Platyhelminthes, Nemertea, & Gastrotricha The Flat Worms

Phylum Platyhelminthes Classification: ◦Class Turbellaria ◦Class Monogenea ◦Class Trematoda  Subclass Aspidogastrea  Subclass Digenea

Characteristics Usually flattened dorsoventrally Triploblastic (three germ layers) Acoelomate (lacks a true gut) Bilateral symmetry Unsegemented worms Incomplete gut ◦Gut is absent in Class Cestoidea Protonephria as excretory/osmoregulatory structures

Characteristics Some cephalization present ◦Anterior cerebral ganglion ◦Longitudinal nerve cords Usually monoecious Complex reproductive system Nervous system consists of the pair of anterior ganglion with longitudinal nerve cords connected by transverse nerves located in the mesenchyme

Specialized Structure Platyhelminthes have a specialized structure called the parenchyma This is a loose collection of tissues developed from the mesoderm. The parenchyma fills in all the spaces between other more specialized tissues, organs, and the body wall.

Class Turbellaria Free-living bottom dwellers Found in freshwater and marine environments Named for turbulence their cilia create in the water. ◦L.turbellae, a commotion + aria, like Commonly known as planaria

Class Turbellaria

Turbellarian Nutrition Simple, incomplete digestive tract. ◦Pharynx with branched or lobed digestive tubes Evolutionarily, their digestive tracts may be advantageous. The gastrodermis is closer to the location of digestion and absorption. Most of these worms are carnivorous and many other scavenge for food. A few are herbivorous, feeding on algae.

Gas exchange in Turbellarians Since these organisms lack a respiratory system, all gasses and wastes are exchanged through diffusion across the skin. The protonephridia are basically tubuleswhich function in osmoregulation for freshwater turbellarians. The end of these structures contain flame cells. Flame cells use cilia to move fluids out through the nephridiopore.

Major organ system Digestive Protonephridial

Major organ systems Nervous system Reproductive

Planarian Nervous System Some of have a centralized nerve net and ganglia in the head, aka “cerebral region” Most have just a simple bet with two nerve cords creating a ladder-like structure. Eyespots called ocelli detect light and dark. They want to get away from the light!

Reproduction in Planarians Most common form of is tranverse fission. This type of asexual reproduction forms zooids. Zooids are able to complete regenerate missing parts after the split occurs. They are monoecious, containing male and female reproductive systems, but do not practice self-fertilization. (Penis fencing) Form a cocoon full of eggs or a free-swimming larve know as Müller’s larva.

Class Monogenea Commonly known as flukes Name comes from their life cycle. They on produce one egg, or generation, in their life cycle. External parasites that lives on freshwater and marine fishes ◦Feed on epithelial cells, mucus, or blood ◦Opisthaptor attaches to fish gill filaments Eggs hatch a ciliated larva called a oncomiracidium, which will attached to another adult’s opisthaptor and develop into an adult

Class Monogenea EctoparasiteMonogenea anatomy

Class Trematoda Trematodes, Gr., perforated form Also known as flukes because of their wide, flat shape. Trematodes are internal parasites, aka endoparasotes. ◦Immature flukes are found inside vertebrates and invertebrates, or even encysted on plants. ◦Adult flukes are found inside vertebrates. ◦Feed on cells and cell fragments.

Class Trematoda Digestive system is similar to other Platyhelminthes ◦One special structure: two blind-ending, branched pouches called cecum. Also has a special structure in the body wall called the tegument.

Trematoda Body Wall The body wall is aids in nutrient absorption for trematodes. The outer layer of the body wall is the tegument, which consists of: ◦Glycocalyx: organic layer of proteins and carbohydrates that aids in transport of nutrients, wastes, and gases ◦Microvilli: facilitate nutrient exchange ◦See Fig on pg. 155

Subclass Apidogastrea Small, endoparasitic flukes Found in molluscs Oral sucker is absent Life cycle may involve one or two hosts ◦One host: only the mollusc ◦Two hosts: Initial host is the mollusc and the final host is a vertebrate.

Subclass Apidogastrea

Subclass Digenea Most flukes belong in this class. Di, two + genea, birth (Gr) Named for their life cycle which includes a larval and adult stage. Adults are endoparasites in vertebrates. May have one or more intermediate hosts in the larval stage.

Digenea lifecycle

Noteworthy Parasitic Trematodes Chinese liver fluke, Clonorchis sinensis- lives in bile ducts of liver. Humans become infected from eating poorly cooked fish. Sheep liver fluke, Fasciola hepatica- common in sheep-raising areas. Humans and sheep become infected by consuming infected plants. Schistosomes, or blood flukes, several species- humans become infected by entering or ingesting water where flukes live.

Class Cestoidea Kestos, girdle + eidos, form (Gr) These flatworms are commonly known as tapeworms Endoparasites residing in vertebrate digestive systems. Lack a mouth and digestive tract Adults have a series of proglottids, each of which contains a complete set of reproductive structures.

Subclass Cestodaria Endoparasites of primitive fish Only 15 species exist Only one set of both reproductive systems is present instead of proglottids

Class Eucestoda True tapeworms Most specialized parasitic animal Body made of a scolex (head), neck, and strobila (series of proglottids) Nervous and protonephridial systems are reduced, reproductive system is advanced.

Noteworthy tapeworms Beef tapeworm- cattle become infested by grazing in pastures contaminated by human feces, humans from eating infected, poorly cooked beef. Pork tapeworm- similar to beef tapworm Broad fish tapeworm- fish become infected by consuming infected copepods, humans by consuming infected, undercooked fish

Phylum Nemertea Nemertes, a Mediterranean sea nymph (Gr) Elongated, flattened, marine worms. Found in mud and sand. Have a long proboscis which is held in a sheath called the rhyncocoel. The proboscis is used to capture annelids and crustaceans.

Nemertean characteristics Triploblastic (three germ layers) Acoelomate (no body cavity) Bilaterally symmetrical, unsegmented worms Has a ciliated epidermis containing mucous glands Complete digestive tract with anus Dioecious

Nemertean characteristics Protonephridia Cerebral ganglion with longitudinal nerve cords Closed circulatory system ◦First animals to develop this system containing two lateral blood vessels but no heart Body musculature is divided into two or three layers

Phylum Gastrotricha Gastros, stomach + trichos, hair (Gr) 500 species of freshwater and marine free-living worms Cilia used as sensory structures and for movement Nervous system similar to Platyhelminthes Complete digestive system & digestion is extracellular

Gastrotricha characteristics Most reproduce sexually and are monoecious Asexually reproduction can occur ◦Parthenogenesis-laying two unfertilized eggs which will then hatch as females. No larval stage

Nemertea & Gastrotricha