Invertebrates A Survey of Invertebrates

Trends in Invertebrate Evolution 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 Evolution Second trend – existence of mesoderm- lined 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

Coelom 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 Evolution 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 Evolution 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 Evolution 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 Asymmetric 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 (flatworms) 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 (echinoderms) Star fish, sea cucumbers, sea urchins) Live entirely in water Spiny skin Have five-part radial symmetry

Phylum Arthropoda (arthropods) Most successful animals - largest and most diverse phylum Jointed-leg animals Exoskeletons Divided into three groups

Phylum Arthropoda (arthropods) 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