Flatworms, Mesozoans, and Ribbon Worms

Chapter 14

Bilateria

Most animals have bilateral symmetry.

The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development.

Bilateral Symmetry

Radially symmetrical animals have the world coming at them from all directions.They can catch prey coming from any

direction.Animals that begin to move about

actively seeking food need a different body organization.Distinct head end with sensory structures.Cephalization

Bilateral Symmetry

Animals with bilateral symmetry have a distinct head end and can be divided into right and left halves.

Acoelomate Bilateral Animals

Animals that have no space between their gut and body wall are said to be acoelomate.

These animals are also triploblastic – they have three embryonic germ layers.

Organ-system level of organization – more division of labor among their organs.

Acoelomates

Although flatworms undergo triploblastic development, they are acoelomates.

Acoelomates

These acoelomate phyla are protostomes and have spiral cleavage.Most have determinate cleavage.

These are the simplest animals with an excretory system.

Acoelomate phyla belong to the superphylum Lophotrochozoa

Phylum Acoelomorpha

Group contains ~350 species. Members were formerly in Class

Turbellaria within phylum Platyhelminthes Small flat worms less than 5 mm in length.

Typically live in marine sediments; few are pelagic. Some species live in brackish

water. Most symbiotic but some parasitic. Have a cellular ciliated epidermis. Parenchyma layer contains small

amount of ECM and circular, longitudinal, and diagonal muscles.

Phylum Acoelomorpha - Digestion and Nutrition

Incomplete digestive system - no anus.

In many acoels, the gut and pharynx are absent. Phagocytotic cells

digest food intracellularly when food is passed into temporary spaces.

Phylum Acoelomorpha - Reproduction

MonoeciousFemale produces yolk-filled, endolecithal

eggs.Following fertilization some or all

cleavage events produce a duet-spiral pattern of new cells.May be a defining character for

acoelomorphs.

Phylum Platyhelminthes

Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats.

Phylum Platyhelminthes

Flatworms are flattened dorsoventrally and have a gastrovascular cavity. Extracellular

digestion. Undigested food is

egested through the pharynx.

Phylum Platyhelminthes

The osmoregulatory system consists of protonephridia (excretory or osmoregulatory organs closed at the inner end) with flame cells. Most metabolic

wastes removed by diffusion across the body wall.

Phylum Platyhelminthes

The nervous system consists of a ladder-like network of nerves and a bilobed brain.

Many have large ocelli – light sensing organs.

Phylum Platyhelminthes

Many can reproduce asexually as well as sexually. Asexual

reproduction via fission.

Sometimes the new individuals remain attached – chains of zooids.

Monoecious

Taxonomy

Flatworms (phylum Platyhelminthes) are divided into four classes:Class Turbellaria – ex. Planaria

Not monophyleticClass Trematoda – parasitic flukesClass Monogenea – parasitic monogenetic

flukesClass Cestoda - tapeworms

Phylum Platyhelminthes

Class Turbellaria

Turbellarians are nearly all free-living and mostly marine.

Class Turbellaria

The best-known turbellarians, commonly called planarians, have light-sensitive eyespots and centralized nerve nets.

Class Trematoda

Trematodes live as parasites in or on other animals.

They parasitize a wide range of hosts.

Class Trematoda

Subclass Digenea, digenetic flukes, have a complex life cycle with a mollusc (snail) as the first host and a vertebrate as the final, or definitive, host.

Class Monogenea

All monogeneans are parasites. Often found in the

gills or external surfaces of fishes.

Class Cestoda

Tapeworms (Class Cestoda) are also parasitic and lack a digestive system.

The scolex is equipped with suckers and hooks for attachment to the host.

Each proglottid contains a set of reproductive organs.

Class Cestoda

Cestodes usually require at least two hosts. Adult cestodes are

parasites in the digestive tracts of vertebrates.

Phylum Mesozoa

Phylum Mesozoa is considered a “missing link” between protozoa and metazoa.

Have a simple level of organization. Minute, ciliated, and wormlike animals.

All live as parasites in marine invertebrates. Most composed of only 20 to 30 cells arranged in two

layers. Layers are not homologous to germ layers of other

metazoans. Two classes, Rhombozoa and Orthonectida, are so

different that some authorities place them in separate phyla.

Phylum Mesozoa

Rhombozoans live in kidneys of benthic cephalopods.

Adults called vermiforms and are long and slender.

Inner, reproductive cells give rise to vermiform larvae.

When overpopulated, reproductive cells develop into gonad-like structures producing male and female gametes.

Larvae are shed with host urine into the seawater.

Phylum Mesozoa

Orthonectids parasitize a variety of invertebrates.

Reproduce sexually and asexually. Asexual reproduction consists of a

multinucleated mass called a plasmodium.

Phylogeny of Mesozoans

Some consider these organisms primitive flatworms and place them in phylum Platyhelminthes.

Molecular evidence groups them with flatworms in superphylum Lophotrochozoa.

However, molecular phylogeny that included an orthonectid and two species from a rhombozoan subgroup, the dicyemids, did not show members of the two classes to be sister taxa. The phylum may not be monophyletic.

Phylum Nemertea

Ribbon worms, phylum Nemertea, use a proboscis to capture prey. Almost completely

marine. Active predators. General body plan

similar to turbellarians.

Phylum Nemertea

An anus is present providing these worms with a complete digestive system.

Nermeteans are the simplest animals to have a closed loop blood-vascular system.

Phylogeny

A planuloid ancestor (like the planula larva of cnidarians?) may have given rise to a branch of descendents that were sessile or free floating and radialCnidaria.

Another branch acquired a creeping habit and bilateral symmetry

Bilateria.