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Chapter 33 Invertebrates. Two views of animal phylogeny: Which is right? Both are useful?. Parazoa Phylum Porifera:  Sponges 1.  Non-moving (sessile) animals 2.  No nerves or muscles (no tissue differentiation) 3.  Mostly marine 4.  Filter feeders:  Collect food particles from water

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Chapter 33 Invertebrates

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Chapter 33


Two views of animal phylogeny: Which is right? Both are useful?


Phylum Porifera:  Sponges

1.  Non-moving (sessile) animals

2.  No nerves or muscles (no tissue differentiation)

3.  Mostly marine

4.  Filter feeders:  Collect food particles from water

5.  Most sponges are hermaphrodites.  Hermaphrodites function as both male and female in sexual reproduction by producing eggs and sperm.

**All other animals are in the clade Eumetazoa (true tissues).

Animals probably evolved from colonial, flagellated protists, like this choanoflagellate colony.


Polyp and medusa forms of cnidarians.

Radial symmetry with central digestive (gastrovascular) cavity.

One opening in the gastrovascular cavity serves as both mouth and anus.


Phylum name comes from specialized cells called cnidocytes. 

Cnidocytes are stinging cells used for defense and to capture prey.

Purple striped jelly, Pelagia panopyra

Polyps: Sea anemones

Phylum Ctenophora

The Comb jellies

Resemble cnidarian medusas.

Use cilia for locomotion.

Protostomia:  Lophotrochozoa

Phylum Platyhelminthes:  Flatworms

Sizes range from microscopic up to 20 meters long (tapeworms).

Many are parasites.

Class Turbellaria (flatworms)

i.  Mostly free-living (non-parasitic)

ii.  Feed on small animals, dead animals

iii.  Very flat for O2 exchange.  They have no gas exchange organs.

Classes Monogenea/Trematoda

i.  Live as parasites

ii.  Trematodes parasitize vertebrates. 

For example, humans but often with intermediate hosts. 

The life history of a blood fluke (Schistosoma mansoni).

Class Cestoidea – Tapeworms

i.  Live as parasites

ii.  Head contains suckers and hooks that lock onto the intestinal lining of the host.

iii.  The rest of the body is mostly units called proglottids that are sex organs.

iv.  Eggs transferred to new hosts by consuming fecal contaminated water.

Anatomy of a tapeworm

Phylum Rotifera:  Rotifers


Sizes range from 0.5 to 2 mm

Complete digestive tract

A rotifer

Lophophorate Phyla:  Bryozoans, Phoronids, & Brachiopods

All of these groups have a lophophore. 

The lophophore is a horseshoe shaped fold near the mouth that is surrounded by ciliated tentacles.

Bryozoans resemble mosses. 

 - They have a hard exoskeleton.

           - They are important as reef builders.

Phoronids are marine worms.

Brachiopods resemble clams.

A bryozoan

A brachiopod

Phylum Nemertea:  Proboscis (ribbon) worms

Up to 30 meters in length

These worms have a hydraulically-operated proboscis that is used to capture prey.

Closed circulatory system.

Nemertea – Proboscis worms

Phylum Mollusca:  Mollusks

Snails, clams, octopi, squids, oysters

There are at least 150,000 known species

All mollusks have similar body plans:

a.  Muscular foot

b.  Visceral mass with organs

           c.  Mantle that secretes the shell


Shells divided into two parts. 

Gills are used for feeding and gas exchange.


Clams, oysters

Dreissena polymorpha

Zebra mussel


Rapid movement; well-developed nervous systems. 

Example:  Octopus, squid, nautilus

Phylum Annelida:  Segmented worms

Sizes range from 1 mm to 3 meters in length.

Each segment contains a pair of excretory tubes called metanephridia.

Annelids are hermaphrodites that cross-fertilize.

Three classes:

a.  Oligochaeta – earthworms

           b.  Polychaeta – mostly marine

           c.  Hirudinea – leeches

Protostomia:  Ecdysozoa

Phylum Nematoda:  Roundworms


Some are important parasites of animals.

Pinworms and hookworms, e.g., Trichinella, obtained by eating undercooked pork; juveniles infect all body organs and tissues.


Phylum Arthropoda:  Arthropods (crustaceans, spiders, insects)

Hard exoskeleton, segmented bodies, jointed appendages

Arthropods are the most successful of all animal phyla based on diversity, distribution, and numbers.

Nearly one million species identified so far, mostly insects.

The exoskeleton, or cuticle, is composed of protein and chitin.

Molting of the cuticle is called ecdysis.

Extensive cephalization.

Open circulatory systems in which a heart pumps hemolymph through short arteries and into open spaces (sinuses).

Aquatic members- gills for gas exchange; terrestrial members- tracheal system of branched tubes leading from surface throughout body.

Four evolutionary arthropod lineages

a.  Trilobites – extinct

b.  Chelicerates – horseshoe crabs, spiders

c.  Uniramians – centipedes, millipedes, insects

d.  Crustaceans – crabs, lobsters, barnacles 

From these lineages arose five major classes of arthropods.

Insecta (insects)

Outnumber all other forms of animals.

Evolved flight during Carboniferous period.  Flight was followed by an explosion of diversity.

Coevolution of flowering plants and insects.

Wings are extensions of the cuticle.

Waste is removed from hemolymph by excretory organs called malpighian tubes.  

Nervous system has a pair of ventral nerve chords that join in the head to form a cerebral ganglion (brain) that is close to the sensory organs in the head (cephalization).

  • Note: Many insects undergo metamorphosis during their development.

  • Incomplete metamorphosis is a process whereby the young look like the adults, but have different body proportions.

  • Complete metamorphosis is a process where the larval stages (larva, maggot or grub) are specialized for eating.

    • The adult stage is specialized for reproduction and dispersal (e.g. flight).

    • The process of metamorphosis occurs during a pupal stage.

Class Crustacea

E.g., crabs, lobsters

Almost all are aquatic.

Ca. 40,000 species.

Includes krill eaten by whales, and daphnia, copepods, & amphipods in Lake Erie, pill bugs, etc.

Deuterostomia: Phylum Echinodermata (Echinoderms)

Water vascular systems – network of hydraulic canals used for locomotion, feeding, and gas exchange.  It extends into tube feet that are used for locomotion and feeding.

Echinoderms appear to be radial, but are bilateral in larval stages.

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