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
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.
Cnidarians
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.
Carnivores.
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
Aquatic
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
Bivalvia
Shells divided into
two parts.
Gills are used for
feeding and gas
exchange.
Example:
Clams, oysters
Dreissena polymorpha
Zebra mussel
Cephalopoda
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
Non-segmented
Some are important parasites of animals.
Pinworms and hookworms, e.g., Trichinella, obtained by
eating undercooked pork; juveniles infect all body organs
and tissues.
Nematodes
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.
b.
c.
d.
Trilobites – extinct
Chelicerates – horseshoe crabs, spiders
Uniramians – centipedes, millipedes, insects
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.