Animal Diversity I: Invertebrates – Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Modern medicine often relies on advanced technology, but some treatments use invertebrates such as the medicinal leech (Hirudo medicinalis).

Medicinal leeches have been used for over 2,000 years to treat various diseases.
By the 20th century, their use declined but has since returned for specific medical applications.
Leeches are used to treat venous insufficiency in reconstructive surgery, especially when veins cannot be reconnected.
They promote blood flow by making small incisions and sucking blood, which helps prevent tissue death.

Animals are a diverse group of organisms with several defining characteristics.

Eukaryotic: Animal cells contain a nucleus and membrane-bound organelles.
Multicellular: Animals are composed of multiple cells.
Lack cell walls: Unlike plants and fungi, animal cells do not have rigid cell walls.
Heterotrophic: Animals obtain energy by consuming other organisms.
Sexual reproduction is typical, though some can reproduce asexually.
Motility: Most animals are capable of movement at some stage of life.
Rapid response to stimuli: Most animals have nervous systems for quick reactions.
Evolutionary history: Most major animal phyla appeared during the Cambrian period (~541 million years ago).

Animal evolution is marked by key anatomical innovations that define major groups.

Tissues: The appearance of tissues (groups of similar cells functioning together) was a major innovation. Sponges lack true tissues; all other animals have them.
Body Symmetry:
- Radial symmetry: Body parts arranged around a central axis (e.g., cnidarians, ctenophores).
- Bilateral symmetry: Only one plane divides the body into mirror-image halves (most other animals).
Germ Layers:
- Radially symmetrical animals: Two germ layers (ectoderm and endoderm).
- Bilateral animals: Three germ layers (ectoderm, mesoderm, endoderm).
Cephalization: Concentration of sensory organs and nervous tissue at the anterior (head) end, seen in most bilaterally symmetrical animals.
Body Cavities:
- Coelomates: True body cavity (coelom) completely lined with mesoderm (e.g., annelids, arthropods, mollusks, echinoderms, chordates).
- Pseudocoelomates: Body cavity partially lined with mesoderm (e.g., roundworms).
- Acoelomates: No body cavity (e.g., flatworms).
Developmental Patterns:
- Protostomes: Mouth develops first (e.g., annelids, arthropods, mollusks, flatworms, roundworms).
- Deuterostomes: Anus develops first (e.g., echinoderms, chordates).
Protostome Subgroups:
- Ecdysozoans: Arthropods and roundworms; shed outer covering.
- Lophotrochozoans: Flatworms, annelids, mollusks; have lophophore or trochophore larva.

Animals are classified into about 32 phyla, with the vast majority being invertebrates (lacking a backbone).

Vertebrates: Animals with a backbone (phylum Chordata; includes fish, amphibians, reptiles, mammals).
Invertebrates: All other animals; make up over 97% of animal species.

Phylum	Tissues	Symmetry	Body Cavity	Development	Examples
Porifera (Sponges)	No	None	None	None	Sponges
Cnidaria	Yes	Radial	None	None	Jellyfish, corals
Platyhelminthes	Yes	Bilateral	None (acoelomate)	Protostome	Flatworms
Nematoda	Yes	Bilateral	Pseudocoelomate	Protostome (Ecdysozoan)	Roundworms
Annelida	Yes	Bilateral	Coelomate	Protostome (Lophotrochozoan)	Segmented worms
Mollusca	Yes	Bilateral	Coelomate	Protostome (Lophotrochozoan)	Snails, clams, octopuses
Arthropoda	Yes	Bilateral	Coelomate	Protostome (Ecdysozoan)	Insects, spiders, crustaceans
Echinodermata	Yes	Bilateral (larvae), Radial (adult)	Coelomate	Deuterostome	Sea stars, sea urchins
Chordata	Yes	Bilateral	Coelomate	Deuterostome	Vertebrates, tunicates

Coelom: A fluid-filled body cavity completely lined by mesoderm.
Pseudocoelom: A body cavity partially lined by mesoderm.
Acoelomate: Lacking a body cavity.
Protostome: Animal whose mouth develops from the first opening in the embryo.
Deuterostome: Animal whose anus develops from the first opening in the embryo.
Cephalization: Evolutionary trend toward the concentration of sensory and neural organs at the anterior end.
Germ layers: Layers of cells in the embryo that give rise to tissues and organs (ectoderm, mesoderm, endoderm).

Earthworm (Annelid): True coelom, segmented body, closed circulatory system.
Roundworm (Nematode): Pseudocoelom, unsegmented, no circulatory system.
Flatworm (Platyhelminth): Acoelomate, unsegmented, simple nervous system.

Understanding these features helps in classifying animals and tracing evolutionary relationships.
These anatomical innovations are foundational for the diversity of animal life on Earth.