Pseudocoelomate Body Plan - Aschelminths

Chapter 11

Aschelminths GENERAL CHARACTERISTICS

The aschelminths are the first assemblage of animals to possess a distinct body cavity, but they lack the peritoneal linings and membranes, called mesenteries, found in more advanced animals. As a result, the various internal (visceral) organs lie free in the cavity. Such a cavity is called a pseudocoelom or pseudocoel and the animals are called pseudocoelomates

Aschelminths Characteristics

First animal to posses a body cavity

Pseudocoelom Distinct body cavity with

no peritoneal lining and mesenteries.

Visceral (Internal) organs lie free in the cavity

Most phylum's contain a complete tubular digestive tract

Aschelminths Eutely – Same number

of cells for each animal and for each given organ

Ex. Caenorhabditis elegans (a type of nematode) has 959 cells

Every worm has 80 cells in their pharnyx

Aschelminths Microscopic to meters in length Bilaterally symmetrical Unsegmented and cylindrical in cross

section Most are freshwater Osmoregulation through protonephridia

(mainly in freshwater) Triploblastic Most are dioecious No separate gas exchanges Some cephalization is present Cuticle present – may bear spines or

scales and is useful for protection and taxonomic identification

Phylum Rotifera Phylum Rotifera

Characteristics Corona – ciliated organ

around the head used for locomotion and food gathering

0.1 to 3 mm in length Most are freshwater (less

than 10% marine) 2,000 species Usually solitary, free

swimming animals although there are a few colonical members

Phylum Rotifera Characteristics (cont.)

Triploblastic, bilateral, unsegemented, pseudocoelomate

Complete digestive sys. Post. End with toes and

adhesive glands Developed cuticle Protonephridia Pathogenesis common,

males reduced in this phylum

Phylum Rotifera External features

Epidermally secreted cuticle used for protection

Lorica – thickened cuticle that makes an encasement used for protection and support

Epidermis is synctial Head has a mouth,

brain, sensory organs Foot has 1-2 toes

Foot has pedal glands

Phylum Rotifera Phylum Rotifera (rotifer-

ah) A cilated corona surrounding a mouth; muscular pharynx (mastax) present with jawlike features; nonchitinous cuticle; parthenogenesis

Class Monogononta (mono-go-nontah) Rotifers with one ovary; mastax not designed for grinding; produce

Class Seisonidea (syson-ide-ah) A single genus of marine rotifers that are commensals of crustaceans; large and elongate body with reduced corona. Seison.

Class Bdelloidea (del-oide-ah)

Anterior end retractile and bearing two trochal disks; mastax

Phylum RotiferaBody parts

Head - Anterior Corona Mouth Buccal field

Trunk Middle

Foot Toes Adhesive glands

Phylum RotiferaDigestion

Most are free-living Small microorganisms Organic material

Mastax jaw that grinds food

Phylum Rotifera

Other organs Protonephridia with flame

cells Functions for

osmoregulation

Phylum Rotifera Reproduction

Some perform sexual reproduction (several use pathogenesis)

Class Seisonida 2 Species Marine Haploid eggs that must be fertilized Males have no mouth, digestive

tract, or bladder Class Bdelloidea

All females are parthenogenic No males present

Class Monogononta Amitic eggs – diploid eggs Mictic eggs – haploid eggs, can

become amitic Small sized males

Phylum KinorhynchaPhylum Kinorhyncha

Characteristics < 1mm in length Marine Bilateral symmetry Burrowers – mud and

sand 150 species Dioecious

Phylum Kinorhyncha Composed of 13 or 14 zonites

Definite units called zonites Zonite 1 can retract into zonite 2 Spines line most zonites

Pair of lateral spines and one dorsal spine

Protonephridia in Zonite 11 Brain and ventral nerve cord with

a ganglion in each zonite The body wall consists of a cuticle, epidermis,

and two pairs of muscles: dorsolateral and ventrolateral.

The pseudocoelom is large and contains amoeboid cells.

Phylum Nematoda Phylum Nematoda

Characteristics These are the

roundworms Up to 5 billion per

acre of land 16,000 to 500,000

species exist Microscopic to meters

long

Phylum NematodaParasites and free-

livingLack cilia except in

their sensory structures

Marine, freshwater, and soil habitats

Triploblastic, bilaterally symmetrical

Unsegmented vermiform (wormlike organism)

Phylum NematodaExternal features

Noncellular, collagenous cuticle

May have spines, bristles, papillae, or warts

Maintains internal hydrostatic pressure

Mechanical protection Resists digestion by the

host Longitudinal muscles

are used for locomotion

Phylum NematodaSome nematodes have

lips, some have spines or teeth on those lips

Sensory organs Amphids – chemoreceptors

along the cuticle Phasmids - chemoreceptors

near the anus Ocelli – eyespots found in

aquatic nematodes

Phylum NematodaDigestion

All have a complete digestive tract

Carnivores Herbivores Omnivores Saprobes -

decomposers Parasites

Phylum Nematoda Other organs

Glandular system Aquatic nematodes Renettes – absorb

nitrogenous waste Tubular system

Parasitic nematodes Renettes form a canal

Nervous system Nerve ring from anterior to

posterior Also have neuroendocrine

secretions involved in growth, molting, cuticle formation, and metamorphisis

Phylum Nematoda Females

Pair of convoluted ovaries

Oviducts become uterus

Several hundred to several hundred- thousand eggs per day

Ovovivparity – giving birth to larvae that hatched from an egg

ReproductionDioeciousMales are

smaller than females

MalesOne

testisBursa

Phylum NematodaTransmitted through

feces contactOnce the eggs are

ingested, they hatch and travel to the lungs

After two moltings, they travel to the trachea where they are swallowed

Important ParasitesAscaris lumbricoides

“The Giant Intestinal” roundworm

800 million infectedLive in small

intestines of humans

Phylum Nematoda

Phylum NematodaEnterobius

vermicularis Pinworm Staple size (1-2mm) Live in rectum At night females migrate

out of the anus and lay eggs on skin

Phylum Nematoda Necator americanus

Hookworm Penetrates skin, usually feet Females may lay 10,000 eggs/day

Phylum Nematoda

Phylum Nematoda Trichinella spiralis

Porkworm Transmission through

eating uncooked pork Can cause loss of

muscle control or death

Now, less than 12 cases per year

skeletal muscle

Phylum NematodaWuchereria

Filarial worms Infect the lymph

vessels which are responsible for returning fluid to the circulatory system

Cause trichinosis or elephantiasis

Heartworms in dogs

Phylum NematomorphaPhylum Nematomorpha

Characteristics 250 species Horsehair worms Adults are free-living Juveniles are parasitic No distinct head Dioecious

Phylum Acanthocephala Phylum Acanthocephala

Characteristics Spiny-headed worm Proboscis – anterior region covered

with spines This structure is retractable and is

responsible for attachment 1,000 species Most are < 40 mm long Parasites Dioecious Macracanthorhynchus hirudinaceus

80 cm long As many as 10 million eggs

Phylum LoriciferaPhylum Loricifera

Characteristics Discovered in 1983 Live in spaces between marine

gravel 14 species Spiny-head called an introvert

which is retractable

Phylum PriapulidaPhylum Priapulida

Characteristics 16 species Live buried in mud in the

seafloor 2 mm to 8 cm Cold marine climates

VINIA M. BADATO

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