Bilateral Symmetry The remaining phyla are all bilaterally symmetrical or at least have primary bilateral symmetry Also called Bilateria the development of bilateral symmetry is one of the most important traits in higher animals it implies that a single line or plane will divide the body into equal halves or mirror images

bilateral symmetry has allowed several other important changes in body structure Cephalization- concentration of the nerve tissue into a head; which allow for a single organ to direct the functions of the animal allow for greater organ development allows for greater differentiation of structure; such as appendages Bilateral Symmetry

Advantages of bilateral symmetry: better coordinated movement much quicker and more precise response to stimulation; since it is directed by a central nerve center. Bilateral Symmetry

Acoelomates Includes phyla Platyhelminthes and Nemertea those organisms that do not have a true coelom or body cavity are entirely solid except for the gastric cavity or coelenteron

PHYLUM PLATYHELMINTHES THE FLATWORMS

bilaterally symmetrical triploblastic; 3 distinct tissue or germ layers –Ectoderm –Mesoderm –Endoderm Dorsoventrally flattened lack an anus; incomplete digestive tract CHARACTERISTICS OF PHYLUM

coelom- solid mesenchyme (mesoderm); first phylum to show a definite cellular mesoderm have no true respiratory system or circulatory system –Have cutaneous respiration first phylum to show distinct excretory system; get rid of nitrogenous waste are usually hermaphroditic CHARACTERISTICS OF PHYLUM cont.

They consist of 4 classes of flatworms; 3 of which are entirely parasitic, the other free-living –Turbellaria- free-living –Monogenea- parasitic, flukes –Trematoda- parasitic, flukes –Cestoda- parasitic, tape worms PLATYHELMINTHES

CLASS TURBELLARIA

consists of all of the free-living flatworms, but some are found on aquatic hosts as ectoparasites or commensals few live in freshwater (ie. Planaria), marine as well as moist terrestrial habitats

Morphology the outer surface of turbellarians consists of ciliated epidermis usually best developed on the ventral surface and function in locomotion epidermis also has a large number of mucous glands that secrete mucous that is used by the cilia in locomotion - Rhabdites

Important Structures Eyespots- for light reception; phototaxis Mouth with pharynx Incomplete digestive tract No anus Morphology

Locomotion/Movement Below epidermis are a series of muscles –Dorso-ventral or oblique muscles (=parenchymal) –circular muscles –longitudinal muscles Movement is combination of these muscles contracting

Feeding Turbellarians primarily carnivorous capture of prey is done by wrapping themselves around it and entangling it with mucous they ingest the whole prey or suck its juices through a hardened stylet (modified pharynx) in many species the pharynx is completely eversible and can envelope the entire prey there is no anus so ingestion and egestion are through the mouth

Excretion/Osmoregulation done by specialized cell called flame cells or protonephridia –first group with specialized excretory structures

Nervous System and Sensory Structures Ladder-like arrangement show a beginnings of a well developed central nervous system- cepahlization a variety of sensory cells and glands; most are chemo- or tactile receptors –2 eye spots or ocelli, which can discriminate varying light intensities

Reproduction the most complex organ system most are hermaphroditic cross fertilization is the most common mode of reproduction some self fertilization can and does occur usually the eggs and sperm are produced at different times in the individual

fertilization is internal fertilized eggs are usually deposited in clusters winter eggs have a hard outer covering that can survive desiccation and freezing in freshwater forms eggs hatch into miniature adult forms; direct development in marine forms (not all) a larva is produced called a Müller's larva which is free swimming Reproduction cont.

Asexual Reproduction fragmentation and regeneration many studies have been done on regeneration in Planaria

Life as a Parasite Loss of sensory structures –Eye spots; tactile sensors Loss of some organ systems –Digestive system Increased reproductive potential –Insures survival of species

Parasites and Hosts Host- organism parasite lives on or in –Definitive or Primary host- where parasite has sexual reproduction –Secondary or Intermediate hosts- parasite has asexual reproduction Endoparasite/Ectoparasite- in or on host Facultative/Obligatory parasites –Facultative- on host only part time; e.g., leech –Obligatory- on or in host entire life; e.g., fluke

Classes Trematoda and Monogenea (Flukes) all are parasitic (endoparasitic and ectoparasitic) –most parasitic to vertebrates; especially fish –most have intermediate hosts as invertebrates –many species are economically and medically important

Characteristics of Flukes Have many of the same characters as turbellarians usually have an anterior sucker around mouth and a posterior sucker- used to attach to host tissue –suckers best developed in Monogenea where they are called Opisthaptors body does not have ciliated epidermis as turbellarians

Reproduction of Flukes Sexual in definitive host –is generally through copulation with cross fertilization; sometimes self fertilization occurs Asexual in intermediate hosts

Typical Life Cycle Egg Miracidium larva Sporocysts Redia Cercaria Metacercaria Adult Usually 2 but as many as 4 hosts –Often intermediate host is a snail Two intermediate hosts

Another Example: this one with one intermediate host

CLASS CESTODA (tapeworms) All are endoparasites the body is covered by a cuticle like the trematodes they differ from all of the other flatworms in that they do not have a digestive tract

Characteristics of Tapeworms Features similar to other flatworms –E.g. respiration, and excretion Morphological features include: –Scolex Rostellum Suckers and hooks –Proglottids (repeated segments) Immature; sexually immature Mature; sexual structures present Gravid; filled with fertilized eggs

Typical Life Cycle Egg Onchoshere larva Cysticercus larva in cysts Adult Usually only 2 hosts Intermediate host usually warm blooded

Hydatid cysts can form when larva is ingested by wrong host; example here is in liver of human

Example here is dog tapeworm hydatid cysts in brain of human, which are often inoperable and fatal

PHYLUM NEMERTEA THE PROBOSCIS OR RIBBON WORMS

Almost all are marine, one freshwater genus and one terrestrial genus most are free-living, bottom dwellers NEMERTEA

closely related to flatworms but differ in a number of ways have a circulatory system have tubular complete (**first phylum with complete gut) gut- mouth and anus have an eversible proboscis PHYLUM NEMERTEA

the most diagnostic feature is the eversible proboscis used to capture small prey or for browsing on dead and decaying organisms; are carnivorous lies in a fluid filled cavity (rhyncocoel) in some species the proboscis is armed with barbs or spines and may inject a toxin once food is captured the food is passed into the mouth and gut PHYLUM NEMERTEA

REPRODUCTION are dioecious and fertilization is external –reproductive organs are simple masses of mesenchymal cells –eggs and sperm are released to outside through them –Produce a free living larva- Pilidium in some species fragmentation is common especially when worms are disturbed –usually only anterior end can regenerate to produce a new posterior end Pilidium larva

OTHER FEATURES Excretion- Flame cells Respiration- osmosis Locomotion- gliding Feeding- proboscis, ingestion of prey Regeneration