Chapter 19: Invertebrates



A Survey of Invertebrates

Trends in Invertebrate


Common ancestors of multicellular animals had already evolved two distinct cell layers called germ layers, separated by a jelly-like middle layer

Germ Layers

The ectoderm (outer layer) will develop into skin and other body coverings, glands, and nervous system tissues

The endoderm (inner layer) grows into tissues and organs of the digestive tract and respiratory system

Germ Layers

Middle layers varies by animal

Sponges and jellyfish only have a primitive layer of jelly-like cells

Higher animals have an actual third cell layer called the mesoderm which develops into skeletal, muscle, and circulatory tissues, and reproductive system

Trends in Invertebrate


Second trend – existence of mesodermlined cavity called coelom

Provides space for organs to grow and function without being twisted or squeezed by body movements

Fluid within coelom helps to carry food, wastes, dissolved gases


Acoelomates = animals without a coelom, like the hydra

Pseudocoelomates = have a body cavity between mesoderm and endoderm, like roundworm

Coelomates = have a true coelom that houses digestive tract and organs, like annelids (earthworm)

Trends in Invertebrate


Third trend is the evolution of a body

plan, built of several compartments

Compartments called segments, allow for increase in body size

In higher animals, segments are specialized and more complex

Trends in Invertebrate


Many organisms also have symmetry

Radial symmetry = body parts repeated around an imaginary line drawn through center of the body

Bilateral symmetry = left and right sides are identical

Trends in Invertebrate


Organisms with bilateral symmetry also show cephalization, which means that they have a distinct front end and back end (anterior and posterior)

Invertebrate Phyla

Phylum Porifera (sponges)

Phylum Cnidaria (cnidarians)

Phylum Platyhelminthes (flatworms)

Phylum Nematoda (nematodes or roundworms)

Phylum Mollusca (mollusks)

Phylum Annelida (annelids)

Phylum Echinodermata (echinoderms)

Phylum Arthropoda (arthropods)

Phylum Chordata (invertebrate chordates)

Phylum Porifera (sponges)

Simplest multicellular animals


Mostly independent, but can live together

Mostly marine, some in freshwater

Phylum Cnidaria (cnidarians)

Jellyfish, corals, sea anemones, hydras

Mostly marine, few freshwater

Radial symmetry

Phylum Plathyhelminthes


Bilateral symmetry

Show cephalization

Mostly parasitic

Phylum Nematoda (nematodes

AKA roundworms)

First to show a tube-like digestive system (mouth at one end, anus at the other end)

Some are microscopic, some can be a meter long

Phylum Mollusca (mollusks)

Clams, snails, squids

Can live everywhere, even on land

Many have outer shells for protection

Sea snails secrete chemicals to avoid predators

Clams, mussels, oysters, scallops are bivalves

(double shells) and can shut when in danger

Squids and octopuses also emit dark ink to confuse predators

Phylum Annelida (annelids)

AKA segmented worms

Many are detritus feeders

Live on land and in water

Phylum Echinodermata


Star fish, sea cucumbers, sea urchins)

Live entirely in water

Spiny skin

Have five-part radial symmetry

Phylum Arthropoda


Most successful animals - largest and most diverse phylum

Jointed-leg animals


Divided into three groups

Phylum Arthropoda


Subphylum Chelicerata includes spiders and scorpions

Named by mouth parts (chelicerae)

Mostly carnivorous, can be posionous

Class Crustacea includes lobster, shrimp, crab

Class Insecta contains ½ of all arthropods

Phylum Chordata

Invertebrate chordates

Link between invertebrates and vertebrates

Endoskeleton with notochord, to which muscles are attached

Ex: Sea squirt