Invertebrates

General Characteristics of Animals

Animals are classified in the Kingdom Animalia and share several fundamental characteristics. Understanding these traits is essential for distinguishing animals from other life forms.

• Multicellular: Composed of multiple cells with specialized functions.

• Heterotrophic: Obtain energy by consuming other organisms.

• Reproduction: Most reproduce sexually; some can reproduce asexually.

• Cell Structure: Animal cells lack a cell wall.

• Motility: Most animals are capable of movement.

• Response to Stimuli: Animals can sense and respond to environmental changes.

Phylogenetic Tree of Major Animal Phyla

The evolutionary relationships among animal phyla are depicted in a phylogenetic tree, which highlights the development of tissues, symmetry, and embryonic layers.

Symmetry in Animals

Animals with tissues exhibit either radial or bilateral symmetry, which influences their body plan and behavior.

• Radial Symmetry: Body parts arranged around a central axis; common in cnidarians.

• Bilateral Symmetry: Body has distinct left and right sides, with a head (anterior) and tail (posterior); allows cephalization (development of a head region).

Embryonic Germ Layers

Animal embryos develop distinct tissue layers, which contribute to the formation of organs and systems.

• Radially Symmetric Animals: Two germ layers: ectoderm (outer) and endoderm (inner).

• Bilaterally Symmetric Animals: Three germ layers: ectoderm, endoderm, and mesoderm (middle layer; forms muscles, circulatory, and skeletal systems).

Protostomes vs. Deuterostomes

Bilateral animals are further classified based on embryonic development:

• Protostomes: Mouth forms before anus during development.

• Deuterostomes: Anus forms before mouth during development.

Body Cavities in Bilateral Animals

Bilateral animals differ in the presence and type of body cavities, which affect their internal organization and complexity.

Major Invertebrate Phyla

Invertebrates are animals without a backbone. There are 24 phyla, but eight are most commonly studied:

1. Phylum Porifera (sponges)

2. Phylum Cnidaria (hydra, corals, sea anemones, jellyfish)

3. Phylum Platyhelminthes (flatworms)

4. Phylum Nematoda (roundworms)

5. Phylum Mollusca (snails, clams, squid)

6. Phylum Annelida (segmented worms)

7. Phylum Arthropoda (insects, spiders, crustaceans)

8. Phylum Echinodermata (sea stars, sea urchins)

Phylum Porifera: Sponges

Characteristics and Structure

Sponges are simple aquatic animals lacking true tissues and organs. They exhibit no body symmetry and are primarily marine.

• Attachment: Adults are sessile, attached to surfaces.

• Feeding: Filter water through pores; collar cells move water and filter particles.

• Reproduction: Both asexual (budding) and sexual (hermaphroditic).

• Support: Spicules (protein, siliceous, or calcareous fibers) help maintain structure.

Phylum Cnidaria: Sea Anemones, Corals, Hydras, Jellyfish

Body Structure and Function

Cnidarians are radially symmetrical and possess two tissue layers (ectoderm and endoderm) separated by mesoglea. They capture food with tentacles and exhibit both medusa and polyp forms.

• Symmetry: Radial

• Feeding: Tentacles bring food to the mouth; mouth also expels waste.

• Support: Corals secrete calcium carbonate skeletons.

• Special Cells: Cnidocytes (stinging cells) kill prey.

Coral Bleaching

Coral bleaching is a major environmental issue caused by the loss of symbiotic algae due to stress (temperature, pH, pollution). Without algae, corals lose color and may die if stress persists.

Phylum Platyhelminthes: Flatworms

Characteristics and Body Systems

Flatworms include free-living and parasitic species. They are bilaterally symmetrical and can reproduce both sexually and asexually.

• Body Systems: Digestive, excretory, and nervous systems present; lack circulatory and respiratory systems.

• Hermaphroditic: Most species possess both male and female reproductive organs.

Tapeworm Lifecycle

Parasitic tapeworms have complex lifecycles involving multiple hosts, including humans and pigs.

Phylum Nematoda: Roundworms

Characteristics and Parasitism

Roundworms are mostly harmless soil dwellers, but some are important parasites. Their bodies are covered by a cuticle that must be molted.

• Systems: Lack circulatory and respiratory systems.

• Parasitic Species: Pinworm, hookworm, trichinosis, dog heart worm.

Phylum Mollusca

General Features

Mollusks include snails, slugs, bivalves, and cephalopods. Most are aquatic, with some terrestrial species. They reproduce sexually and may be hermaphroditic or have separate sexes.

• Circulatory System: Open in most, closed in cephalopods.

• Mantle: Extension of body wall; forms gill chamber and secretes shell.

Gastropods: Snails and Slugs

• Shell: Snails have external shells; slugs lack shells but may be toxic.

• Feeding: Use radula to scrape food.

• Locomotion: Muscular foot.

• Respiration: Gills, skin, or lungs.

Bivalves: Clams, Oysters, Mussels, Scallops

• Shell: Two-sided shell connected by hinge.

• Feeding: Filter feeders using gills.

• Locomotion: Muscular foot or swimming.

• Pearl Formation: Oysters secrete nacre around irritants.

Cephalopods: Octopuses, Squid, Nautiluses, Cuttlefish

• Habitat: Marine

• Feeding: Carnivorous predators with tentacles and suction disks.

• Defense: Ink secretion and chromatophores for color change.

• Intelligence: Highly developed brains and eyes.

Phylum Annelida: Segmented Worms

Body Structure and Systems

Annelids include earthworms, polychaete worms, and leeches. Their bodies are divided into segments and they possess a closed circulatory system.

• Segmentation: Repeated body segments.

• Circulatory System: Blood travels within vessels.

• Digestive System: Mouth, pharynx, esophagus, crop, gizzard, intestine, anus.

• Excretory System: Nephridia filter wastes.

• Nervous System: Anterior brain and ventral nerve cords.

Phylum Echinodermata: Sea Stars, Sea Cucumbers, Sea Urchins

Characteristics and Water Vascular System

Echinoderms are radially symmetrical marine animals covered with calcium carbonate spines. They lack a circulatory system and can regenerate body parts.

• Feeding: Sea stars are carnivorous; others feed on algae.

• Respiration: Breathe through tube feet.

• Movement: Water vascular system enables locomotion via tube feet.

Comparison of Major Animal Phyla

Body Plan and Internal Systems

The following tables summarize the key features of major animal phyla, including level of organization, symmetry, body cavity, segmentation, and internal systems.

Additional info: Tables above are reconstructed from the original images and text, summarizing the main features of each phylum for exam preparation.