Ch. 26-28 Aquatic Invertebrates

Learning objective:
• For each of the phylums and classes we look at today,
we will identify:
• Method of fertilization and reproduction
• Type of body cavity
• Skeletal structure
• Body symmetry
• Body coverings
• Locomotion
• Special features

I. Aquatic invertebrates
A. Phylum PORIFERA
• Porifera – literally means
“pore-bearers” – sponges have
tiny pores, or openings, all over
their body.
• Ex. Sponges
• All aquatic (water-dwelling) –
most are marine (salt-water)

REPRODUCTION:
• Asexual – budding or gemmules (can survive harsh
conditions)
• Sexual – sperm fertilizes egg INSIDE body wall of sponge =
INTERNAL FERTILIZATION.
Giant barrel sponge spawning

STRUCTURE:
• Water enters through pores, and
leaves through large hole in the
top, the OSCULUM.
• Cells called choanocytes line the
inside of the sponge. They use
their flagella to sweep water in
through the pores.
• Sponges are FILTER FEEDERS –
they filter out materials from the
water as it passes through.

STRUCTURE:
• Sponges have spike-shaped
structures called spicules.
Purpose: to deter predators
from eating sponge.
• Softer sponges have a
“skeleton” of spongin –
network of protein fibers.
• These are the sponges used
for natural bath sponges.

Porifera –the sponges
• Sponges are
asymmetrical – no
symmetry
• Sessile (non-moving)
adults; free-swimming
larva
• Simplest animals – no
tissues
• Probably the most ancient
animals – may have been
the first “animals” to
appear on Earth.

Ping-pong tree sponge

B. Phylum Cnidaria
• Examples: jellyfish, sea anemones, corals, hydra

Name origin:
• Cnidarians get their names from cnidocytes, or stinging
cells, that are located along their tentacles.
• Within each cnidocyte is a nematocyst – a stinging structure
that contains a tightly coiled dart.
• The nematocyst is released when something touches it, and
will shoot the barbed dart into the prey.
• The barb contains a poison that will kill or paralyze small
prey.

Characteristics of the phylum cnidaria:
• All aquatic, mostly marine.
• Radial Symmetry
• Evolutionarily, cnidarians represent the first animals to
develop tissues, and an opening for eating and excreting
waste.
• Single body opening through which food enters and waste
leaves.

Reproduction in Cnidarians:
• Sexual Reproduction
• Corals “spawn” by releasing sperm and egg into the
water where EXTERNAL FERTILIZATION occurs.
• Asexual Reproduction
• Some cnidarians can also reproduce asexually through
budding.

Cnidarian Life cycle:
• Jellyfish are motile (medusa) as adults, but sessile
(polyps) as juveniles.
• Some hydra – sessile (polyps) as adults, motile
(medusa) as juveniles.

C. Phylum Mollusca
• Name origin: Latin word “molluscus”, meaning soft
• Mollusks are soft-bodied animals that usually have an internal or
external shell.
• They are coelomates–they have a true body cavity (coelom)
• Bilateral symmetry
• A few mollusks are hermaphrodites – they have both male and
female gonads. Usually, they do not fertilize themselves.
• Gastropods, cephalopods, and bivalves all belong to the phylum
mollusca.

• Some mollusks have external shells made of calcium
carbonate. These shells are secreted by a portion of
their body, and are attached.

1. Class Cephalopoda
• Name means: “head-footed”
• Examples: cuttlefish, squid,
octopuses, nautilus
• Reproduction: Sexual, with
internal fertilization
• Protective structure: small
internal shell or no shell at
all. (except nautilus –
external shell)

Cephalopod characteristics:
• Special body structures: tentacles with sucking
discs for catching prey.
• Locomotion: VERY motile – they use jet propulsion
to move around
• Well-developed nervous system and sensory organs
(eyes). Very intelligent.
• Can excrete ink to get away from predators.

2. Class Gastropoda
• Name means: “stomachfooted”
• Ex. Snails, slugs
• Many are NOT aquatic,
and dwell on land
• Reproduction: Sexually,
external fertilization
• Some snails may have
internal fertilization

Gastropods:
• Have one or no shell
• Motile; some secrete
slime, and then glide
upon their slime trail
• Feed by scraping food off
of surfaces with a rough
tongue called a radula

3. Class Bivalvia
• Name means: “2 valves/shells”
• Ex: clams, oysters, scallops,
mussels
• Reproduction: sexual – external
fertilization
• They have 2 shells that are held
together by one or two powerful
muscles

Bivalves:
• Most bivalves stay in one place
for most of the time.
• Scallops can move very quickly
to escape by clapping their
shells together.
• Bivalves are filter feeders -
they filter out particles in the
water, and can take in
pollutants and viruses. You
can get sick from eating raw
oyters.

D. Phylum Echinodermata
• Name means: “spiny skin”
• Ex. Sea stars, sand dollars,
sea urchins, sea cucumbers
• Echinoderms are the only
phylum that is entirely
marine.
• Reproduction: Sexual
(external fertilization)
• Capable of regeneration

Echinoderms:
Sea cucumber
Sea cucumber

Characteristics of echinoderms:
• 5 part radial symmetry
• Internal skeleton
• Water vascular system – pumps water through body to move
tube feet. It also functions in respiration and circulation.
• Tube feet – little tube-like extensions on bottom of arms, used
to move and grasp prey.

Ecological importance of echinoderms:
• A “keystone” in an archway is a wedge-shaped piece of stone in
the center of the arch. If you remove the keystone, the arch will
collapse.
• Echinoderms are often keystone species – removal of them
from the ecosystem will cause that ecosystem to collapse.
• Keystone predator– helps keep competition between prey species
even, so that one prey species doesn’t outcompete the others.
• On the West Coast of North America, for instance, removal of a
certain species of starfish caused a rapid growth in the numbers of
mussel upon which the starfish fed. This can throw off the balance
of an ENTIRE ECOSYSTEM>