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Chapter 44

BIOL412 Chapter Notes - Chapter 44: Endodermis, Agnatha, Swim Bladder

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BIOL412 Chapter 44- Animal Diversity
44.1 A Tree of Life for More Than a Million Animal Species
The organisms most closely related to animals are choanoflagellates (type of protist).
Choanoflagellates are unicellular, whereas animals are multicellular. does not mean that
multicellular = animal, as plants and fungi are also multicellular.
What makes animals different from plants is that animals are heterotrophs. This means they cannot
create energy themselves, and instead rely on consuming plants or other animals as a source of carbon
and energy.
What makes animals different from fungi (which are
also heterotrophs) is that animals have a specific
pattern of early embryological development. This
pattern includes gastrulation. That’s a character not
found in fungi or other multicellular groups.
The fundamental mechanism by which biological
diversity increases is speciation (= the divergence of
two populations from a common ancestor).
Speciation creates a tree-like pattern of relatedness,
also called a phylogenetic tree.
The branches of a phylogenetic tree do not say much
about time, but rather the complexity of that
organism. For example, sponges are much older
than cnidarians and bilaterians even though the tree
places them at the same ‘‘length’’.
Structural complexity is one characteristic that varies among animals. Radial symmetry means that the
animal has a body axis that runs from mouth to base and many planes of symmetry around the axis.
Bilateral symmetry (which is more common) means that a single plane of symmetry runs from mouth
to tail. Bilaterians have a distinct front, back, top, and bottom (also called anterior, posterior, dorsal
and ventral, respectively) as well as right and left.
Bilaterians can be grouped as follows
1. Those without a body cavity (acoelomates)
2. With a body cavity (pseudocoelomates)
3. With a body cavity (coelomates) These differ in the embryonic origin of the
cells lining the cavity
Some animals that look very different as adults have similar patterns of embryological development.
Cnidaria (includes jellyfish and sea anemones) have two germ layers (=diploblastic) in their embryos.
These are called the endoderm (inside layer) and ectoderm (outside layer).
Bilaterians have three layers (=triploblastic) with the third layer called the mesoderm in between the
ectoderm and endoderm. The mesoderm develops into muscles and connective tissues. Bilaterians can
be divided into two groups:
1. Protostomes the earliest forming opening to the internal cavity of the embryo (=blastopore)
becomes the mouth.
2. Deuterostomes the blastopore becomes the anus
Note: Cnidarians and Bilaterians are often grouped together as the eumetazoa.
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44.2 The Simplest Animals: Sponges, Cnidarians, Ctenophores, and Placozoans
Unicellular ancestors were able to metabolize and reproduce.
Sponges retain this, but there is also coordination among cells allows for cell specialization,
meaning cells can have different functions. Multicellularity allows sponges to grow above the
seafloor, which gives them access to food suspended in the water.
Choanocytes is the lining of cells which have flagella and function in nutrition and gas exchange.
Mesohyl is the gelatinous mass which lies between the interior and exterior cell layers. The function of
mesohyl is to aid in skeleton formation and the dispersal of nutrients. Sponges feed though
endocytosis. The exchange of gases occurs through diffusion.
Sponges reproduce by releasing sperm cells into the water, which fuses with an egg cell released by
another sponge.
Spicules are skeletons of simple structures. SiO2 or CaCO3 is often found in these skeletons. Other
sponges make their skeletons with protein. The structures may house microorganisms, allowing for a
symbiotic relationship.
Jellyfish and anemones are both cnidarians. An anemone is also called a polyp and a jellyfish is called
a medusa. The mouth of both species opens into a closed internal gastric cavity. This is where
extracellular digestion and excretion takes place. Rather than having each cell feed individually,
digestive enzymes in the gastric cavity allow for even whole fish to be digested. This allows
cnidarians to consume food unavailable to sponges.
The cnidarian body develops from a diploblastic embryo. The endoderm develops into what is called
the endodermis and the ectoderm develops into the epidermis. They enclose the ‘jelly’ of the jellyfish,
which is called the mesoglea.
The cells that form the epidermis and endodermis occur as closely packed layers of cells forming an
epithelium. Epithelial layers line compartments within animals. Sponges do not form epithelia.
Cnidarians also have a wider variety of cell types than sponges more sophisticated tissue function.
Examples include muscle cells and nerve cells; which sponges do not have. They also have specialized
cells on the tentacles that contain a harpoon-like organelle called a nematocyst, which is tipped with a
neurotoxin. This allows cnidarians to capture their prey and provide defense. Waste leaves by way of
the mouth, and again gas exchange occurs by diffusion.
Many cnidarians reproduce asexually, often through budding. In some colonies, different individuals
develop distinct morphologies
Ctenophores & Placozoans
Ctenophores (comb-jellies) resemble cnidarians in body plan, and largely function the same. However,
they move using their cilia on the surface of the epidermal cells. More importantly, they have both a
mouth and anus. Ctenophores are sometimes seen as the sister group to all animals.
Placozoans are very simple, tiny animals. They contain only a few thousand cells, have no specialized
tissues, and have few differentiated cell types. Also move using cilia on the surface of their cells. Feed
mostly by surrounding food particles and secreting enzymes to break them down. They do contain a
genome that contains many of the genes for transcription factors and signaling molecules that are
present in cnidarians and bilaterians animals. The role of this remains unclear.
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