Animal Diversity

What are animals (Metazoans)?

Multicellular eukaryotes in Kingdom Animalia, typically in the Unikonta → Opisthokonts clade.

What are key traits of animals?

Multicellularity with no cell walls, tissues, heterotrophy (ingest food), nervous and muscle cells, and gametic meiosis.

What supports animal extracellular structure?

Glycoproteins such as collagen provide structural support instead of cell walls.

What is gametic meiosis?

A life cycle where only haploid gametes are produced by meiosis and fuse to form a diploid zygote.

What are Hox genes?

A set of regulatory genes that control body plan development and segment identity during embryogenesis.

How do animals reproduce?

Primarily sexual reproduction with gametic meiosis producing sperm and eggs.

What is cleavage in animal development?

Rapid cell divisions of the zygote without growth that produce many smaller cells.

What is a blastula?

A hollow multicellular ball formed after cleavage in animal embryos.

What is gastrulation?

A developmental process where the blastula folds to form embryonic tissue layers.

Name the three primary embryonic tissue layers.

Ectoderm, mesoderm, and endoderm.

What does the ectoderm form?

The outer layer; gives rise to the outer body surface and sometimes the nervous system.

What does the endoderm form?

The inner layer; forms the lining of the digestive tract and associated organs.

What does the mesoderm form?

The middle layer in triploblastic animals; forms muscles and many internal organs.

What is a larva?

A sexually immature, morphologically distinct juvenile stage that often undergoes metamorphosis.

What is metamorphosis?

The developmental process by which a larva transforms into the adult form.

Which animals lack true tissues?

Sponges (Porifera) lack distinct tissues and are largely cellular in organization.

What is symmetry in animals?

The spatial arrangement of body parts around an axis; can be radial, bilateral, or absent (asymmetrical).

What is radial symmetry?

Body parts arranged around a central axis; common in cnidarians and ctenophores.

What is bilateral symmetry?

Body plan with distinct left and right sides and dorsoventral orientation; common in triploblastic animals.

What is asymmetry?

No symmetry, as seen in many sponges.

What are tissues?

Collections of specialized cells organized to perform common functions, formed during gastrulation.

What does diploblastic mean?

Having two embryonic tissue layers: ectoderm and endoderm (e.g., cnidarians, ctenophores).

What does triploblastic mean?

Having three tissue layers: ectoderm, mesoderm, and endoderm; characteristic of Bilateria.

What is a body cavity?

A fluid- or air-filled space between the digestive tract and body wall that cushions organs and aids support/transport.

What is a coelom?

A body cavity formed from mesoderm that fully surrounds internal organs; found in many triploblastic animals.

What are functions of a coelom?

Cushions organs, allows independent movement/growth of organs, and aids transport.

What is a hemocoel?

A body cavity derived from blastocoel or between tissues that contains hemolymph; found in many arthropods.

What is an acoelomate?

An animal with no body cavity between digestive tract and outer body wall (e.g., flatworms).

What is the role of hemolymph?

Fluid in a hemocoel that transports nutrients and wastes in many invertebrates.

Give an example of an acoelomate.

Flatworms are acoelomate and often have thin, flat bodies without a dedicated circulatory system.

What are protostomes?

A developmental mode where the blastopore becomes the mouth; includes arthropods and mollusks.

What are deuterostomes?

A developmental mode where the blastopore becomes the anus and the mouth forms second; includes vertebrates.

What is the difference between protostome and deuterostome development?

In protostomes the mouth develops from the blastopore; in deuterostomes the anus does.

Name two major protostome groups.

Lophotrochozoa and Ecdysozoa (examples include mollusks, annelids, and arthropods).

What is the current estimated species diversity of animals?

About 1.3 million described species, with many more likely undiscovered.

When did the common ancestor of animals likely live?

Estimated around ~770 million years ago.

What was the Cambrian Explosion?

A rapid period of animal diversification when many large, mineralized-bodied animals first appear in the fossil record.

What factors may have driven the Cambrian Explosion?

Rise in atmospheric oxygen, predator–prey interactions, and evolution of developmental genes like Hox.

When did animals move onto land?

Approximately 450 million years ago for major animal groups like arthropods and later vertebrates.

What major animal radiations occurred after the move to land?

Arthropod diversification and vertebrate radiations, with mammals radiating in the Cenozoic.

Who are choanoflagellates in relation to animals?

A protist group considered the closest living relatives of animals.

What evidence supports choanoflagellates as close relatives of animals?

Similar collar cells to sponge choanocytes and shared molecular/ genetic features.

What is eumetazoa?

The clade of animals with true tissues (all animals except sponges).

Which animals are in Cnidaria and Ctenophora in terms of tissue layers?

They are diploblastic, typically having ectoderm and endoderm.

What is Bilateria?

A clade of animals that are bilaterally symmetric and triploblastic.

Name the three major clades within Bilateria.

Deuterostomia, Lophotrochozoa, and Ecdysozoa.

What is the sponge (Porifera) body plan like?

Simple, porous bodies with choanocytes for feeding and no true tissues.

What key cell type do sponges have for feeding?

Choanocytes, collar cells that create water flow and capture food particles.

What is the importance of Hox genes in animal evolution?

They pattern the body axis and segment identity, enabling diverse body plans and regional specialization.

What is the difference between ectoderm-derived and endoderm-derived structures?

Ectoderm forms outer surfaces and nervous tissue; endoderm forms digestive tract lining and internal organs.

Why do many animals have efficient digestive systems?

To process ingested food internally, enabling extraction of nutrients for active lifestyles.

What is the role of nervous and muscle cells in animals?

Nerve cells detect and process stimuli; muscle cells enable movement and capture of prey.

What does "cells organized into tissues" enable animals to do?

Specialize functions, form organs, and perform more complex behaviors and physiology.

How does bilateral symmetry benefit animals?

Facilitates directed movement and concentration of sensory structures at the front (cephalization).

What are examples of ecological or evolutionary consequences of animal multicellularity?

Improved mobility, larger body sizes, tissue specialization, and complex life cycles.

What is the typical sequence from zygote to gastrula?

Zygote → cleavage → blastula → gastrulation → germ layer formation.

What is the significance of larvae in life cycles?

Larvae allow dispersal and exploitation of different ecological niches before metamorphosis to adults.

What is an example of a larval stage in animals?

Many marine invertebrates and insects have distinct larval forms that later metamorphose.

What major transitions in animal history are highlighted in the notes?

Origin of multicellularity, Cambrian diversification, and terrestrial colonization.

What are key points to remember about animal phylogeny from the notes?

All animals share a common ancestor, sponges are sister to other animals, eumetazoans have tissues, and Bilateria contains most animal diversity.

What is meant by "do tissues form from layers of embryonic cells"?

During gastrulation embryonic germ layers (ecto/meso/endo) form and give rise to tissues and organs.

What is glyconectin in the notes likely referring to?

A glycoprotein component of extracellular matrix in some early-diverging animals (related to cell adhesion).