Invertebrate Phyla and Characteristics

Invertebrate Phyla

• Kingdom Animalia includes invertebrates and vertebrates
• Invertebrates lack a backbone
• Eight invertebrate phyla are: Porifera (sponges), Cnidaria (cnidarians), Platyhelminthes (flatworms), Nematoda (roundworms), Annelida (segmented worms), Mollusca (mollusks), Arthropoda (arthropods), and Echinodermata (echinoderms).

Invertebrate Characteristics

• Animals are eukaryotic, multicellular, and heterotrophic
• Animals have specialized cells

Trends in Animal Evolution

• Cell specialization
• Early development (protostomes vs. deuterostomes)
• Body symmetry (asymmetry, radial, bilateral)
• Cephalization (concentration of sensory organs at the head)
• Body cavity (acoelomate, pseudocoelomate, coelomate)

Phylum Porifera (Sponges)

• Porifera are aquatic, filter feeders
• They have a porous body and lack true tissues or organs.
• Their bodies are asymmetrical
• Sponges have a water flow system
  • Water enters through pores, flows through the central cavity, and exits through an opening called the osculum.
• Sponges have amoeboid cells that build spicules which help digest food

Phylum Cnidaria (Cnidarians)

• Cnidarians have radial symmetry
• Cnidarians are primarily carnivorous
• Their bodies are composed of two layers of cells
• Cnidarians have a gastrovascular cavity with one opening
  • Tentacles have nematocysts which inject toxins into prey
• Cnidarians can exist in polyp or medusa body forms

Phylum Platyhelminthes (Flatworms)

• Flatworms have bilateral symmetry
• They are unsegmented and have a simple body plan.
• Flatworms have a one opening digestive system
• Flatworms are free-living or parasitic
• Flatworms have a layered structure
  • Ectoderm, mesoderm, and endoderm

Phylum Nematoda (Roundworms)

• Roundworms have bilateral symmetry
• They are unsegmented and have a complete digestive system with two openings.
• Roundworms have a fluid-filled pseudocoelom
• Roundworms are free-living or parasitic.

Phylum Annelida (Segmented Worms)

• Segmented worms exhibit bilateral symmetry; their bodies have numerous segments.
• Their digestive systems have two openings.
• They have a closed circulatory system and nephridia, excretory organs that filter wastes from the blood
• Annelids have a coelom body cavity

Class Polychaeta (Polychaetes)

• Polychaetes are primarily marine
• They have bristles on their segments called parapodia.
• They are mostly filter feeders

Class Oligochaeta (Oligochaetes)

• Oligochaetes include earthworms; they live in terrestrial and freshwater environments
• They have fewer bristles than polychaetes.
• Oligochaetes are largely detritivores

Class Hirudinea (Leeches)

• Leeches are mostly freshwater; some are terrestrial
• Leeches have suckers at both ends of their bodies and lack bristles
• Leeches are blood-sucking external parasites

Phylum Mollusca (Mollusks)

• Mollusks have a soft body
• Some mollusks, like the clams, have a shell
• A mollusk's body consists of a foot, mantle, shell, and visceral mass.
• Mollusks have bilateral symmetry.
• Mollusk digestive systems have two openings.

Class Bivalvia (Bivalves)

• Bivalves include clams, oysters, scallops, and mussels.
• They have two-part hinged shells and wedge-shaped feet for burrowing.
• They are mostly sessile as adults.

Class Gastropoda (Gastropods)

• Most have spiral, chambered shells
• Some gastropods lack shells completely; examples are snails and slugs.
• A distinctive head is present in gastropods
• Aquatic or terrestrial

Class Cephalopoda (Cephalopods)

• Cephalopods, such as octopuses, squids, nautiluses, and cuttlefish, have a closed circulatory system
• Their foot is divided into tentacles, and they have well-developed nervous systems.

Phylum Arthropoda (Arthropods)

• Arthropods have segmented bodies with jointed appendages
• Arthropods have exoskeletons made of chitin
• Arthropods possess an open circulatory system
• They undergo metamorphosis in some instances
• Arthropods have three germ layers, a coelom body cavity, bilateral symmetry, and a cephalized nervous system

Subphylum Trilobita (Trilobites)

• Trilobites are extinct marine arthropods; they were known from fossil records.
• Each appendage of the trilobite was divided into a gill and a walking leg

Subphylum Chelicerata (Chelicerates)

• Chelicerates have mouthparts called chelicerae (for biting or sucking) and pedipalps (for prey capture)
• Chelicerates may lack antennae but have two-part bodies

Subphylum Uniramia (Uniramians)

• Uniramians are primarily terrestrial; they have almost all terrestrial representatives
• They have one pair of antennae
• Centipedes, millipedes, and insects are all uniramians

Class Chilopoda (Centipedes)

• Centipedes have long bodies consisting of numerous segments and one pair of legs per segment
• They have poison claws for feeding
• Centipedes are carnivorous

Class Diplopoda (Millipedes)

• Millipedes possess long bodies with numerous segments and two pairs of legs per segment.
• Millipedes are mostly herbivorous

Class Insecta (Insects)

• Insects have three-part bodies (head, thorax, abdomen)
• Insects usually have three pairs of legs and one or two pairs of wings.
• Some insects undergo complete metamorphosis

Subphylum Crustacea (Crustaceans)

• Crustaceans are mostly aquatic and have two pairs of antennae
• Mouthparts are called mandibles, and their bodies are covered by a carapace

Phylum Echinodermata (Echinoderms)

• Echinoderms have radial symmetry as adults
• Echinoderms have a water vascular system that functions in locomotion and feeding; it transports fluid around an organism.
• Echinoderms have a calcareous endoskeleton

Sea Lilies and Feather Stars

• Sea lilies and feather stars are filter feeders
• Their feeding and respiration occur through feathery arms, and they have long feathery arms.

Sea Stars

• These echinoderms are star-shaped, carnivorous, and bottom dwellers
• Their mouths are on the bottom/lower part of their bodies, and their anuses are on top. Examples are crown-of-thorns, sea stars, and sun stars

Brittle Stars and Basket Stars

• Brittle stars and basket stars have small body disks and lack an anus
• Most of them are filter feeders or detritus feeders

Sea Urchins and Sand Dollars

• These echinoderms are covered in spines that comprise their shell
• They are mostly herbivorous or detritus feeders

Sea Cucumbers

• Sea cucumbers have leathery, cylindrical bodies with feeding tentacles on one end
• They are mostly detritus or filter feeders; the endoskeleton is greatly reduced

Invertebrate Systems

• Six major invertebrate systems include skeletal, digestive, circulatory, respiratory, excretory, and nervous.
• Invertebrates exhibit various skeletal systems, including exoskeletons and endoskeletons.