The Animals: Classification and Phylogeny

Animal Classification and Phylogeny

Evolutionary Origins

• Common Ancestor: Colonial, flagellated protist (approx. $700$ million years ago).

• Early Development: Indented for feeding, forming specialized feeding cells and later nerve cells.

Nervous System Development

• Nerves developed early, coordinating movement and sensing the environment.

• Absence of Nerves: Phylum Porifera (sponges).

Body Symmetry

• Radial Symmetry: Body arranged around a central axis (e.g., squid). Only Phylum Cnidaria.

• Bilateral Symmetry: Body divided into right and left mirror images (e.g., lobster). Further divided into protostomes and deuterostomes.

Gastrulation and Early Development

• Gastrulation: Embryo transforms from blastula to gastrula.

• Triploblastic Animals: Develop $3$ tissue layers (reptiles, birds, mammals).

• Deuterostomes: Anus develops from the gastrula opening.

• Protostomes: Mouth develops from the gastrula indented space.

General Animal Characteristics

• Multicellular, heterotrophs, aerobic respiration, specialized cells, diploid.

• Develop via blastula formation.

Cell Specialization and Germ Layers

• Most animals (excluding sponges and Cnidaria) have $3$ germ layers: Ecto (outer), Meso (middle), Endo (inner).

Coelom

• Definition: Fluid-filled body cavity for organ development.

• Acoelomate: Flat, softer bodies (e.g., flatworms, jellyfish).

• Coelomate: Rounder, firmer bodies (e.g., insects, humans).

The Simplest Invertebrate Organisms

Phylum Porifera (Sponges)

• Two cell layers, no nerves, choanocytes, hermaphroditic, sessile adults.

Phylum Cnidaria (Corals, Jellyfish, Sea Anemones)

• Two cell layers, radial symmetry, specialized tissues, tentacles with nematocysts.

Protostome Invertebrate Phyla

• Phylum Arthropoda: Segmented bodies, jointed appendages, exoskeleton (chitin), complex sensory systems. Only invertebrates capable of flight (insects).

• Phylum Nematoda (Roundworms): Unsegmented cylindrical bodies, complete digestive tracts, many are parasites.

• Phylum Annelida (Segmented Worms): Segmented bodies and organs, complete digestive systems, bristles for movement.

• Phylum Mollusca (Mollusks): Three main unsegmented body parts (foot, visceral mass, mantle), specialized radula, complete digestive and circulatory systems.

• Phylum Rotifera (Rotifers): Small aquatic animals, use cilia for feeding, no respiratory or circulatory system, important consumers.

• Phylum Platyhelminthes (Flatworms): Flattened, unsegmented, most have a single-opening digestive cavity, no coelom, many are parasites.

Deuterostome Phyla

Phylum Echinodermata (Sea Stars, Urchins)

• Anus forms before mouth (like vertebrates).

• Immature stages are bilaterally symmetrical; adults are radially symmetrical with $5$ or more arms.

• Move using a water-filled vascular system.

Phylum Chordata (Animals with a dorsal backbone or a notochord)

• Includes fish, amphibians, reptiles, birds, mammals.

• All have a backbone or notochord (flexible rod).

• Vertebrate Success: Internal skeletons, nervous system protection (spinal column/skull), terrestrial adaptations (limbs, protective skin, enhanced lungs, amniotic eggs).

Classes of Chordata

1. Agnatha (Jawless Fishes): Cartilaginous skeleton, no jaws, notochord in adults, no paired appendages.

2. Chondrichthyes (Cartilaginous Fishes): Cartilaginous skeleton, jaws, vertebrae, paired appendages.

3. Actinopterygii (Bony Fishes): Bony skeletons, most have a swim bladder for buoyancy.

4. Amphibia (Amphibians): Aquatic larval stage with gills, adults are tetrapods, breathe through lungs and/or skin.

5. Reptilia (Reptiles): Terrestrial tetrapods, dry scaly skin, lungs, internal fertilization, soft-shelled amniotic eggs.

   • Aves (Birds): Forelimbs as wings, feathers, endothermic, large brains, hard-shelled amniotic eggs.

6. Mammalia (Mammals): Hair, nurse young with milk (mammary glands), endothermic, large brains, most give birth to live young.