Flatworms, Mesozoans, and Ribbon Worms Chapter 14

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.

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 monophyletic Class Trematoda – parasitic flukes Class Monogenea – parasitic monogenetic flukes Class 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 radial Cnidaria. Another branch acquired a creeping habit and bilateral symmetry Bilateria.