Chapter 26Animal Evolution: The Invertebrates

Fig. 26.2, p. 423

I. What is an Animal?

1. Multicellular eukaryote without cell wall

2. Aerobic Heterotrophs

3. Require oxygen (electron acceptor in

respiration)

4. Sexual reproduction (many can also go through asexual reproduction)

5. Motile during some stage of life

II. Trends in Animal Evolution

1. Symmetry: Most animals display a. Radial Symmetry (organized around a central point) or b. Bilateral Symmetry (mirror image, right and left). Those with bilateral

symmetry will have dorsal, ventral, anterior, and posterior surfaces. Bilateral allowed for specialized head (paired limbs, lungs, etc)

Fig. 26.3, p. 424

dorsal

posterior

ventral

anterior

II. Trends in Animal Evolution

2. Gut: Where food is digested and then absorbed by the body. There are two forms:

a. Saclike/Incomplete: one opening

b. Complete: two openings (mouth and anus)

II. Trends in Animal Evolution

3. Body Cavity: NOT the gut; think of your stomach, it resides within a larger body cavity. The gut exists inside the coelom, which contains a peritoneum, a membrane lining organs and holding them in place.

a. No body cavity b. Coelem: Cavity between body wall and organs with a peritoneum. c. Pseudocoel: A body cavity with no peritoneum.

Fig. 26.4a, p. 425

epidermisgut cavity

no body cavity; region between gut and body wall packed with organs

Fig. 26.4b, p. 425

epidermis

gut cavity

unlined body cavity (pseudocoel) around gut

Fig. 26.4c, p. 425

epidermis

gut cavity

peritoneum

lined body cavity (coelom); lining also holds internal organs in place

II. Trends in Animal Evolution

4. Segmentation: Compartments or specialized tissue.

5. Cephalization: Formation of head (concentration of neurons)

A. Porifera (The Sponges)

• The simplest animals known. • No symmetry, tissue, specialized gut, body cavity or

body segments. • Largely consist of protein and spicules of glass-like

silica. • They are able to pull water into a central cavity by

motion of flagella and captured food is absorbed and digested by amoeba-like cells within the sponge, but there is no distinct digestive organ.

• They are able to reproduce sexually and, as larvae, they are motile (mobile) animals.

B. Cnidarians

• Tentacled aquatic animals with radial symmetry.• Examples are anemones, sea jellies and corals. • Tissues are evident (tissue is made up of similar cells which

perform a similar function). • Three notable aspects of cnidarian anatomy: a) nematocysts, complicated triggers with nerves that shoot out

a barbed stinger in reaction to a stimuli; b) saclike gut, with one opening; and, c) epithelium, tissue made of cells with one side facing an exposed environment (gut lining, skin, etc). • No true body cavity. • Maintain their shape by moving water within a hydrostatic

skeleton.

Polyp vs Medusa Stage

C. “Minor” Animals

1. Aceolomates: example, Flatworm (phylum Platyhelminthes)

2. Roundworms: pseuodceol

Fig. 26.4a, p. 425

epidermisgut cavity

no body cavity; region between gut and body wall packed with organs

Fig. 26.4b, p. 425

epidermis

gut cavity

unlined body cavity (pseudocoel) around gut

D. Annelids

• Examples) Worms and leeches.

• Segmentation, a complete gut, true coelom, and a developed organ system.

• Worms often have a series of aortic arches (hearts) that move blood through a closed circulatory system (contain vessels allowing the flow of blood to specific locations within the body).

Fig. 26.29a, p. 441

circular muscles

longitudinal muscles dorsal blood vesel coelom cuticle

nephridiumnerve cordventral

blood vessel

seta (retracted)

E. Mollusks

• This very diverse group includes snails, clams, squid, cuttlefish and octopuses.

• The name (think mollify) refers to their soft body.

• The mollusks are the

first animals with a

true coelom.

E. Mollusks

• This body includes most organs commonly associated with animals: heart, stomach, mouth (often a series of teeth for grinding or scraping, known as a radula), anus, and gills (thin tissue for gas exchange

• All mollusks have a visercal mass (consisting of organs), a mantle (tissue around the visceral mass consisting of gills and the cells capable of depositing the calcium carbonate shell) and often a ‘foot’ used for movement.

Classes within Mollusks

A. Gastropods: Slugs, Snails, ‘Seashells’

B. Chitons:

C. Bivalves: Clams

D. Cephalopods: Squid, octopus, cuttlefish

Fig. 26.25, p. 439

mouthretractor muscle

left mantle

retractor muscle

foot

palpsleft gill shell

water flows out through exhalant siphon

water flows in through inhalant siphon

F. Arthrophods

1) Hardened exoskeleton,

2) Jointed appendages (literally means ‘jointed legs’)

3) Fused/modified segments,

4) Unique respiratory structures (gills I n some, tracheas located on side of body in others),

5) Developed nervous/sensory systems.

F. Arthropods, 1. Crustaceans

Crustaceans (example, Lobster)–

• A very hard exoskeleton,

• Jointed legs (walks on five pairs),

Fig. 26.33a, p. 444

antennae (two pairs)

one of two eyes

fused segments of cephalothorax

segments of abdomen

tail fin

swimmerets

five walking legs (five airs total)

first leg

food-handling appendages (three pairs)

F. Arthropods, 2. Chelicerates

• Ex) spiders, chiggers and ticks, horseshoe crabs, and scorpions.

• As a group, they are diverse, making it difficult to classify them by a few shared characteristics.

Fig. 26.39, p. 448

F. Arthropods, 3. Insects

The insects have three body parts (head, thorax and abdomen) and extensive cephalization. They usually walk with three legs and they often display wings.

In their lifecycle, they will go through complete (egg, larvae, pupa, adult) or incomplete (egg, nymph, adult) metamorphosis.

Insects usually breath through openings in the body (trachea) that allow oxygen to diffuse directly into body tissue, they excrete nitrogenous wastes (from protein synthesis) in the form of uric acid crystals (saving water) and they have a series of Malpighian tubules that allow them to channel wastes through the gut.

Protostomes vs

Deuterostomes

G. Echinoderms

• Include sea stars, sea urchins, and sea apples/cucumbers.

• Often display radial symmetry, but always begin bilateral,

• Full gut, coelom, nerves, a water-vascular system that helps them move, and bilateral symmetry of larvae.

• Internally, most are rather complex.

Fig. 26.44, p. 451

upper stomach

anus

lower stomach

spine ossicle

ossicle (stiffening, support structure)gonad

coelom

digestive gland

eyespot

spine

part of the water-vascular system

sieve plate

ring canal

ampulla