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Chapter 11 The Psuedocoelomate Body Plan: Aschelminthes (Lophotrochozoan and Ecdysozoan Phyla) Evolutionary Perspective • 7 Phyla linked as the aschelminths for convenience • Fossil record is meager • 2 Ideas of possible relatedness 1st Hypothesis • The phyla are related based on the presence of the following structures: a pseudocoelom, a cuticle, a muscular pharynx and adhesive glands. 2nd Hypothesis • Contends that the various aschelminth phyla are not related to each other; thus, are probably polyphyletic. Conclusions? • The absence of any single unique feature found in all groups strongly suggests independent evolution of each phylum. • The similarities found among these phyla may simply be the result of convergent evolution as these animals adapted to similar environments General Characteristics • 1st animals to possess a distinct body cavity • Lack peritoneal linings found in more advanced animals • Organs lie free in cavity • Cavity is called a pseudocoelom or pseudocoel • Thus AKA Psuedocoelomates General Characteristics (cont.) • Pseudocoel often fluid-filled or may contain a gelatinous substance with mesenchyme cells – Helps in circulation – Aids in digestion – Acts as an internal hydrostatic skeleton that functions in locomotion General Digestion • Most aschelminths have a complete tubular digestive tract that extends from an anterior mouth to posterior anus – Exceptions: • Acanthocephalans • Nematomorphs Eutely • A condition in which the number of cells is constant both for the entire animal and for each given organ in all animals of that species – Ex. Caenorhabditis elegans • Somatic cells = 959 • Cells in pharynx = 80 Shapes • • • • • • Most are microscopic Some can be over a meter in length Bilaterally symmetrical Unsegmented Triploblastic Cylindrical in cross section Other Stuff • Protonephridia present • Mostly diecious • Cuticle may bear spines, scales or other forms of ornamentation that protect the animal are useful to taxonomists • Some aschelminths shed their cuticle in a process called molting or ecdysis in order to grow Location, Location, Location • Most aschelminths are freshwater animals; very few marine species • The nematomorphs & acanthocephalans and many nematodes are parasitic • The rest of the aschelminthes are mostly free-living • Some rotifers are colonial Aschelminthes That Do Not Molt (Lophotrochozoan Phyla) • Phylum Rotifera • Phylum Acanthocephala • Some scientists combine them into a single Phylum Syndermata Phylum Rotifera • Pseudocoelomates with jaws, crowns of cilia and complete digestive tracts • Possess a pair of corona, ciliated organs that aid in locomotion and food gathering • Cilia of the corona do not beat in synchrony; instead, each cilium is at a slightly earlier stage in the beat cycle that the cilium next to them – Gives the appearance of spinning wheels – Once called “Wheel animalicules.” Phylum Rotifera (cont.) • 0.1 to 3 mm in length • Mostly freshwater – Less than 10% marine • 2000 species divided into 3 classes • Body has approx. 1000 cells – Organs eutelic • Usually solitary, free-swimming animals – Some colonial forms known Characteristics of the Phylum Rotifera • Triploblastic, bilateral, unsegmented, pseudocoelomate • Complete digestive system, regionally specialized • Anterior end often has a ciliated organ called a corona • Posterior end with toes and adhesive glands • Well-developed cuticle • Protonephridia with flame cells • Males generally reduced in number or absent; parthenogenesis common Feeding and Digestion • The coronal cilia create a current that brings food particles to the mouth • The pharynx contains a unique structure called a mastax (jaws) • The mastax is a muscular organ that grinds food Class Seisonidea • A single genus (Seison) of marine rotifers that are commensals of crustaceans; large and elongate body with reduced corona. Only 2 Species. Class Bdelloidea • Anterior end retractile and bearing 2 trochal disks • Mastax adapted for grinding • Paired ovaries; cylindrical body • Males absent • About 590 species • Adineta, Philodina, Rotaria Class Monogononta • • • • • • Rotifers with 1 ovary Mastax not designed for grinding Produce mictic and amictic eggs Males appear only sporadically 1400 species Conochilus, Collotheca, Notommata Phylum Acanthocephala • Akantha = spine or thorn • Kephale = head • “Spiny headed worms” Phylum Acanthocephala (cont.) • Endoparasites in the intestinal tract of vertebrates (especially fishes) • 2 hosts per life cycle • Juveniles found in crustaceans and insects • About 1000 species Size • Usually less than 40 mm – Exception • • • • Macracanthoryncus hirudinaceus Found in pigs Can be up to 80 cm long Can hold up to 10 million eggs Looks • Body of an adult is elongate and composed of a short anterior proboscis, a neck region, and a trunk. – Proboscis covered with re-curved spines – Proboscis provides a means of attachment in the host’s intestine • Females larger than males Aschelminths that Molt (Ecdysozoan Phyla) • • • • • Phylum Nematoda Phylum Nematomorpha Phylum Kinoryncha Phylum Loricera Phylum Priapulida Characteristics of Phylum Nematoda • Triploblastic, bilateral, vermiform (resembling a worm in shape; long and slender), unsegmented, pseudocoelomate • Body round in cross section and covered by a layered elastic cuticle; molting usuallu accompanies growth in juveniles • Complete digestive tract, mouth usually surrounded by lips bearing sense organs • Most with unique excretory system comprised of 1 or 2 renette cells or a set of collecting tubules • Body wall has only longitudinal muscles Phylum Nematoda • Nematodes or Roundworms • 16,000 species • 2 Classes – Secernentea – Adenophorea Class Secernentea • Formerly known as Phasmidea • Possess paired glandular or sensory structures called phasmids in the tail region • Poorly developed amphids on anterior • Free living and parasitic • 5000 species • Ascaris, Enterobius, Rhabditis, Tubatrix, Necator, Wuchereria. Class Adenophorea • • • • • Formerly known as Aphasmidia Phasmids absent Most free-living, some parasitic 3000 species Dioctophyme, Trichinella, Trichuris. External Features • Body is typically slender, elongate, cylindrical and tapered at both ends. • Amphids are anterior depressions in the cuticle that contain modified cilia and function in chemoreception. • Phasmids are near the anus and also function in chemoreception • Paired ocelli (eyes) are present in aquatic nematodes Important Nematode Parasites of Humans • High reproductive potential • Life cycles that increase the likelihood of transmission from one host to another • Enzyme resistant cuticle • Resistant eggs • Encysted larvae Ascaris lumbricoides • The Giant Intestinal Roundworms of Humans • 800 million people may be infected worldwide • Live in small intestine of humans Ascaris lumbricoides life cycle • Human ingests embryonated eggs • Eggs hatch in intestine • Larvae penetrate intestinal wall and carried via circulation to the lungs • Larvae molt twice in lungs • Migrate up the trachea and are swallowed • Attain sexual maturity in the intestine, mate, and begin egg production Enterobius vermicularis • The Human Pinworm • Most common roundworm parasites in the US • Lives in lower region of large intestine E. vermicularis life cycle • At night, gravid females migrate out of the rectum to the perianal area, where they deposit eggs containing the 1st larval stage and then die • The females and eggs produce an itching sensation • When a person scratches the itch, the hands and the bedclothes become contaminated E. vermicularis life cycle (cont.) • If the hands touch the mouth and the eggs are swallowed, the eggs hatch • The larvae molt 4 times in the small intestine • Larvae leave the small intestine for the large intestine • Adults mate and females begin egg production Necator Americanus • • • • • The New World Hookworm Found in SE US Adults live in small intestine Feed on blood and tissue fluids Females can produce up 10,000 eggs daily – Passed through feces Necator Americanus life cycle • Humans become infected when the filari-form larva penetrates the skin, usually between the toes • Outside defecation and subsequent walking barefoot through the immediate area maintains the life cycle in humans • The larvae burrows deep into the skin to reach the circulatory system • The rest of the life cycle is similar to that of Ascaris Trichinella spiralis • The Porkworm • Live in the mucosa of the small intestine of humans and other carnivores and omnivores • In the intestine, the adult females give birth to young larvae that then enter into the circulatory system and are carried to striated skeletal muscle Trichinella spiralis (cont.) • The young larvae encyst in the skeletal muscle and remain ineffective for many years • The disease this nematode causes is called trichinosis • Another host must ingest infective meat (muscle) to continue the life cycle • Humans often become infected by eating improperly cooked pork products • Larvae encyst in stomach; move to small intestine; molt 4 times and develop into adults Wuchereria spp. • The Filarial Worms • 250 million human infected worldwide • 2 Examples – Wuchereria bancrofti – Wuchereria malayi Wuchereria spp. (cont.) • Elongate, threadlike nematodes live in the lymphatic system, where they block the vessels • Because lymphatic vessels return tissue fluids to circulatory system, when the filarial nematodes block these vessels, fluids and connective tissue tend to accumulate in peripheral tissues – Elephantiasis Elephantiasis • In the lymphatic vessels, filarial nematode adults copulate and produce larvae called microfilariae • The microfilariae are released into the bloodstream of the human host and migrate to the peripheral circulation at night • Mosquito ingests microfilariae • Molt 2 times in mosquito stomach • Injected back into humans Heartworm Disease • Dirofilaria immitis • A parasite of dogs • Live in the heart and large arteries of the lungs • Prevention with medicine Phylum Nematomorpha • 250 species • Horsehair worms or Gordian worms – The hairlike nature of these worms is so striking that they were formerly thought to arise spontaneously from the hairs of a horses tail in drinking troughs or other stock-watering places • No distinct head • Body wall has a thick cuticle, a cellular epidermis, longitudinal cords, and a muscle layer of longitudinal fibers • Two separate sexes Phylum Kinorhynca • • • • 1 mm in length Bilaterally symmetrical Exclusively marine, in mud and sand No arms or cilia – Burrow through the sand with their snouts • 150 species Phylum Kinorhynca • 13 or 14 definitive units called zonites • Zonite 1 bears the mouth, and oral cone, and spines • Zonite 2, represented by the neck, contains spines called scalids and plates called placids • The trunk consists of the remaining 11 or 12 zonites and terminates wi the anus • Zonite 11 contains protonephridia • Diecious Phylum Loricifera • Recently described (1983) • Example Species – Nanaloricus mysticus • 14 species Phylum Priapulida • • • • 16 species 2mm to 8cm Marine worms found in cold water Priapos = Phallus – Priapos the Greek god of reproduction • Separate sexes but not superficially distinguishable