Noncoelomate Invertebrates Chapter 33

Invertebrate Phylogeny Revolution Scientists generally agree about the taxonomic classification of 36 animal phyla -They do disagree, however, about how these are interrelated Traditional reconstructions lump together phyla that share major features of body plan New reconstructions employ molecular comparisons of rRNA and other genes

Invertebrate Phylogeny Revolution Both old and new phylogenies agree that: -Porifera first separated from Eumetazoa -Among the eumetazoans, Cnidaria and Ctenophora branch out before Bilateria -Bilateria are divided into two groups: protostomes and deuterostomes Modern phylogeny differs radically from traditional phylogeny in construction of the protostome lineage

Invertebrate Phylogeny Revolution Traditionally, the branches of animals are based on the nature of the body cavity: -Acoelomates = Have no body cavity -Pseudocoelomates = Have pseudocoel -Coelomates = Have coelom Acoelomates and pseudocoelomates are always protostomes Coelomates are either protostomes or deuterostomes

Invertebrate Phylogeny Revolution

Invertebrate Phylogeny Revolution Modern protostome phylogeny distinguishes between two major clades that have evolved independently: -Spiralians -Ecdysozoans

Invertebrate Phylogeny Revolution Spiralians -Grow by adding mass to an existing body -Two main groups -Lophotrochozoa: mostly coelomates -Include the phyla Brachiopoda, Mollusca and Annelida -Platyzoa: mostly acoelomates -Include the phyla Platyhelminthes, and Micrognathozoa

Invertebrate Phylogeny Revolution Ecdysozoans -Increase in size by molting their external skeletons -Two phyla have been particularly successful -Nematoda: pseudocoelomates -Roundworms -Arthropoda: coelomates -Insects, crustaceans, and others

Invertebrate Phylogeny Revolution

Parazoa Parazoans lack tissues, organs and a definite symmetry -However, they have complex multicellularity Sponges, phylum Porifera, are parazoans -Include marine and freshwater species -Larval sponges are free-swimming, but adults are anchored onto submerged objects

Parazoa Adult sponges are shaped like a vase Walls of the “vase” have three functional layers

Parazoa 1. Inner layer = Specialized flagellate cells called choanocytes, or collar cells 2. Central layer = Gelatinous, protein-rich matrix called the mesohyl 3. Outer layer = Protective epithelium Mesohyl may contain spicules and/or fibers of a tough protein called spongin -These strengthen the body of the sponge

Parazoa

Parazoa Sponges are “filter-feeders” -Beating of the choanocyte flagella draws water through numerous small pores -Brings in food and oxygen and expels wastes -Small organisms are filtered from the water, which flows through passageways -Eventually exits from the osculum

Parazoa Sponge reproduction -Asexual = Fragmentation -Sexual = Egg and sperm -Larval sponges use cilia for swimming -Settle down on a substrate -Transform into adults

Eumetazoa Eumetazoans are animals with distinct tissues Embryos have distinct layers -Inner endoderm = Forms the gastrodermis -Outer ectoderm = Forms the epidermis and nervous system -Middle mesoderm (Only in bilateral animals) -Forms the muscles

Eumetazoa Eumetazoans also evolved true body symmetry -Radiata = Exhibit radial symmetry -Phylum Cnidaria -Phylum = Ctenophora -Bilateria = Exhibit radial symmetry -All other animals

Phylum Cnidaria Cnidarians are nearly all marine -Bodies have distinct tissues but no organs Cnidarians are diploblastic -Ectoderm (epidermis) and endoderm (gastrodermis) -In between, a layer of gelatinous material (mesoglea)

Phylum Cnidaria Cnidarians are carnivores They have two basic body forms -Polyps = Cylindrical and sessile -Medusae = Umbrella-shaped and free-living

Phylum Cnidaria

Phylum Cnidaria Some cnidarians exist as either medusae or polyps, while others alternate between the two phases Polyps may reproduce sexually or asexually -Asexual reproduction occurs by budding and may produce a new polyp or medusae Medusae reproduce sexually -Form free-swimming larvae called planulae

Phylum Cnidaria Cnidarians have internal extracellular digestion -Digestion begins with extracellular fragmentation in the gastrovascular cavity -This is followed by phagocytosis and intracellular digestion Cnidarians have no circulatory, respiratory or excretory systems

Phylum Cnidaria Cnidarians have unique specialized cells on their epidermis called cnidocytes The nematocyst is a special type of cnidocyte -Contains a small but powerful harpoon -Used for food acquisition and defense

Phylum Cnidaria

Four Classes of Cnidarians Hydrozoa (hydroids) Scyphozoa (jellyfish) Cubozoa (box jellyfish) Anthozoa (corals and sea anemones)

Phylum Ctenophora A small phylum whose members are known as comb jellies, sea walnuts or sea gooseberries Structurally more complex than cnidarians Capture prey with a colloblast -Contains strong adhesive material

Phylum Ctenophora Ctenophores propel themselves through water with eight rows of fused cilia They are triploblastic -Have true muscles cells derived from the mesoderm Their mode of symmetry is not strictly radial as in cnidarians

The Bilaterian Acoelomates The Bilateria are characterized by bilateral symmetry -Allowed for high levels of specialization Bilaterians are traditionally classified by the condition of their coelom -Acoelomates -Pseudoceolomates -Coelomates

Phylum Platyhelminthes The flatworms are soft-bodied animals -Many species are parasitic -Others are free-living Move by ciliated epithelial cells Have developed musculature

Phylum Platyhelminthes Flatworms have an incomplete digestive cavity with only one opening -Therefore, they cannot feed continuously Muscular contractions in the pharynx allows food to be ingested and torn into small bits Tapeworms (parasitic flatworms) lack digestive systems -Absorb food directly through body walls

Phylum Platyhelminthes Flatworms have an excretory and osmoregulatory system -Network of fine tubules runs through body -Flame cells located on the side branches -Primary function = water balance -Secondary function = excretion Metabolic wastes are excreted into the gut and eliminated through the mouth

Phylum Platyhelminthes Flatworms lack a circulatory system -Flattened body shape and highly-branched gut utilize diffusion They have a simple nervous system -Eyespot can distinguish light from dark Most are hermaphroditic -Undergo sexual reproduction -Also have capacity for asexual regeneration

Phylum Platyhelminthes

Phylum Platyhelminthes

Phylum Platyhelminthes Flatworms have been traditionally grouped into four major classes -Turbellaria -Monogenea -Trematoda -Cestoda Free-living Parasitic New studies group all three parasitic classes into a single group, Neodermata

Phylum Platyhelminthes Class Turbellaria -Free-living flatworms -Recently shown to be polyphyletic -Found in freshwater, marine and even terrestrial environments

Phylum Platyhelminthes Classes Monogenea and Trematoda -Flukes -Live as ectoparasites or endo-parasites in the bodies of other animals -Attach by suckers, anchors or hooks -Most have a life cycle that involves two or more hosts

Phylum Platyhelminthes Clonorchis sinensis: Oriental liver fluke -Complex life cycle -Miracidium – Ciliated stage in egg -Rediae – Nonciliated larvae produced within sporocyst -Cercaria – Tadpole-like larval stage -Metacercaria – Adult stage produced within cysts

Phylum Platyhelminthes Other important flukes are the blood flukes of the genus Schistosoma -Three species cause the disease schistosomiasis, or bilharzia -Worms coat themselves with host’s own antigens -Thus, are immunologically invisible

Phylum Platyhelminthes Class Cestoda -Tapeworms -Live as parasites within the bodies of other animals -Most species occur in the intestines of vertebrates

Phylum Platyhelminthes Class Cestoda -Long flat bodies divided into three zones -Scolex = Attachment organ -Neck = Unsegmented portion -Proglottids = Repetitive sections -Each a complex hermaphroditic unit -Taenia saginata is the beef tapeworm

Phylum Platyhelminthes

Phylum Platyhelminthes Acoel flatworms were once considered basal members of the phylum Platyhelminthes -Have a primitive nervous system and lack a digestive cavity Are now classified in their own phyla, Acoela

Phylum Nemertea Nemerteans are often called ribbon worms or proboscis worms Are long animals that can stretch to several meters

Phylum Nemertea Nemerteans possess a fluid-filled sac, the rhynchocoel: a true coelomic cavity Are the simplest animals with a complete digestive system (mouth and anus) Have a closed circulatory system They are not related to flatworms -Are now classified with other coelomate protostomes in the Lophotrochozoa

Phylum Cycliophora Discovered in 1995 Acoelomates that live on the mouthparts of claw lobsters Have unique anatomy and form of sexual reproduction

The Pseudocoelomates Possess a pseudocoel, which is a cavity between the mesoderm and endoderm -Serves as a hydrostatic skeleton, against which the animal’s muscles can work Pseudocoelomates lack a defined circulatory system -Role is performed by fluids that move within the pseudocoel

Phylum Nematoda Nematodes are roundworms comprising many species -Found in marine, freshwater and soil habitats Are bilaterally symmetrical and unsegmented

Phylum Nematoda Nematodes are covered with a flexible, thick cuticle Are ecdysozoans -Molt cuticle four times Lack specialized respiratory organs and exchange oxygen through their cuticles

Phylum Nematoda Mouth is equipped with piercing organs called stylets Food passes through the mouth by the sucking action of the pharynx Undigested material is eliminated through the anus

Phylum Nematoda

Phylum Nematoda Nematodes usually reproduce sexually -Sexes are separate Adults of some species exhibit eutely -Have a fixed number of cells -Caenorhabditis elegans has only 959 cells

Phylum Nematoda Many nematodes are active hunters, preying on protists and other small animals Others are parasites of plants Still others live within the bodies of larger animals

Phylum Nematoda Nematodes can cause diseases in humans -Trichinella causes trichinosis -Forms cysts in muscles -Ascaris lumbricoides – Intestinal roundworm

Phylum Rotifera Rotifers are bilaterally symmetrical unsegmented pseudocoelomates -They have complex internal organs They propel themselves through water by rapidly beating thick cilia at their heads -“Wheel animals” They gather food via a conspicuous organ called the corona

Phylum Rotifera