Dr. T logo BIO 101 Principles of Biology II
The Invertebrates


Invertebrates are defined as those animals that don't have a backbone (internal skeleton)

Invertebrates comprise more than 95% of all animal species.

•  There are so many phyla of invertebrates, it can be difficult to keep all the details straight. Try to identify the main features of each group using the cladograms and phylum comparison tables in the book as guides.

Parazoa - no tissues

•  This group contains only one phylum.

•  Phylum Porifera - sponges

•  There are about 8,000 species of sponges

•  Nearly all are marine animals.

•  Adult forms are sessile, but they have a free-swimming larva stage.

•  Basic body plan of a sponge

Structure of a sponge
Basic structure of a typical sponge
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Sponges are filter-feeders.

•  Beating flagella of choanocytes produce a current that forces water through the body.

•  Water enters through pores (ostia) and is expelled trough the osculum.

•  Food particles are filtered out by choanocytes.

•  Amoebocytes are mobile and carry food to other cells.

•  Some sponges have hard spiny spicules.

Eumetazoa - tissues present

•  The Eumetazoa include all remaining phyla (all except Porifera)

•  These animals are have cells organized into definite tissues

•  Eumetazoa are divided first based on body symmetry, radial or bilateral.

Radiata - radial symmetry

•  Phylum Cnidaria - anemones, corals, jellyfish, and others

•  Some examples

•  Sea anemones

Sea Anemones
Several species of sea anemones
Photograph by John Tiftickjian

•  Corals

Corals
Corals
Photograph by John Tiftickjian

•  Can be sessile (polyp) or motile (medusa)

•  Polyps can live alone (sea anemones) or in colonies (corals)

•  Corals deposit calcium carbonate (the hard part of a coral), when extensive, can form reefs.

•  Body plan of a cnidarian

Two forms of cnidarians
The polyp and medusa forms of cnidarians
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Body has two layers

•  Epidermis - from ectoderm of embryo

•  Gastrodermis - from endoderm of embryo

•  Food is ingested through the mouth and digested extracellularly in the gastrovascular cavity.

•  Wastes are expelled through the mouth (there is no anus)

•  Food is captured by tentacles which surround the mouth.

•  Cnidarians have special stinging cells - cnidocytes.

•  These contain stinging capsules called nematocysts that can be expelled to catch prey.

•  Some species have both polyp and medusa stages.

Life cycle of Obelia
Life cycle of the cnidarian Obelia.
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Phylum Ctenophora - comb jellies

Bilateria - bilateral symmetry

•  In the traditional system, the bilateria were divided based on the presence of a body cavity (coelom). But this has been modified based on newer molecular evidence. We will follow the newer system to stay in synch with the textbook.

•  The bilateria comprises two groups, the protostomes and deuterostomes.

Protostomia - embryo with spiral cleavage

•  Lophotrochozoa

•  Phylum Platyhelminthes - flatworms

•  Lack respiratory and circulatory systems

•  Relatively small and flat to speed up diffusion

•  Body plan of a flatworm

A planarian (flatworm)
The body plan of a typical flatworm.
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Three body layers: ectoderm, endoderm, mesoderm

•  This allows for more complex organs to develop
•  No true body cavity (coelom)

•  Muscles are present.

•  Primitive nervous system.

•  Sensory organs (eyespots) in head.

•  First animals to have active predatory lifestyle.

•  Phylum includes free-living planarians and parasitic flukes and tapeworms.

•  Phylum Rotifera - rotifers ("wheel animals")

•  Head has a ciliated crown the propels it and brings food to the mouth.

•  Digestive system has two openings: mouth and anus (why is this an advantage?).

•  Has a body cavity called a pseudocoelom.

•  It is not completely lined with mesoderm, so not a true coelom.

•  Body plan of a rotifer

A rotifer
The body plan of a typical rotifer
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Lophophorates - bryozoans, tube-worms, others (3 phyla)

•  Have a lophophore

•  This structure contains tentacles that act in feeding and respiration.

•  A bryozoan

A bryozoan
A bryozoan, showing extended lophophore
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Phylum Mollusca - snails, clams, octopus

•  Very large group - over 100,000 species

•  Many fossils (about 35,000 species) are known (why are so many fossils available in this group?)

•  Most species produce hard calcified shells.

•  Body is composed of: foot (for movement), visceral mass (organs), and mantle (secretes the shell).

•  Mollusks have a circulatory system and gills for gas exchange.

•  Mouth parts may have a radula (special toothed device for scraping food off surfaces).

•  Body parts of a mollusk

Mollusk body plan
The body plan of a typical mollusk (gastropod)
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Includes

•  Gastropods (snails)
•  Have a one-piece shell
•  Some have lost the shell through evolution, e.g. slugs
•  Bivalves (clams, mussels, oysters)
•  Have a two-piece shell that can open and close
•  Cephalopods (octopus, squid)
•  The most complex of invertebrates
•  Have tentacles with suckers
•  Have a closed circulatory system

•  Annelida - segmented worms

•  The segmented body is a crucial evolutionary step that gives several advantages.

•  Some body parts are repeated in each segment (if a part in one segment fails, other segments may still remain active.
•  Some segments can be specialized.
•  Fluid-filled coelom creates a hydrostatic skeleton that allows sophisticated locomotion. Waves are contraction propel the body.

•  Body plan of an earthworm

Annelid body plan
The body plan of a typical annelid (earthworm)
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Ecdysozoa

•  Animals that carry out ecdysis (molting)

•  Body is covered by a nonliving cuticle

•  Because the cuticle cannot increase in size once formed, molting is required to grow

•  The old cuticle is shed, the body rapidly enlarges while the new cuticle is soft.

•  Molting is repeated several times over the life of the animal.

•  Phylum Nematoda - roundworms

•  Found in nearly every habitat on the planet

•  Can reach large population density. One shovelful of soil contains many million nematodes.

•  Cuticle is composed of collagen (a protein also present in vertebrates)

•  Electron micrograph of a nematode worm

A nematode (SEM)
A nematode worm as seen with the scanning electron microscope

•  Some are free-living

•  Many are parasites of plants and animals

•  Ascaris - parasite of small intestine
•  Hookworms - also intestinal parasites
•  Pinworms
•  Wuchereria - parasite of lymphatic system, causes elephantitis

•  Phylum Arthropoda - insects, spiders, crustaceans, etc.

•  This is the largest animal phylum. 75% of all species are in this phylum.

•  Name means "jointed foot"

•  Body is covered by hard exoskeleton

•  Exoskeleton contains chitin and protein
•  May be strengthened with calcium carbonate as in the hard shells of crabs and other crustaceans.
•  Impermeable to water, an important adaptation for colonizing land (like cuticle in plants)

•  Segmented body plan (like annelids)

Insect body plan
Body plan of a typical insect
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Segments may have appendages for locomotion and other functions.

•  Well developed sensory structures

•  Special gas exchange organs

•  Gills in aquatic species
•  Tracheal system in terrestrial species
•  Oxygen enters through spiracles then is carried directly to tissues by tubelike trachea.

•  Complex digestive and excretory systems

•  Major classes of arthropods

Arthropods
The major classes of arthropods
Brooker Biology textbook, Copyright McGraw-Hill companies
•  Arachnida - spiders, scorpions, ticks, mites
•  Diplopoda - millipedes
•  Chilopoda - centipedes
•  Insecta - insects
•  Huge number of species
•  Large impact on humans
•  Browse the various orders in the text, page 703
•  Crustacea - crabs, shrimp, crayfish, barnacles, etc.

•  Insect life cycles

•  Incomplete metamorphosis
Incomplete metamorphosis
Incomplete metamorphosis in the grasshopper
Brooker Biology textbook, Copyright McGraw-Hill companies
•  Complete metamorphosis
Complete metamorphosis
Complete metamorphosis in the butterfly
Brooker Biology textbook, Copyright McGraw-Hill companies

Deuterostomia - embryo with radial cleavage

•  Phylum Echinodermata - sea stars, urchins, sea cucumbers

•  Name means "spiny skin"

•  Body has modified radial symmetry.

•  Radial symmetry is a derived trait because larvae have radial symmetry.

•  So echinoderms evolved from other bilateria, not from the radiata.

•  Most have a 5-part symmetry (e.g five arms on a sea star).

•  A unique feature is the water vascular system

•  This is a system of canals that branch into tube feet.

•  Muscle contractions generate hydraulic pressure in the canals causing the tube feet to slowly propel the animal along a surface.

•  Body plan of a typical echinoderm

Echinoderm body plan
Body plan of typical echinoderm (sea star)
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Phylum Chordata - a few invertebrates (+ all vertebrates)

•  Defining characteristics of chordates

Chordate features
Major features of chordates
Brooker Biology textbook, Copyright McGraw-Hill companies

•  Notochord

•  A stiff flexible rod (primitive endoskeleton)
•  In vertebrates, it is replaced by a jointed backbone.

•  Dorsal hollow nerve cord

•  Nonchordates have ventral, solid nerve cords

•  Pharyngeal slits

•  Slits develop in the pharynx just inside the mouth.
•  In primitive chordates, these become a filter-feeding device.
•  In more advanced chordates (like fish), they develop into gills.

•  Postanal tail

•  A tail develops behind the anus.
•  May be used for swimming or other functions.
•  May be absent (as in humans)

•  Few chordates have all of these features in the adult form, but all of them exhibit them during embryonic development.

•  The Chordata contains two subphyla of invertebrates along with the vertebrates (including humans).

•  Subphylum Urochordata - tunicates or sea squirts

•  You would never guess from the adult form that these are chordates.
A tunicate
Body plan of a tunicate
Brooker Biology textbook, Copyright McGraw-Hill companies
•  Adult forms are sessile, filter-feeders.
•  But the tadpole-like larva has all four chordate features.

•  Subphylum Cephalochordata - lancets

•  Like the tunicates, are also filter-feeders, but they look more like chordates and have all four chordate trademarks.

•  Subphylum Vertebrata - vertebrates

•  To be covered in subsequent chapters

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