Biology Guided Notes
Chapter 26
Sponges/Cnidarians
Intro to the Animal Kingdom
All members of the animal kingdom share certain characteristics. All animals are multicellular,
eukaryotic heterotrophs whose cells lack walls.
More than 95 % of all animal species are grouped in one informal category: invertebrates. Invertebrates
are animals that have no backbone, or vertebral column. The other 5% of animals are vertebrates.
Vertebrates are animals that have backbones.
An animal’s structure, or anatomy, allows it to carry out body functions. Many body functions help
animals maintain homeostasis. Homeostasis is the process by which organisms keep internal
conditions stable. Often this involves use of internal feedback mechanisms. For example, feedback
inhibition occurs when the product of a process stops or limits the process itself.
Animals carry out the following essential functions:
 Feeding: Animals must feed, or eat food. Herbivores eat plants, carnivores eat animals, and
parasites live and feed on other organisms.
 Respiration: all animals respire, or take in oxygen and give off carbon dioxide.
 Circulation: Most animals have a system that circulates materials within their bodies.
 Excretion: Most animals have an excretory system that helps eliminate wastes for their body.
 Response: animals respond to their environment using nerve cells.
 Movement: most animals move from place to place using muscles and some type of skeletal
system.
 Reproduction: most animals reproduce sexually by producing haploid gametes. Sexual
reproduction helps create and maintain genetic diversity in populations. Many invertebrates
can reproduce asexually as well. Asexual reproduction produces offspring that are genetically
identical to the parent.
Complex animals tend to have high levels of cell specialization and internal body organization. They
also tend to have bilateral symmetry, a front end or head, with sense organs, and a body cavity.
Early development:
Animals that reproduce sexually start life as a fertilized egg, or zygote. The zygote divides
several times to form a blastula, a hollow ball of cells. The blastula then folds in on itself to form the
digestive tract. In time, the blastopore forms either a mouth or anus. The anus is the opening through
which waste leaves the digestive tract. If the blastopore forms a mouth, the animal is a protostome. If
the blastopore forms an anus, the animal is a deuterostome.
The cells of most animal embryos differentiate into three germ layers. The endoderm, is the innermost
germ layer. The middle layer is the mesoderm. The ectoderm is the outermost germ layer.
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Body cavity formation: Most animals have a body cavity. A body cavity is a fluid filled space that
lies between the digestive tract and the body wall.
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Body symmetry and cephalization: all animals, except sponges, show some type of body
symmetry. In radial symmetry, any number of imaginary planes can be drawn through the
center of the animal to divide it into equal halves. In bilateral symmetry, only one plane can
divide the body into two equal halves. Animals with bilateral symmetry typically have
cephalization. (brain)
Cephalization: is a concentration of sense organs and nerve cells at the front of the body. More
complex animals have bilateral symmetry.
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Sponges:
Sponges make up the phylum Porifera. Like all animals, sponges are multicellular and heterotrophic.
They have no cell walls and have only a few specialized cells. Sponges live their entire adult life attached
to a surface.
Sponges sift microscopic food particles from water moving through them. Specialized cells called
choanocytes move a current of water through the sponge body. The water enters through the pores in
the body wall. It leaves through the osculum, a large hole at the top of the central cavity. Digestion is
intracellular; it takes place inside the cells. The movement of water through a sponge also supplies a
sponge with everything in needs for respiration, circulation and excretion.
Sponges reproduce both sexually and asexually. In sexual reproduction, eggs are fertilized internally.
The zygote then develops into a larva. A larva is an immature stage of an organism that looks different
from the adult form. The larva is motile. They are carried by the ocean currents until they settle to the
sea floor and grow into a mature sponge. Sponges reproduce asexually by budding.
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Cnidarians
The phylum Cnidaria includes hydras, jellyfish, sea anemones, and corals. Cnidarians are soft-bodied
carnivores that have stinging tentacles arranged in circles around their mouths. They are the simplest
animals having body symmetry and specialized tissues.
Cnidarians exhibit radial symmetry. They have a central mouth surrounded by numerous tentacles.
Cnidarians get their name from cnidocytes, or stinging cells, on their tentacles. Most cnidarians have a
life cycle with two very different stages: a polyp and a medusa.
 A polyp: has a cylindrical body with arm-like tentacles and usually does not move. A polyp lives
attached to a surface with its mouth pointing upward.
 A medusa, has a bell shaped body with a mouth at the bottom. Medusas are free swimming.
Cnidarians have a gastrovascular cavity – digestive chamber with one opening. Food and waste enter
and leave through this opening. Digestion is extracellular, meaning it takes place outside cells.
After digestion, nutrients are transported throughout the body by diffusion. Cnidarians take in oxygen
and released wastes by diffusion in their body walls.
Cnidarians gather information from their environment using specialized sensory cells. Both polyps and
medusas have a nerve net. A nerve net is a network of nerve cells that together let cnidarians detect
stimuli.
Some cnidarians have a hydrostatic skeleton. This skeleton consists of a layer of circular muscles and a
layer of longitudinal muscles that, together with the water in the gastrovascular cavity, enable the
cnidarian to move.
Cnidarians reproduce both sexually and asexually. Polyps reproduce asexually by budding. Sexual
reproduction takes place with external fertilization in water.
There are 3 classes of cnidarians. They include jellyfish, hydras and their relatives, and sea anemones
and corals.
 Jellyfish live mostly as medusas
 Hydras and related animals grow in branching colonies. The Portuguese man-of-war is a colonial
hydrazoan composed of many specialized polyps.
 Sea anemones and corals have only the polyp stage in their life cycles. Most corals are colonial.
Their polyps grow together in large numbers. As the colonies grow, they secrete an underlying
skeleton of calcium carbonate (limestone). Coral colonies from structures called coral reefs.