The Triploblastic, Acoelomate Body Plan

Phyla Involved

• Platyhelminthes

• Nemertea

• Gastrotricha

Characteristics of All 3

• 1st to exhibit bilateral symmetry

• Triploblastic

• Acoelomate

Platyhelminthes

• Turbellarians– Free-swimming

• Flukes– Parasitic

• Tapeworms– Parasitic

Platyhelminthes (cont.)

• (plat”e-hel-min’thez)

• 34,000 species

• Range in size from less than 1mm to 25m

• Their mesodermally derived tissues include a loose tissue called the parenchyma.

Parenchyma

• Depending on the species, the parenchyma may provide:

– Skeletal support– Nutrient storage– Motility– Reserves of regenerative

cells– Transport of materials– Structural interactions with

other tissues– Modifiable tissue for

morphogenesis– Oxygen storage– Other functions

General Charateristics of the Phylum Platyhelminthes

• Usually flattened dorsoventrally, triploblastic, acoelomate, bilaterally symmetrical

• Unsegmented worms

• Incomplete gut usually present

• Somewhat cephalized, with an anterior cerebral ganglion and usually longitudinal nerve cords

General Charateristics of the Phylum Platyhelminthes (cont.)

• Protonephridia as excretory/osmoregulatory structures

• Most forms are monoecious; complex reproductive systems

• Nervous system consists of a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves and located in the mesenchyme

Class Turbellaria: The Free-Living Flatworms

• (tur’’bel-lar’e-ah)• Free-living bottom dwellers in freshwater or

marine environments, a few terrestrial in humid and tropical climates

• 3000 species• Predators and scavengers• Usually less than 1cm, some can be up to 60cm

in length• Normally colored black, brown or gray,

occasionally brightly colored

Movement

• The 1st group of bilaterally symmetrical animals to appear

• Bilateral symmetry is usually a characteristic of an active lifestyle – Turbellarians glide over the substrate– They use cilia and muscular undulations to

move– As they move, they lay down a sheet of

mucus that aids in adhesion and helps the cilia to gain traction

Digestion & Nutrition

• Some feed on small, live invertebrates or scavenge on larger, dead animals

• Some are herbivores and feed on algae that they scrape from rocks

• Sensory receptors (chemoreceptors) found on their head help them to detect food at considerable distances

• Digestion is primarily extracellular• Pharyngeal glands secrete enzymes that help

break down food

Exchanges with the Environment

• Do not have respiratory organs thus they “breathe” through their body walls by diffusion– Wastes are also removed by diffusion

• In marine environments– Inverts are in osmotic equilibrium

• In freshwater environments– Inverts are hypertonic

Coping with Freshwater

• In order to move into bodies of freshwater turbellarians had to develop something that would regulate their osmotic concentration

• They developed protonephridia– Protos = 1st

– Nephros = kidney– Networks of fine tubules that run the length of

the turbellarian

Reproduction and Development

• Many produce asexually by transverse fission

• Fission usually begins with a constriction point behind the pharynx

• The two (or more) animals that result from fission are called zooids and they regenerate the missing parts.

• Turbellarians are monoecious– Reciprocal sperm exchange

Class Monogenea

• Named because they only have one generation in their life cycle– One adult develops from one egg

• Mostly external parasites (ectoparasites)• Attaches to the gill filaments and feed on

epithelial cells, mucus or blood of freshwater and marine fishes

• Attachment is possible due to the presence of an opisthaptor, a posterior attachment organ

Class Trematoda

• (trem’’ah-todah)• 8000 species; parasitic• Called flukes• Almost all adult flukes can be found in

vertebrates, whereas their immature forms can be found in inverts and verts

• Flat, oval and elongated• 1mm to 6cm in length• Digestive tract includes a mouth and a muscular

pumping pharynx

Subclass Aspidogastrea

• Mostly endoparasites of molluscs

• Possess large opisthaptor

• Most lack an oral sucker

• Aspidogaster, Cotylaspis, Multicotyl

Subclass Digenea

• Adults are endoparasites in verts

• At least two or more life cycle forms in two or more hosts

• Have oral sucker and acetabulum

• Schistosoma, Fasciola, Clonorchis

Trematode Parasites of Humans

• The Chinese Liver Fluke, Clonorchis sinensis, is a common human parasite in Asia, where over 30 million people are infected.

• They live in the bile ducts of the liver where it feeds on epithelial tissue and blood.

• Eggs are eliminated in the feces.• People are infected by eating infected raw or

undercooked fish (sushi, sashimi, ceviche).

Trematode Parasites of Humans (cont.)

• Fasciola hepatica is called the sheep liver fluke because it is common in sheep raising areas and uses sheep or humans as its definitive host

• Adults live in the bile ducts of the liver

• Snail – Plant – Sheep

• Snail – Plant - Human

Class Cestoidea:The Tapeworms

• (ses-toid’e-ah)

• Most highly specialized of the flatworms

• 3500 species

• Endoparasites, usually in vertebrate digestive system

• Range from 1mm to 25m in length

Adaptations for the Parasitic Lifestyle

• Tapeworms lack a mouth and a digestive tract in all of their life-cycle stages; absorb nutrients directly across the body wall

• Most adult tapeworms consist of a long series of repeating units called proglottids; each proglottid contains one or two complete sets of reproductive structures

Home Sweet Home

• Live in a very stable environment because the vertebral digestive system has very few environmental variations that would require the development of great anatomical or physiological complexity in any single tapeworm system

• It is thought that tapeworms may have lost some characteristics that were present in the ancestral species

• Tapeworms are a good example of evolution not always resulting in greater complexity

Subclass Cestodaria

• Endoparasites in the digestive tracts of primitive fishes

• 15 species

Subclass Eucestoda

• True tapeworms• Holdfast structure called the scolex• The scolex narrows to form a neck, which gives rise to

the strobila• The strobila consists of a series of linearly arranged

proglottids, which function as reproductive units• New proglottids are added in the neck region pushing

older proglottids posteriorly– Neck = immature– Strobila = mature– End = gravid

Reproduction of Tapeworms

• Monoecious• Made to breed• Multiple testes• Single pair of ovaries• Usually breed with other mature

proglottids on the same organism or a mature proglottid on a different tapeworm in the same organism

• Cross-breeding leads to “Hybrid Vigor”

Tapeworm parasites of humans

• Taeniarhychus saginatus, the beef tapeworm– 25m in length– About 80,000 eggs per proglottid

• Taenia solium, the pork tapeworm– Usually 2-3m, can be up to 10m

Nemertea

• A small group of elongate, segmented, soft bodied worms that are mostly marine and free-living

• 900 species

• AKA proboscis worms

• From a few mm to several cm in length

Characteristics of the Phylum Nemertea

• Triploblastic, acoelomate, bilaterally symmetrical, unsegmented worms possessing a ciliated epidermis containing mucous glands

• Complete digestive tract with an anus• Protonephridia• Cerebral ganglion, longitudinal nerve cords, and

transvere commisures• Closed circulatory system• Body musculature organized into 2 or 3 layers

Stuff

• The most distinctive feature of nemerteans is a long proboscis held in a sheath called a rhyncocoel

• The proboscis may be tipped with a barb called a stylet

• Also, nemerteans are diecious

Breakthroughs

• They have a mouth for ingesting food and an anus for eliminating wastes– This allows for the mechanical breakdown of

food, digestion, absorption, and feces formation to proceed sequentially in an anterior to posterior direction

• Also, they have a circulatory system consisting of two lateral blood vessels– No heart present

Nemertean Fact

• Include the longest invertebrate animal, Lineus longissimus, which can be 30m long

Gastrotricha

• Includes members that inhabit the space between bottom sediments

• 500 free-living freshwater and marine species

• From 0.1 to 4mm in length

• Contains a single class divided into 2 orders

Reproduction

• Most marine species reproduce sexually and are hermaphroditic

• Most freshwater species reproduce asexually by parthenogenesis; the females can lay 2 kinds of unfertilized eggs– Under favorable conditions they lay thin-

shelled eggs that hatch into females– Under non-favorable conditions they lay thick

shelled eggs that hatch into females