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Invertebrate Notes
Invertebrate Phyla:
Porifera-sponges
Cnidaria:
sea anemone
hydra
Coral
Platyhelminthesflatworms
Fluke
Jellyfish
Tapeworm
Nematodaroundworms
Trichinella
Ascaris
Hookworms
Rotifera--rotifers
Annelida-segmented
worms
earthworm
leech
Mollusca
Pinworms
clam
snail
octopus
Arthropoda
Subphylum:
Subphylum:
Subphylum: ChelicerataMandibulata-Trilobita-horseshoe crabs, spiders, crustaceans, insects,
trilobites (extinct) scorpions, mites, & ticks
millipedes,
centipedes
Trilobite
Horseshoe crab
Echinodermata: starfish,
sea cucumbers, sea lilies
Starfish
Millipede
Sea Cucumber
Sea Lily
About 97% of all animals are invertebrates. Invertebrates are
animals which do not have a backbone. There are nine phyla of
invertebrates: Porifera, Cnidaria, Platyhelminthes, Nematoda,
Rotifera, Mollusca, Annelida, Arthropoda, & Echinodermata.
Sponges
The phylum Porifera are sponges. There are about 800 different
species of sponges, and 88% are marine. "Marine" means that they
live in salt water, such as an ocean or a sea. Freshwater sponges
are smaller and less brightly colored than marine sponges. Sponges
are filter feeders. This means that they use their body as a filter
to trap their food, microscopic plankton. Sponges are asymmetrical
and live attached to one spot as adults making them sessile animals.
Sponges have a skeleton composed of a flexible protein material
called spongin & hard fibers called spicules composed of calcium
carbonate or silicon dioxide. The body of a sponge is filled with
holes or pores through which water enters their hollow bodies.
Sponges lack the tissue level of organization but they do have some
specialized cells. Choanocytes are specialized cells that line pores
in a sponge and have a flagellum that spins to pull in water and food.
Collar cells at the base of choanocytes capture plankton & start
digesting it. Amebocytes are specialized cells that carry food to all
other parts of a sponge's body. Wastes and excess water leave a
sponge through an opening at the top called the osculum. Sponges
reproduce asexually by internal or external buds and by
fragmentation whenever a piece of the sponge breaks off. Each
piece can form a new sponge. This is how sponges form colonies.
Sponges are hermaphrodites that produce both eggs and sperm.
They do not fertilize their own eggs, but they exchange sperm
with other sponges. Sponges reproduce sexually by dispensing eggs
and sperm into the water. If the freshwater supply evaporates,
freshwater sponges become dormant and form an internal bud or
gemmule which is release when the sponge dies. The gemmule is a
small freshwater sponge covered with hardened mucus which
prevents it from drying out. When the freshwater returns, the
gemmule becomes an active sponge
Cnidarians
The phylum Cnidaria include sea anemones, hydra, corals and
jellyfish. All Cnidaria are marine except hydra, which is a
freshwater organism. Cnidarians have radial symmetry and are
carnivorous using tentacles that surround their mouth to get food.
Cnidarians exhibit two body forms - the sessile polyp with tentacles
& mouth at the top or the motile medusa with tentacles & mouth on
the bottom. Cnidarians may exist in one of these two stages or go
through both stages in their life cycle such as the jellyfish.
Cnidarians have 2 layers of cells --- gastrodermis & epidermis.
Cnidarians have a hollow gastrovascular cavity on the inside lined
with gastrodermis. Epidermis covers the outside and a jellylike
material called mesoglea is between the layers. Mesoglea is thin in
polyp forms but thick in medusa forms. Cnidarians have stinging
cells called nematocysts or cnidocytes on their tentacles that are
poisonous & shoot out like a harpoon to kill or paralyze prey. Their
mouth is the only opening to their body so they have a two-way
digestive system. The also have a simple nerve net . Cnidarians
reproduce asexually by budding or sexually producing fertilized
eggs whenever males release sperm and females release eggs into
the water. Some cnidarians like coral build a limestone case that
makes an underwater reef.
Platyhelminthes (flatworms)
The phylum Platyhelminthes are dorsoventrally flattened and have a
definite anterior and posterior end giving them bilateral symmetry.
Their bodies are solid so they are said to be acoelomate. Some
flatworms are parasites, while others are free-living carnivores or
scavengers. Examples of parasitic flatworms are flukes and
tapeworms. Flatworms also have only a mouth for both food and
wastes. Their nervous system is composed of a nerve net and
sometimes light-sensitive eyespots at the anterior end. Specialized
flame cells help get rid of wastes. The planarian is the most
common free-living flatworm found in water or moist places. They
are hermaphrodites producing both eggs and sperm, but they
exchange sperm with each other during sexual reproduction.
Planarians also reproduce asexually by fragmentation. Flukes and
tapeworms often live in their host's digestive tract resistant to
the host's enzymes. They do not have a digestive system allowing
the host to digest their food.
Tapeworms are divided into sections called proglottids that each
have a complete reproductive system producing fertilized eggs.
Tapeworms are hermaphroditic (one body having both sexual parts),
and they fertilize their own eggs. Ripe proglottids with their eggs
pass out with the host's feces. Tapeworms anterior end is called
the scolex and is modified with both hooks and suckers to attach
to the host's intestines. Humans most often get tapeworms from
infected, undercooked pork, beef. or fish. Tapeworm eggs can
withstand boiling water so it is important to cook these meats well
enough to destroy the eggs. Children sometimes get tapeworms by
playing with the feces in the litter box of a cat, getting the eggs
on their hands, and placing their hands or fingers in their mouth.
The longest tapeworm ever passed by a person was 39 meters.
Flukes have complex life cycles that involve more than one host. A
fluke causes Schistosomiasis, a disease that affects 250 million
people world wide. This blood fluke attacks the kidneys, liver, and
intestines causing progressive weakness. It often takes 20 years to
die from Schistosomiases, & there is no cure.
Nematoda (roundworms)
The phylum Nematoda are the roundworms. Roundworms are
cylindrical in shape, tapered at both ends, and vary in length
from being microscopic to 20 inches long. Roundworms are
pseudocoelomate having a body cavity that is not completely lined.
The body cavity or pseudocoel serves as a hydrostatic skeleton
against which muscles can contract. Unlike flatworms, roundworms
have a complete gut. This means that they have a one-way
digestive tract with a gut that begins with a mouth and ends with
an anus. Therefore, they are usually able to digest food. However,
roundworms have no blood or heart. Nutrients are distributed by a
non-blood fluid which is not pumped. Most roundworms are parasites
and are found in all habitats. They are bilaterally symmetrical and
unsegmented. Although they are cylindrical in shape, they usually
taper at both ends. They are covered with a thick protective
cuticle that is flexible and can be molted. They have separate
sexes generally and reproduce sexually. The roundworm Trichinella,
causes the disease called trichinosis. People get trichinosis from
eating infected, undercooked pork. Trichinella gets into muscles
and leaves calcium deposits which effect muscle contraction.
Trichinosis can affect the heart. Another roundworm, Ascaris,
parasitizes human lungs. The Filaria worm attacks the lymphatic
system causing great swelling. Hookworms and pinworms are also
roundworms which parasitize humans.
Rotifers
The phylum Rotifera includes microscopic worms found in aquatic
and soil habitats. They have a crown of cilia at their head end
surrounding their mouth for movement and feeding. Their bodies
are covered with an external layer of chitin. Having separate
sexes, they reproduce sexually. Some species contain only females
and reproduce by parthenogenesis (unfertilized eggs developing into
females).
Mollusks
The phylum Mollusca contains snails and slugs, bivalves, octopus,
squid, and the chambered nautilus. Many members of this phylum
have durable limestone shells and are found in all habitats.
Members of this group are economically important as sources of
human food , pearl and shell production, crop & flower damage,
destruction to submerged wooden structures, and intermediate
hosts for some parasitic diseases. The giant squid and giant clam
are the two largest invertebrates. Mollusks have bilateral
symmetry and a visceral mass containing their body organs.
Mollusks also have a muscular foot for movement which can be
modified into arms or tentacles in some species. Mollusks breathe
through gills or lungs located below a protective layer called the
mantle. The mantle forms the shell in some species and also
protects the body organs. All mollusks except bivalves contain a
rasping, tongue-like radula for scraping food. The circulatory
system consists of a three-chambered heart and open-flowing
system except for octopus & squids which have a closed circulatory
system. Reproduction is sexual even in hermaphroditic forms.
Mollusks go through a free swimming larval stage called the
trochophore.
The class of mollusks called gastropods have a foot on their belly.
An example of a gastropod is the snail. When a snail lacks a shell it
is called a slug. Snails and slugs walk on their belly. Most snails are
marine, but some do live on land. Marine snails have gills. Land
snails are called pulmonate snails and have an air hole for breathing.
Snails can be very large. The helmet snail can be as big as 15
pounds.
The class of mollusk called Bivalvia includes clams, oysters,
mussels , and scallops. These mollusks have two shells hinged
together by a ligament. Strong adductor muscles open and close
the shells. Incurrent and excurrent siphons circulate water
containing food and oxygen through the bivalve. Gills extract the
oxygen from the water, and they move by jet propulsion. Their
muscular foot can be extended from the shell for movement or
anchoring.
The class of mollusks called cephalopods have their muscular foot
modified into tentacles on their head. Examples of cephalopods are
octopus, squid and nautilus. Most cephalopods have beaks,
tentacles and jaws and are active predators. Their muscular foot
has been modified into arms or tentacles. They lack external
shells except for the nautilus. These are the most intelligent of all
invertebrates. They used their siphons to move by jet propulsion.
Octopus have their shell inside of their body. Octopus secrete an
inky substance which they spit out to help them escape from
predators. The giant squid is the largest cephalopod. It can be up
to 60 meters in length and has been known to eat whales.
Annelids (segmented worms)
The phylum Annelida are the segmented worms and are abundant in
all habitats. External segments are characterized by ringlike
structures called metameres along the body, and corresponding
internal segments are called septa. Segmentation gives worms more
flexibility in movement. If one segment is damaged, it isn't usually
fatal to the animal because their organs are duplicated in other
segments. Annelids have a A tube within-a-tube body plan known
as a coelom which is fully lined and contains the body organs. The
coelom runs from the mouth to the anus. Annelids have bilateral
symmetry, and a well-developed brain and diverse sense organs
showing cephalization. Coelomic fluid serves as a hydrostatic
skeleton. Earthworms belong to this phylum. Each segment of the
earthworm has setae or external bristles made of chitin. These
bristles allow the earthworm to move and to burrow into soil.
Earthworms have a head and a central nervous system. Earthworms
respire through their moist skin as they dig through the soil and
help loosen it. They have a closed circulatory system in which blood
is pumped by five pairs of hearts. Most earthworms feed on
decomposing vegetation causing it to decompose faster. A pharynx
sucks in the organic debris which the muscular gizzard grinds.
Earthworms bring the nutrients from the subsoil to the top soil,
thereby helping plants to grow. Undigested materials or castings
are deposited outside burrows.
Leeches are also in the phylum Annelida. Most leeches live in water
and have suckers at both ends of their bodies. The tail suckers are
used to latch on to a host, while the head suckers are used to suck
blood from the host. Most leeches are predators or scavengers,
but some suck blood. Because of this, blood sucking leeches are
collected for anticoagulant. Leeches bodies are flattened
dorsoventrally and lack setae except for one species. Like
earthworms, leeches are hermaphrodites that exchange sperm with
other members of their species.
Polychaetes are marine annelids that have their setae modified into
paddle-like structures called parapodia. Parapodia improvement
movement and give more area for gas exchange. Polychaetes often
live commensally with sponges, mollusks, echinoderms, and
crustaceans. Sexes are separate with external fertilization.
Arthropods
The members of the phylum Arthropoda all have jointed
appendages. In fact, the word "arthropod" means jointed leg.
There are more species of arthropods than any other phylum.
Arthropods have these characteristics:
a. hard exoskeleton which is usually composed of substance
called chitin
b. go through periodic ecdysis (molting) as they shed or molt
their exoskeleton
c. they have specialized body segments (head, thorax,
cephalothorax, & abdomen)
d. jointed appendages such as legs, antenna, and mouthparts.
e. open circulatory system (blood is pumped out of blood
vessels into the body)
The phylum Arthropoda is divided according to their type of
appendages. The subphylum Chelicerata possess chelicerae or
fangs and no antenna, while the subphylum Mandibulata have
antenna and mandibles or jaws. Crustaceans have pincers called
chelipeds. The subphylum Trilobita are an extinct group with a head
and trunk with a pair of legs on each segment. Terrestrial
arthropods like insects, millipedes, & centipedes have a system of
hollow air tubes called tracheae as their respiratory system.
Aquatic chelicerates like the horseshoe crab have book gills, while
terrestrial chelicerates such as spiders, ticks, mites, & scorpions
use book lungs. Book lungs have numerous blood vessel lined
surfaces which look like the pages in a book & get oxygen from air.
Crustaceans respire through gills. Gills are folded tissue which are
lined with blood vessels which remove oxygen from water.
Terrestrial mandibulates are uniraimous with one-branched
appendages, but aquatic mandibulates like crustaceans are
biramous or two-branched. Arthropods have a brain and nervous
system and possess a variety of sensory receptors such as simple
eyes called ocelli or compound eyes, tympanic membranes for
hearing, and antenna that can smell and taste. Excretory
structures in arthropods vary, but terrestrial arthropods have
Malpighian tubules to filter nitrogenous wastes.
The subphylum Chelicerata (ki-LISS-uh-ruh) include the class
Xiphosura or horseshoe crabs which have a cephalothorax and
abdomen, live in marine environments breathing through book gills,
lack antenna, but have chelicera & 4 pairs of walking legs. The class
Arachnida containing spiders, scorpions, mites, and ticks are also
chelicerates that lack antenna, have chelicera (fangs) and 4 pairs of
legs, but they live in terrestrial habitats and breathe through book
lungs or trachea. Chelicerates also have appendages on their head
called pedipalps that are sensory and can help move food into their
mouth. Unlike most arthropods, spiders do not see well; however,
they are good at detecting movement. Spiders have glands called
spinnerets on the posterior end of their abdomen that produce silk
to make webs. When prey get caught in a spider's web, it is the
movement which alerts the spider to the captured prey. Most
spiders also have hairs on their body to assist them in feeling
movement. Spiders poison their prey once they are caught in their
webs. Spiders are very beneficial because they catch and eat
insects. Two spiders which are dangerous are the black widow and
the brown recluse. Both of these spiders have distinct markings on
the underside of their abdomen. Spiders differ from insects in
having eight, not six legs, having simple eyes and not compound
eyes, and having only 2 body regions (cephalothorax & abdomen)
instead of 3 regions ( head, thorax, & abdomen).
The subphylum Mandibulata contains the class Crustacea. Most
crustaceans live in the water and include crabs, shrimp, lobster,
crayfish, & barnacles. Terrestrial crustaceans include pillbugs and
sowbugs. Crustaceans have a pair of antenna to smell and detect
chemicals and a shorter pair of antennules used for balance. They
have 2 body regions (cephalothorax and abdomen), and their
mouthparts include mandibles, maxilla, and maxillipeds. They also
have pincers called chelipeds to help them catch food. Aquatic
crustaceans have a shell called a carapace that they regularly shed
as they grow to produce a larger one. Crustaceans are
economically important to man as a food source.
The classes Chilopoda and Diplopoda are also in the subphylum
Mandibulata. Chilopoda or centipedes are poisonous predators
feeding on other terrestrial arthropods. Centipedes have fangs,
venom glands, and a pincer on their tail. They have a single pair of
legs per body segment. Diplopoda or millipedes are vegetarians or
scavengers feeding on decaying vegetation that have two pairs of
legs per body segment.
The class Insecta in the subphylum Mandibulata includes all of the
insects. This is the largest and most successful group of
arthropods. Insects usually have six legs, a pair of antenna, and a
pair of wings although some species may be wingless such as
silverfish and termites. Flies have their second pair of wings
modified into a balancing structure called halteres. Insect's
mouths usually have four parts - the mandible or jaw, maxilla,
labium or lower lip, and labrum or upper lip and are adapted for a
particular food. For example, grasshoppers have chewing
mouthparts for eating grass, mosquitoes have sucking mouthparts
for sucking blood, butterflies have siphoning mouthparts for getting
nectar from flowers, and the house fly has spongy mouth parts for
soaking up liquid food. Wings and legs are attached to the
midsection or thorax, antenna, eyes, and mouthparts are attached
to the head, and the abdomen on females may have an egg-laying
tube called the ovipositor. Insects communicate by producing
sounds and by making chemicals called pheromones. Tympanic
membranes on the abdomen and sensory hairs detect sound waves.
Spiracles line the sides of the insect's abdomen and open into their
breathing tubes or trachea. Insects may go through stages
(metamorphosis) in their life cycle. Butterflies, bees, flies, and
beetles go through the egg, larva, pupa, and adult stages. This is
known as complete metamorphosis. Dragonflies and grasshoppers go
through egg, nymph, and adult stages known as incomplete
metamorphosis. Insects such as silverfish and fleas do not go
through metamorphosis. Metamorphosis and molting are controlled
by hormones.
Echinoderms
The phylum Echinodermata include the starfish, sea urchins and
sea cucumbers. The word "echinoderm" means spiny skin.
Echinoderms are the most advanced invertebrates. All other
invertebrates are protostomes in which the blastopore (first
opening in embryo) in their development becomes the mouth.
Echinoderms, like chordates, are deuterostomes in which the
blastopore becomes the anus. Echinoderms have an endoskeleton
composed of movable or fixed calcium plates called ossicles. The
members of this phylum have radial symmetry with a five part
body plan (pentaradial). Adults have no head or brain and move be
extendable tube feet. Echinoderms also possess a water vascular
system made up of a system of canals that help the organism feed
and move. Water enters through an opening called the madreporite
into a short stone canal into the ring canal. Radial canals connect
to the ring canal and determine the five-part symmetry. This
hydraulic water system is strong enough to help starfish open clam
shells. Skin gills are used for respiration and waste removal.
Echinoderms are capable of extensive regeneration whenever parts
are dropped. They can reproduce asexually by fragmentation or
sexually with external fertilization.
Starfish are in the class Asteroidea and are active marine
predators with 5 arms set off from a central disk and their mouth
located on the underside or oral surface. Bivalve mollusks are a
favorite food of the starfish, and they consume them by turning
their stomach inside out and sticking it into the clam shell to
digest the clam. Sea urchins and sand dollars are in the class
Echinoidea and they lack distinct arms. Five rows of tube feet
protrude through their skeletal. They use the spines of their skin
and tube feet to move about and graze on algae, coral, or dead
fish. Triangular teeth around the mouth scrap or crush food. The
class Crinoidea contains sea lilies and feather stars with highly
branched arms around their mouth for filter feeding. Sea lilies are
attached by a stalk to the substrate, but feather stars are able to
detach and move about. Brittle stars in the class Ophiuroidea have
slender arms attached to their central disk and can move faster
than starfish. Sea cucumbers are in the class Holothuroidea and
are soft, sluglike organisms with leathery outer skin. Sea
cucumbers usually lie on their sides on the ocean bottom and can
eject part of their intestines in order to scare away a predator.
They also move by tube feet or by wiggling their entire body. Some
of these are hermaphroditic which is unusual for echinoderms.