Chapter 26

What is cephalization?

The concentration of sense organs and nerve cells at the front (head) of an organism.

Asymmetry body type

no symmetry (e.g., sponges)

Radial symmetry body type

body arranged around a central point (e.g., jellyfish)

Bilateral symmetry body type

mirror-image left and right sides (e.g., humans, worms)

What is a hydrostatic skeleton?

A flexible skeleton supported by fluid pressure inside the body (common in worms and cnidarians).

What are evolutionary trends in animals from simple to complex?

• Development of symmetry

• Cephalization

• Body cavity formation

• Segmentation

• More complex organ systems

Body cavity

a fluid-filled space that houses internal organs and provides cushioning and protection

What is a protostome?

An animal in which the mouth forms from the blastopore during development (e.g., insects, worms).

What is a deuterostome?

An animal in which the anus forms from the blastopore (e.g., vertebrates like humans).

Blastopore

the first opening that appears in an early embryo during gastrulation, a process where the embryo rearranges its cells into multiple layers

What is the function of a choanocyte in a sponge?

It is a flagellated cell that creates water flow and traps food particles.

What are key structures of a sponge?

Pores, central cavity, choanocytes, osculum (opening), and a simple body plan with no symmetry.

What is a hydra and how does it feed?

A small cnidarian that uses tentacles and stinging cells (cnidocytes) to capture prey and bring it into its mouth.

What is the medusa form in cnidarians?

The free-swimming, bell-shaped form (e.g., jellyfish) that usually reproduces sexually.