About 97% of all animals are invertebrates. Invertebrates are animals which do not have a backbone. In this unit we cover five phyla of invertebrates: Porifera, Cindaria, Platyhelminthes, Nematoda, and Annelida.
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. Sponges are filter feeders. This means that they use their body as a filter to trap food. Sponges have a skeleton composed of hard structures called spicules which attach the sponge to the bottom of the sea. The spicules can be composed of calcium, protein or glass. When sponges are used as bath sponges, they must be dried and the spicules crushed before they are made soft.
Sponges are hollow on the inside and have cells called "collar cells" which contain either flagella or cilia. The flagella or cilia move to create a current to assist them in trapping food. Sponges also have "ameboid" cells which actually absorb the food. Sponges reproduce sexually by dispensing eggs and sperm into the water, and asexually by budding. Budding occurs when a sponge becomes broken into pieces. Each piece can form a new sponge.
Freshwater sponges are called demospongia. If the freshwater supply evaporates, demospongia become dormant and form a "gemmule." The gemmule is a demospongia covered with hardened mucus which prevents the demospongia from drying out. When the freshwater returns, the gemmule goes back to active sponge-form.
The phylum Cindaria include sea anemones, hydra, corals and jelly-fish. All Cindaria are marine except hydra, which is freshwater. Many Cindaria have tentacles with hematocysts on their end. The hematocysts are stinging cells.
The phylum Platyhelminthes are the flat worms. Some flat worms are parasites.
Examples of parasitic flat worms are flukes and tapeworms. A fluke causes
Schistosomiasis, a disease that affects 250 million people world wide. This fluke
attacks the kidneys, causing progressive weakness. It often takes 20 years to die
from Schistosomiasis. There is no cure.
Tapeworms are parasites which have no digestive system of their own, so they
attach to the intestines and eat the food which we digest. The longest tapeworm
ever passed by a person was 39 meters. Tapeworms attach to the wall of the
intestines using their "scolex." The scolex is the head of the tapeworm which is
composed of hooks for attaching, and suckers for drinking food. Tapeworms are
composed of segments called "proglottides." Each proglottid grows in size and
ruptures releasing eggs, each of which becomes a new tapeworm. Tapeworms are
hermaphroditic (one body having both sexual parts) and they fertilize their own
eggs. Humans most often get tapeworms from 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 primary symptom of tapeworms is weight loss. Tapeworm eggs have been capsuled and sold as a diet pill. When the desired amount of weight was lost, one took another pill which killed the tapeworms. Effective, but not a good idea medically.
The phylum Nematoda are the roundworms. Roundworms are cylindrical in shape and vary in length from being microscopic to being 20 inches long. Unlike flatworms, roundworms have a complete gut. This means that they have a gut which 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. Roundworms can be parasites. The roundworm trichinella, causes the disease called trichinosis. People get trichinosis from eating undercooked pork. Trichinella get into muscles and leave calcium deposits which effect muscle contraction. Trichinosis can affect the heart. Another roundworm, ascaris, parasitizes human lungs. Hookworms and pinworms are also roundworms which parasitize humans.
The phylum Annelida are the segmented worms. Earthworms belong to this phylum.
Each segment of the earthworm has setae which contain bristles made of chitin.
These bristles allow the earthworm to move and to burrow into soil. However,
earthworms can move just by wiggling. Earthworms have a head and a central
nervous system. They have a closed circulatory system in which blood is pumped
by hearts. Most earthworms feed on decomposing vegetation. They feed on this
vegetation causing it to decompose faster. Earthworms bring the nutrients from
the subsoil to the top soil, thereby helping plants to grow. Leaches are also in
the phylum Annelida. Most leaches live in water and have suckers at both ends of
their bodies. The tail suckers are used to latch on to a host and the head
suckers are used to suck blood from the host.
OUTLINE
Phylum:
1. Porifera--sponges
2. Cindaria
a. sea anemones
b. hydra
c. corals
d. jelly-fish
3. Platyhelminthes--flatworms
a. flukes
b. tapeworms
4. Nematoda--roundworms
a. trichinella
b. ascaris
c. hookworms
d. pinworms
5. Annelida--segmented worms
a. earthworm
b. leaches
The members of the phylum Arthropoda all have jointed legs. 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. as they grow they must shed or molt and grow a new exoskeleton.
c. they have specialized segments, such as a head, a thorax and an abdomen.
Arthropods have a respiratory system. Their respiratory system can be gills or book lungs. Crawfish are arthropods with gills. Gills are folded tissue which are lined with blood vessels. Gills function to remove oxygen from water. Spiders are arthropods with book lungs. Book lungs have numerous blood vessel lined surfaces which look like the pages in a book. Book lungs get oxygen from air.
Arthropods do not hear well. However, they usually have good eye sight. Many arthropods have compound eyes that are made up of many little eyes, each of which sends impulses to their brain. These arthropod brain see a mosaic or a composite of many pictures.
The subphylum Chelicerata (ki-LISS-uh-ruh) include horseshoe crabs, spiders, scorpions, mites and tics. Mites are also called red bugs. Red bugs are bird parasites. However, they sometimes get on people. This is a fatal error, as people will poison and kill them in just a few days. Except for the horseshoe crab, all chelicerates have eight legs. Spiders have four pairs of walking legs, plus other appendages that look like legs. One of these is the chelicera, which is actually the spider's mouth used for sucking juices from the body of its victim and biting and poisoning its victim. The other are pedipalps, which are used to subdue food. Unlike most arthropods, spiders do not see well. However, they are good at detecting movement. When prey get caught in a spider's web, it is the movement which alerts the spider to the captured prey. Most spiders have hair on their body to assist them in feeling movement. Spiders spin silk-like, sticky webs to catch prey. Once caught, they poison the prey with a bite. The web can be used to make silk. 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 red markings of their stomach. Spiders belong to the class known as the arachnids. Spiders are not insects and differ from insects in several ways. Spiders have eight legs and insects have six. Spiders have a simple eye and insects have compound eyes.
The subphylum crustacea include crabs, shrimp, lobster and barnacles. Most crustaceans live in the water. Crabs and lobsters have pincers to help them catch food. Crabs have a shell called a carapace. As they grow, they regularly shed this shell to grow a larger one. When they are without carapace they are called "soft-shell crabs." Crabs have antennae which they use to detect motion and sense chemicals (smell).
The subphylum uniramia include the millipedes and the centipedes. The word
"millipede" means a thousand legs, but in fact they have fewer legs than this.
The word "centipede" means a hundred legs. However, they too have less than this
number. Millipedes are scavengers and eat decayed plant matter. Centipedes are
predators. Centipedes have fangs, venom glands and a pincer on their tail.
Centipedes prey primarily on insects.
The subphylum insecta include all of the insects. Insects usually have six legs,
antennae and wings. Insect's mouths usually have three parts and are adapted for
a particular food. Grasshoppers, for example, have chewing mouth parts for eating
grass. Mosquitos have sucking mouth parts for sucking blood. Butterflies have
siphoning mouth parts for getting nectar from flowers. The house fly has spongy
mouth parts for soaking up liquid food.
The phylum mollusca include clams, oysters, snails and octopus.
Most mollusks have a shell that is made of calcium carbonate. These shells formed
most of the limestone and marble which exists on earth. Marble is formed from
heat and pressure on limestone. Mollusks usually have a mantle. A mantle is a
folding of soft tissue on the edge of their shell. The mantle is the place where
the shell is being secreted. In the mantle are found the gills.
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 and mussels. Clams and oysters live in water, have gills, mantle and eyes. They are filter feeders and move by jet propulsion.
The class of mollusks called cephalopods have a foot on their head. Examples of
cephalopods are octopus, squid and nautilus. Most cephalopods have beaks,
tentacles and jaws. 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.
OUTLINE
phylum: Arthropoda
subphylum: chelicera--horseshoe crabs, spiders, scorpions, mites and tics
class: arachnids (spiders)
subphylum: crustacea--lobsters, shrimp, barnacles, crabs, crawfish
subphylum: uniramia--millipedes, centipedes
subphylum: insecta--insects
subphylum: mollusca--clams, oysters, snails and octopus
class: gastropods--snails
class: bivalvia--clams, oysters and mussels
class: cephalopods__octopus, squid, nautilus
The phylum Echinodermata include the starfish, sea urchins and sea cucumbers. The word "echinoderm" means spiny skin. Echinoderms all have a star somewhere at sometime of their life.
Starfish move by way of tube feet. Tube feet are composed of canals through which water is pumped and the water moves with a great deal of force. This hydraulic system is so powerful that its force can be used to open clam shells. Clams are a favorite food of the starfish. A starfish consumes a clam by turning its stomach inside out and sticking its stomach into the clam shell to digest the clam.
Sea urchins are sometimes called sand dollars. They use the spines of their skin to move about and graze on algae, coral or dead fish. All echinoderms have radial symmetry and have no brain.
The phylum chordata has these characteristics:
1. They have a nerve cord which runs much of the length of their bodies, such as our spinal cord.
2. They have a notochord. A notochord is specialized tissue that supports the nerve cord. Our vertebra make up our notochord.
3. At sometime in their development chordates have pharyngeal pouches. Pharyngeal pouches look like gill slits. In humans, we have these before we are born.
The subphylum of chordates called vertebrata have a notochord composed of vertebra. However, these vertebra do not have to be made of bone. They can be made of cartilage, such as in the shark or the ray. Sharks have no bone. Their skeletal structure is made up entirely of cartilage. Sharks have been in existence for 400 million years. Sharks have no swim (air) bladder to keep them afloat. Therefore they must stay in almost constant motion to avoid sinking to the bottom of the sea. Because they stay in motion, some scientists believe that sharks do not sleep.
The class of vertebrates called Osteichthyes are the bony fish. There are two kinds of bony fish, lung fish and lobed finned fish. Scientists believe that the swim bladder of some fish evolved into lungs, and that the lobed fins evolved into legs. If this occurred it may have resulted in ichthostaga, the first amphibian.
The class of vertebrates known as amphibians include frogs, toads, salamanders and newts. The class known as reptiles include the turtle, tortoise, alligator, crocodile, snakes and lizards. All reptiles have scales. You can see the scales of a snake best when they molt (shed their skin). The scales of the turtle are fused into a shell. Both amphibians and reptiles are egg layers.
The class of vertebrates called aves include the birds. Birds are also egg layers. All birds have feathers, but all do not fly. For example the ostrich and the penguin are birds that do not fly. However, most birds do fly. While many bones are hollow, birds have very hollow bones to give them light weight.
The class of vertebrate known as mammalia have mammary glands which produce milk for the nourishment of their young. All mammals have hair. We are mammals, as are dogs, cats, cows, pigs, whales, deer, etc.
Within the class of mammals is the order of primates. Primates include apes, monkeys and man. Apes are distinguished from monkeys by not having a tail. Most primates can stand erect on their hind legs to allow their arms to be free. Most primates have an opposable thumb to assist in grasping.
Most mammals have good vision, an efficient nervous system and bond in pairs.