Lecture 06 Introduction Animals + Protosomes

What characteristics generally define an animal?

Animals are multicellular, heterotrophic, able to move, and have neurons and muscle cells, except sponges.

Key Innovation

A new trait that opens new possibilities for how an organism lives and can increase the rate of diversification.

Why can a key innovation increase species diversity?

It can open new ecological niches and ways of living, allowing rapid evolution and diversification.

What are major key innovations in animals?

Multicellularity, tissue layers, bilateral symmetry, cephalization, nervous systems, coeloms, and segmentation.

Colonial Flagellate Hypothesis

The hypothesis that multicellular animals evolved from unicellular flagellated protists that formed colonies and eventually developed specialized cells.

What evidence supports sponges being the first animals?

Sponges share ancestral traits with choanoflagellates, tolerate very low O2 similar to Precambrian conditions, and are supported by molecular phylogenies.

Germ Layers

Layers of tissue formed during embryonic development.

What do the three germ layers develop into?

Ectoderm → skin and nervous system; Mesoderm → circulatory system, muscles, and bones; Endoderm → digestive tract and liver.

Diploblast vs. Triploblast

Diploblasts have two germ layers connected by mesoglea, while triploblasts have three germ layers.

What are the three major types of animal symmetry?

Asymmetry = no planes of symmetry; Radial symmetry = more than 2 planes through a single point; Bilateral symmetry = only 1 plane of symmetry.

Why is bilateral symmetry an important animal innovation?

It enables important body structures and organization, including heads, mouths, and limbs.

Cephalization

The concentration of the mouth and sensory organs at the head.

How are bilateral symmetry and cephalization related?

Bilateral organisms are elongated along an anterior-posterior axis, allowing sensory organs and the mouth to become concentrated at the head.

How does the nervous system differ among major animal groups?

Sponges have no nerve cells; cnidarians have a nerve net with no brain; ctenophores have a nerve net and may have ganglia; bilaterians have a CNS with neurons concentrated in the head and nerve cords throughout the body.

Coelom

A body cavity that houses organs.

Coelomate vs. Acoelomate vs. Pseudocoelomate

Coelomates have a coelom completely lined by mesoderm; acoelomates lack a coelom; pseudocoelomates have a cavity only partially lined by mesoderm.

Why is a coelom advantageous?

It allows better movement and the development of diverse organ systems.

Segmentation

The division of the body plan into distinct parts; it enables specialization.

What three major challenges did protostomes face when transitioning from water to land?

Exchanging gases, avoiding drying out, and supporting the body out of water.

How did protostomes adapt to life on land?

They improved surface area for gas exchange, moved respiratory organs inside the body, evolved waxy outer layers or lived in cool moist habitats, reduced body size, and developed structures that resist gravity.