilised from the proglottid of another strobila (Hickman et al. 1998). Chances of finding a new host are slim and proliferation of the gonads in this way greatly increases reproductive output. The more larvae produced the greater the chances of any single larva finding its way to the next host (Grzimek 1974). The free-swimming larvae of the cestodes show similar adaptations for locating and penetrating the next host as the free-swimming larvae of monogeneans and digenians.

The external as well as internal surfaces of many types of animals provide a surprising number of niches. The synctial tegument is probably the single most important adaptation that has allowed the parasitic platyhelminthes to so effectively exploit these niches. The locomotory cilia and well developed sense organs are perhaps the structural elements most fundamental to survival of the free-living turbellaria. The same could also be said of the free-swimming monogenean, digenean and cestodean larvae. Despite the relatively simple structure of the platyhelminthes, the morphological adaptations to lifestyle reviewed above are quite remarkable. They result in a group of animals very well equipped for a predatory life or alternatively to take advantage of the unseen and seemingly hostile but fertile niches provided by the bodies of other organisms. (1947 words).

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