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 14 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