noncoelomate invertebrates chapter 33. porifera parazoa –animals lacking tissues (and therefore...

Noncoelomate Invertebrates

Chapter 33

Porifera• Parazoa

– Animals lacking tissues (and therefore organs) and a definite symmetry

• 7000 marine species; 150 freshwater species• Among the most abundant animals in the deep

ocean

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• Most members lack symmetry

• Various growth forms– Larval sponges free-

swimming– Adults remain

attached – sessile

• Cell types– Truly multicellular– 3 functional layers in

“vase”3

• 3 layers1. Outer epithelium

• Water comes in ostia, exits osculum

2. Mesohyl• Middle layer – gelatinous matrix• Spicules – needles of calcium carbonate• Spongin – reinforcing tough protein fibers

3. Choanocytes• Collar cells• Flagellated – contributes to water circulation• Face internal cavity• Engulf and digest food from passing water

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• Sponge reproduction– Asexual

• Fragmentation

– Sexual• Choanocytes transform into sperm• Sperm captured and passed to egg cell in mesohyl• Development may occur within mother or in open

water• Larva is planktonic; will settle and transform into

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Eumetazoa

• 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

• True body symmetry– Radial symmetry– Bilateral symmetry

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Phylum Cnidaria

• Most marine, few fresh water species• Diploblastic• Bodies have distinct tissues but no organs

– No reproductive, circulatory, or excretory systems

• No concentrated nervous system– Latticework of nerve cells– Touch, gravity, light receptors

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• Cnidarians use nematocysts to capture prey– Secreted within nematocyte

– Mechanism of discharge unknown

– Some carry venom

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• 2 basic body forms– Polyps – cylindrical and sessile– Medusa – umbrella-shaped and free-living

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• Body plan has single opening leading to gastrovascular cavity– Site of digestion– Most gas exchange– Waste discharge– Formation of gametes in many

• 2 layers to body wall1.Epidermis

2.Gastrodermis – Mesoglea between layers

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• Gastrovascular space also serves as hydrostatic skeleton– Provides a rigid structure against which

muscles can operate– Gives the animal shape

• Many polyp species build an exoskeleton of chitin or calcium carbonate around themselves– Some build an internal skeleton

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• Cnidarian life cycle– Some cnidarians occur only as polyps, and

others exist only as medusae, but many alternate between these two phases

• Both phases consist of diploid individuals

– In general, in species having both polyp and medusa in the life cycle, the medusa forms gametes

• Sexes separate• Gonochorism – individual is either male or female• Zygote develops into planktonic planula• Metamorphosis into polyp• Polyp produces medusae or other polyps asexually

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• Major evolutionary innovation in cnidarians is extracellular digestion of food inside the animal– Digestion takes place partly in gastrovascular

cavity– Cells then engulf fragments by phagocytosis

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4 or 5 classes

1. Anthozoa– Sea anemones, most corals,

sea fans– Solitary and colonial polyps– Symbiotic dinoflagellates

(zooxanthellae) photosynthesize and provide nutrients to reef coral

– Coral reefs economically important

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2. Cubozoa– Box jellies– Strong swimmers, voracious

fish predators– Stings may be fatal to humans

3. Hydrozoa – Hydroids, Hydra, Portuguese

man-of-war– Both polyp and medusa

stages– Only class with freshwater

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4. Scyphozoa– Jellyfish– Medusa more conspicuous and

complex– Ring of muscle cells allows for

rhythmic contractions for propulsion

5. Staurozoa– Star jellies– Resembles a medusa in most

ways but is attached to the substratum by a sort of stalk that emerges from the side opposite the mouth

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Phylum Ctenophora

• Known as comb jellies, sea walnuts, or sea gooseberries

• 8 rows of comblike plates of fused cilia that beat in a coordinated fashion

• Many bioluminescent• 2 tentacles covered with

colloblasts– Discharge strong adhesive used to

capture prey

• Phylogenetic position unclear18


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The Bilaterian Acoelomates

• Characterized by bilateral symmetry• Allowed for high levels of specialization• Bilaterians are traditionally classified by

the condition of their coelom– Acoelomates– Pseudocoelomates– Coelomates

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Phylum Platyhelminthes

• Flatworms are ciliated, soft-bodied animals• Bodies are solid aside from an incomplete

digestive cavity• Many species are parasitic• Others are free-living

– Marine, freshwater, moist terrestrial

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• Only one opening to digestive cavity– Muscular contractions in the pharynx allows

food to be ingested and torn into small bits

• Lack circulatory system– Diffusion for gas transport– Gut functions in digestion and food distribution– Some particles digested extracellularly– Cells engulf particles by phagocytosis– Tapeworms (parasitic flatworms) lack digestive

systems – absorb food directly through body walls

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• Have an excretory and osmoregulatory system – Network of fine tubules runs through body– Flame cells located on the side branches

• Flagella move water and excretory substances into the tubules and then to pores located between the epidermal cells through which the liquid is expelled

– Metabolic wastes are excreted into the gut and eliminated through the mouth

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• Simple nervous system– Anterior cerebral ganglion and nerve cords– Eyespot can distinguish light from dark

• Reproduction– Most are hermaphroditic– Undergo sexual reproduction– Also have capacity for asexual regeneration

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• 2 major groups– Free-living Turbellaria

• Probably not monophyletic• Dugesia – common planarian in bio labs

– Parasitic Neodermata• Trematoda – flukes

– Attach within host body by suckers, anchors, or hooks– Life cycle may have 2 or more hosts– Clonorchis sinensis, oriental liver fluke

• Cercomeromorpha – tapeworms

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• One of most important trematodes to human health are blood flukes Schistosoma– Afflict 5% of world’s population– About 800,000 people die each year from

schistosomiasis or bilharzia– Fertilized egg must break through the wall of the blood

vessels in intestine or the urinary bladder to get out

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• Cercomeromorpha – tapeworms– Adult hangs onto inner wall of host intestine

using scolex

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• Most of tapeworm body is proglottids– Complete hermaphroditic unit, containing both male and female reproductive

organs– Formed continuously

• Beef tapeworm, Taenia saginata– Frequent human parasite– From eating uninspected rare beef

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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

Phylum Acoelomorpha

• Based on molecular evidence, similarities are convergent

Phylum Cycliophora

• Discovery reported in 1995

• Striking circular mouth surrounded by a ring of cilia

• Anatomy and life cycle are very unusual

• Live on the mouthparts of claw lobsters on both sides of the North Atlantic

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Pseudocoelomates

• Pseudocoelom – cavity that lies between tissues derived from the mesoderm and those derived from the endoderm

• Pseudocoelomic fluid performs the functions carried out by a circulatory system in most coelomate animals

• Not monophyletic

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Phylum Nematoda

• Vinegar eels, eelworms, and other roundworms

• Members of this phylum are found everywhere – abundant and diverse

• Marine, freshwater, parasites, free-living

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• Bilaterally symmetrical and unsegmented• Covered by a flexible, thick cuticle that is

molted as they grow• Digestive system well developed

– Stylets – piercing organs near mouth– Pharynx – creates sucking action– Anus

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• Sexual reproduction– Most gonochoric– Sexual dimorphism – male smaller with hooked

end– Internal fertilization– Indirect development – egg, larva, adult

• Eutely– Adults consist of a fixed number of cells– Caenorhabditis elegans has only 959 cells– Important in genetic and developmental studies

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• Lifestyles– Many are active hunters, preying on protists

and other small animals– Others are parasites of plants– Still others live within the bodies of larger

animals• Largest known nematode, which can attain a

length of 9 m, parasitizes the placenta of sperm whales

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• About 50 species cause human diseases• Hookworms– Common in southern U.S.– Produce anemia

• Trichinella causes trichinosis– Forms cysts in muscles – Infection from eating undercooked meat

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• Pinworms, Enterobius vermicularis– Infects 30% of children in U.S.– Causes itching of the anus

• Ascaris lumbricoides – intestinal roundworm– Infects 1 in 6 worldwide– Adult female can be 30 cm long– Rare in areas with modern plumbing

• Serious tropical nematode diseases– Filariasis– Elephantiasis

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Phylum Rotifera

• Bilaterally symmetrical, unsegmented pseudocoelomates

• Highly developed internal organs

• Corona – “wheel animals”– Conspicuous ring of cilia at anterior end– Used for locomotion and sweeping food into

the mouth

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noncoelomate invertebrates chapter 33. porifera parazoa –animals lacking tissues (and therefore...

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