Chapter 23 Invertebrate Diversity Section 1 – Diverse animals share several key characteristics Section 2 – Sponges are relatively simple animals with porous bodies Section 3 – Cnidarians are radial animals with stinging cells Section 4 – Flatworms are the simplest bilateral animals Section 5 – Roundworms and rotifers have complete digestive tracts Section 6 – Annelida are segmented worms Section 7 – Mollusks show diverse variations on a common body form Section 8 – Echinoderms have spiny skin and a water vascular system Section 9 – Animal diversity “exploded” during the Cambrian period

What Is an Animal? More than a million living species of animals are organized into about 35 major groups (phyla). Animals share four key characteristics: 1. Animals are eukaryotic. 2. Animal cells lack cell walls. 3. Animals are multicellular. 4. Animals are heterotrophs that ingest food. The fourth characteristic refers to how animals obtain nutrition  most animals take food into their bodies and digest it there (called ingestion). The life cycles of most animals are also different from those of other organisms, especially in the early stages of development.

Invertebrates and Vertebrates One common way to group various animals is based on whether they have a backbone. INVERTEBRATES: animals without a backbone VERTEBRATES: animals with a backbone Invertebrates make up approximately 95 percent of the different kinds of animals on Earth. Most invertebrates live in aquatic or moist terrestrial habitats. The majority of aquatic invertebrate species live in marine waters (sea stars, jellyfish, worms, etc.) Vertebrates make up the remaining 5% of animals found within the animal population. They live in terrestrial environments as well as marine and freshwater habitats.

Chapter 23 Invertebrate Diversity Section 1 – Diverse animals share several key characteristics Section 2 – Sponges are relatively simple animals with porous bodies Section 3 – Cnidarians are radial animals with stinging cells Section 4 – Flatworms are the simplest bilateral animals Section 5 – Roundworms and rotifers have complete digestive tracts Section 6 – Annelida are segmented worms Section 7 – Mollusks show diverse variations on a common body form Section 8 – Echinoderms have spiny skin and a water vascular system Section 9 – Animal diversity “exploded” during the Cambrian period

The Body of a Sponge Sponges (phylum Porifera) are the simplest animals. Definition: member of a group of animals that lack true tissues and organs Unlike most animals, sponges lack true tissues and organs. Most of the different types of cells in a sponge are relatively unspecialized. The body of most sponges consists of two layers of cells separated by a jelly-like material. The outer layer of cells protects the interior of the sponge and also has many pores through which water can enter the sponge. The inner layer of cells (collar cells) lines the central cavity of the sponge. Definition: flagellated cell in a sponge's inner layer

In the jelly-like material are cells (amoebocytes) that pick up food from the collar cells, digest it, and carry the nutrients to other cells. Definition: cell found in sponges and other animals that may digest and distribute food, dispose of wastes, and change into other cell types STRUCTURE: In addition to picking up food, amoebocytes also transport oxygen, dispose of wastes, and can change into other cell types, such as support structures. In some sponges, this support can be rigid. In other sponges, the structures are composed of a flexible protein called spongin. DIET: Water moves into the sponge through the pores and into the central cavity. While the water flows, it is filtered for food particles and amoebocytes engulf those particles and transport them to other cells. The water exits through the large opening at one end of the sponge.

FIGURE 23-4

MOVEMENT: Adult sponges are sessile. REPRODUCTION: Sponges live singly or in clusters formed by budding  budding is a form of asexual reproduction in which new sponges develop from an outgrowth of a parent organism. Small fragments of a sponge body can grow into an entire new sponge. Sponges can also reproduce sexually – the union of egg and sperm cells form zygotes that develop into flagellated larvae. MOVEMENT: Adult sponges are sessile. Definition: anchored in place Sponges have chemical defenses that protect them from possible predators, disease organisms, and parasites. These defenses include toxins that keep predators from eating the sponges and powerful antibiotics that fight bacterial infections.

Diversity of Sponges The 9,000 known species of sponges are diverse in shape, size, and color. Some sponges consist of a single cylinder, while other sponges branch out irregularly over the seafloor or lake bottoms. Some sponges are quite small, while others can reach heights of 2 meters.

Chapter 23 Invertebrate Diversity Section 1 – Diverse animals share several key characteristics Section 2 – Sponges are relatively simple animals with porous bodies Section 3 – Cnidarians are radial animals with stinging cells Section 4 – Flatworms are the simplest bilateral animals Section 5 – Roundworms and rotifers have complete digestive tracts Section 6 – Annelida are segmented worms Section 7 – Mollusks show diverse variations on a common body form Section 8 – Echinoderms have spiny skin and a water vascular system Section 9 – Animal diversity “exploded” during the Cambrian period

The Body of a Cnidarian Two characteristics that all cnidarians share are radial symmetry and tentacles with stinging cells. Definition: member of a group of invertebrates with radial symmetry and tentacles with stinging cells EXAMPLE: the hydra (commonly found in ponds and lakes) has a cylindrical body (about 3 cm long) with projections called tentacles at one end. Other cnidarians include jellyfish, sea anemones, and coral animals. STRUCTURE: All cnidarians have radial symmetry, and all animals with this symmetry lack a head. Definition: body plan in which an organism can be divided into equal parts around a central axis Fluid in the body cavity provides body support and helps give cnidarians their shape.

MOVEMENT: Most cnidarians are slow-moving or sessile and can be found on shallow ocean floors or on the bottom of ponds. All cnidarians have specialized stinging cells used for defense and capturing prey called cnidocytes  they are abundant along the tentacles and a stinging capsule is located within each cnidocyte. Definition: specialized cell in cnidarians that functions in defense and capturing prey The tentacles of a hydra are armed with numerous cnidocytes and each cnidocyte holds a stinging capsule called a nematocyst  when triggered by touch, the fine, coiled tubule within the nematocyst shoots out toward the prey. Definition: stinging capsule found in a cnidocyte Each nematocyst can fire only once.

DIET: Once a cnidarian captures its prey, its tentacles maneuver the food into its mouth, located at the center of the ring of tentacles. The mouth leads to the gastrovascular cavity. Definition: digestive sac The food that enters the digestive sac is digested at that location and undigested material is released back out through the mouth. Circulating fluids inside the cavity transport digested food to the cells lining the cavity. DIFFERENCES: Cnidarians have features that are absent in sponges but present in nearly all other animals. Cnidarians have the presence of a gastrula stage during embryonic development. Cnidarians have an outer covering (epidermis) that has protective and sensing functions. The nerves enable the hydra to respond to stimuli and coordinate its movements.

Diversity of Cnidarians There are about 9,000 known species of cnidarians. These organisms have two kinds of body forms – either the polyp or the medusa. Definition: cnidarian body form consisting of a cylindrical body with tentacles radiating from one end Definition: cnidarian body form that is umbrella-shaped with fringes of tentacles POLYP: mostly sessile organisms (hydra) MEDUSA: move freely about in the water (jellyfish)