Noncoelomate Invertebrates

Noncoelomate Invertebrates

Chapter 33

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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

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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

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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

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Ecdysozoans

-Increase in size by molting their external skeletons

-Two phyla have been particularly successful

-Nematoda: pseudocoelomates

-Roundworms

-Arthropoda: coelomates

-Insects, crustaceans, and others

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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

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Adult sponges are shaped like a vase

Parazoa

Walls of the “vase” have three functional layers

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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

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Parazoa

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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

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Sponge reproduction

-Asexual = Fragmentation

-Sexual = Egg and sperm

-Larval sponges use cilia for swimming

-Settle down on a substrate

-Transform into adults

Parazoa

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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

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Eumetazoans also evolved true body symmetry

-Radiata = Exhibit radial symmetry

-Phylum Cnidaria

-Phylum = Ctenophora

-Bilateria = Exhibit radial symmetry

-All other animals

Eumetazoa

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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

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Cnidarians are carnivores

They have two basic body forms

-Polyps = Cylindrical and sessile

-Medusae = Umbrella-shaped and free-living

Phylum Cnidaria

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Fig. 33.5(left)

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Phylum Cnidaria

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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

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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

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Fig. 33.7

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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

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Phylum Cnidaria

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Hydrozoa (hydroids)

Four Classes of Cnidarians

Scyphozoa (jellyfish)

Cubozoa (box jellyfish) Anthozoa (corals and sea anemones)

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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

The Bilaterian Acoelomates

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The flatworms are soft-bodied animals

-Many species are parasitic

-Others are free-living

Phylum Platyhelminthes

Move by ciliated epithelial cells

Have developed musculature

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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


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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


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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


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-Turbellaria

-Monogenea

-Trematoda

-Cestoda


Free-living

Parasitic

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Class Turbellaria

-Free-living flatworms

-Recently shown to be polyphyletic

-Found in freshwater, marine and even terrestrial environments


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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


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Class Cestoda

-Tapeworms

-Live as parasites within the bodies of other animals


-Most species occur in the intestines of vertebrates

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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


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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

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Nemerteans are often called ribbon worms or proboscis worms

Phylum Nemertea

Are long animals that can stretch to several meters (up to 60 m)

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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

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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

The Pseudocoelomates

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Nematodes are roundworms comprising many species-Found in marine, freshwater and soil habitats

Are bilaterally symmetrical and unsegmented

Phylum Nematoda

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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

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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

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Phylum Nematoda

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Phylum Nematoda

Nematodes usually reproduce sexually

-Sexes are separate

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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

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Phylum Nematoda

Nematodes can cause diseases in humans

-Trichinella causes trichinosis

-Forms cysts in muscles

-Ascaris lumbricoides – Intestinal roundworm

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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

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Phylum Rotifera

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Noncoelomate Invertebrates