Animals

Basic Characteristics of Animals

• Multicellular chemoheterotrophic eukaryotes
  • They must take organic molecules into their body through ingestion (eating).
• Lack cell walls
  • They have cell membranes, as do all living organisms, but nothing external to that.
• They have muscle and nervous tissue.
• Diploid stage of life cycle is dominant.

Origin of Animals

First Appearance of an “Animal”

• About 700 million years ago
• It is believed that an ancestral protist acquired multicellularity. This would have been considered the “first animal.”
• Animals have been evolving ever since this time.
• Four major branch points have occurred during the evolution of animals.

Figure 32.3, page 591, Campbell's Biology, 5th Edition

Branch Point 1: Eumetazoans Separate from Parazoans

• Parazoans: animals without true tissues
  • Word derivation:
    • Para: beside
    • Zoa: animals
    • Don't look like animals, hence their name, “beside the animals.”
  • They are given their own special subkingdom: Parazoa.
  • One phylum: Porifera (pore bearers), the sponges
• Eumetazoans: animals with true tissues
  • Word derivation:
    • Eu: good, true
    • Meta: after
  • A subkingdom within the animal kingdom—the “good” animals—that came after the “first animals” (protozoans)
  • Includes all animals except sponges.

Branch Point 2: Bilateria Separate from Radiata

• Radiata: radially symmetrical
  • Animals that can be divided multiple times through a central axis creating multiple mirror images.
  • Have a top and bottom but no left nor right, no head nor tail.
  • Example: Phylum Cnidaria (jelly fish and sea anemones)
• Bilateria: bilaterally symmetrical
  • Animals that can only be cut in one plane to create a mirror image.
  • They have top (dorsal), bottom (ventral), head (anterior), tail (posterior), right, and left sides.
  • Cephalization: Concentration of nervous tissue in the head region begins to happen—the origin of the “brain.”

Figure 32.4, page 592, Campbell's Biology, 5th Edition

The second major distinction between Radiata and Bilateria is the number of tissue layers (a.k.a. germ layers)

• Radiata: diploblastic—have two tissue (“germ”) layers
  • Ectoderm: “Outer skin”
  • Endoderm: “Inner skin”
• Bilateria: triploblastic—have three tissue (“germ”) layers
  • Ectoderm: “Outer skin”
  • Mesoderm: “Middle skin”
  • Endoderm: “Inner skin”

Branch Point 3: Coelomates Separate from Acoelomates and Pseudocoelomates

• Question: How are all the bilaterally symmetrical animals distinguished from one another?
• Answer: By the presence or absence of a particular body cavity called the coelom.
  • It is a body cavity found within the mesoderm, that splits it.
  • Bilateria are in one of three categories depending on types of body cavities:
    • Acoelomates: “no coelom”
      • Have solid bodies with no body cavities.
        • Note: The digestive tract is not a body cavity.
      • Example: flatworms
    • Pseudocoelomates: “false coelom”
      • Have a body cavity between the mesoderm and endoderm.
      • Example: nematodes (roundworms)
    • Coelomates: “true coelom”
      • Have a body cavity within the mesoderm.
      • Examples: all other animals—annelids (segmented worms), etc.

Figure 32.5, page 593, Campbell's Biology, 5th Edition

Branch Point 4: Deuterostomes Separate from Protostomes

• The final major branch point in animal evolution
• Coelomates are divided into two categories:
  • Protostomes
  • Deuterostomes
• The division into these two categories is based on three events that occur during the early embryological development of these animals.
  • It is important to note that you would never be able to place these animals into these two categories based on the way they look as adults.
  • The embryologist is the scientist who is primarily responsible for categorizing these animals.
• Here are the three things they look for…
  • The cleavage pattern of the zygote
  • How the coelom forms
  • The “fate” of the blastopore

Cleavage

• Spiral and Determinate
  • Seen in Protostomes.
  • New cells sit in grooves of cells beneath them (see picture).
  • Each cell has a predetermined “fate”; hence the term: determinate.
• Radial and Indeterminate
  • Seen in Deuterostomes.
  • New cells sit on top of cells beneath them (see picture).
  • Each cell could potentially become a complete embryo; its “fate” is not yet determined, hence the term “indeterminate.”

Figure 32.6(a), page 594, Campbell's Biology, 5th Edition

Coelom Formation

• Schizocoelous
  • Seen in Protostomes.
  • Schizo: to split (like schizophrenia—“split personality”).
  • Coelom forms by the splitting of the mesoderm.
• Enterocoelous
  • Seen in Deuterostomes.
  • Coelom forms by the budding of the archenteron.

Figure 32.6(b), page 594, Campbell's Biology, 5th Edition

Blastopore Fate

• When the archenteron is created, an opening called the blastopore is formed.
• In protostomes (as seen in the left part of the diagram), the mouth develops from this blastopore and the anus develops secondarily somewhere else. This is why protostomes were named as such:
  • Proto: first
  • Stome: mouth
• In deuterostomes, the mouth is formed from the secondary opening and the blastopore forms the anus, hence the name, deuterostomes:
  • Deutero: second
  • Stome: mouth

Figure 32.6(c), page 594, Campbell's Biology, 5th Edition