Download Name Supplementary Text: Annelids and Arthropods Introduction to

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Supplementary Text: Annelids and
Arthropods
Introduction to Annelids
Annelids are worms with segmented bodies.
The phylum Annelida consists of earthworms
and leaches and other organisms. The body of an
annelid is divided into segments that are
separated by septa, which are internal walls.
Most segments are similar to one another. Some
segments may be modified to perform special
functions. In many annelids, bristles called
setae are attached to each segment which give
the annelid traction with the ground. Annelids
have complex organ systems such as a nervous
system, reproductive system, excretory system,
circulatory system and digestive system.
Comparing the Earthworm to the Hydra
When an earthworm is compared to a hydra
there are important differences in how feeding
and nutrition occur because the earthworm’s
cells are highly organized into a digestive
system made up of specialized organs for
mechanical and chemical digestion. The hydra’s
cells are only at the tissue level of biological
organization . In the hydra, once digested,
nutrients slowly spread from cell to cell by slow
diffusion while in the earthworm, nutrients are
transported quickly to all cells in the body by a
blood circulatory system. In the hydra food
enters and leaves through the same opening (the
mouth/anus) to the gastrovascular cavity. The
more complex earthworms digests food in a tube
called a digestive tract that contains two
openings. Food enters the body through the
mouth, and wastes leave through the anus. A
one-way digestive tract allows there to be
specialized regions that allow food to be
processed more efficiently. This is important in
the type of environment an earthworm lives in,
the type of food of the earthworm, and its
greater energy needs.
Another major difference between the hydra and
the earthworm is that the hydra only uses
chemical digestion in which enzymes turn on
chemical reactions to break down food. This
works well if the food is very small and the
organism does not need as much energy but if
only chemical digestion was available to the
earthworm much of it’s food would pass through
its body without being broken down chemically
because the enzymes would not be able to
penetrate into the large food particles it ingested.
So in an earthworm digestion has two stages-the
mechanical stage, where food is physically
broken down into smaller pieces, and the
chemical stage, where the food mash is broken
down further by chemicals.
The evolutionary advantage of using mechanical
digestion before chemical digestion, is that that
once the food has been mechanically broken into
smaller pieces, they have much more surface
area that can be exposed to digestive enzymes,
making chemical digestion more efficient.
The Earthworm Digestive System
Earthworms eat soil and receive the nutrition
from the organic molecules it contains.
Earthworms ingest their food by using the
muscles of their pharynx to suck it up, mix the
food with saliva and swallow it. Some digestive
enzymes are released into the pharynx to begin
the process of chemical digestion. As the food
passes through the esophagus calcium carbonate
is secreted to reduce the acidity of the food
before it moves into a storage sac called the
crop. Then, food moves into the gizzard. In the
gizzard, strong muscular contractions, along
with the help of small bits of gravel grind the
food into much smaller pieces. This type of
digestion , where food in the gizzard is
physically broken down into smaller pieces, is
called mechanical digestion. More enzymes
are also secreted which can now penetrate into
the food which is mostly broken down to
monomers at this stage. The intestine is where
absorption of the nutrients from the broken
down food takes place. The walls of the
intestine are made of very thin membranes next
to blood vessels. The nutrients diffuse through
the membranes from the intestine into the blood
stream to be transported to all cells of the body.
Introduction to Arthropods and the
Grasshopper
Phylum Arthropoda includes animals such as
crabs, spiders, and insects. Arthropods have a
segmented body, a tough exoskeleton, and
jointed appendages. The arthropod exoskeleton
is made of a tough, water tight carbohydrate
called chitin. The evolution of arthropods has
led to fewer body segments and highly
specialized appendages for feeding, movement
and other functions. We study the grasshopper
as a representative of the insects. Unlike the
hydra, and earthworm who live in either an
aquatic environment or very moist soil
environment, the grasshopper lives in a dry
open-air environment which causes a constant
loss of water from it’s body. The exoskeleton of
the grasshopper is waterproof, which is an
adaptation to prevent water loss. The
disadvantage of this, it that the grasshopper can’t
breath through its skin like the worm, and has
evolved a respiratory system that can enable it to
breath without losing too much water. The
grasshopper must get all its nutrients and water
from its food.
The Digestive system of the Grasshopper
Like the earthworm, the grasshopper has a oneway digestive tract, and extracellular digestion.
Similarly to the earthworm, the grasshoppers
food passes through the esophagus, crop, and
into the gizzard where it is chemically and
mechanically broken down, however the food of
the grasshopper is very different from that of the
earthworm. The grasshopper eats a variety of
plants with tough cell walls made of cellulose.
Since the earthworm’s food of soil doesn’t vary
much and has already been partially
decomposed by bacteria, not as much action is
needed to complete the process in the worm’s
digestive tract. However more digestion is
needed to break down the type of plant matter
that the grasshopper ingests. More specialized
structures have evolved to handle the challenge
of the diverse foods a grasshopper eats.
Mandibles in the Grasshopper’s mouth are
mouth parts with sharp edges that can cut the
food into smaller pieces, beginning the process
of mechanical digestion. Digestive enzymes are
secreted by salivary glands into the mouth to
also begin chemical digestion. The food then
passes through the esophagus and into the crop
where it is temporarily stored before moving
into the gizzard. Teeth made of chitin, in the
gizzard help to mechanically break down the
food as the muscles contract. A one-way door
between the gizzard and stomach called the
pyloric valve prevents backflow of food from
the stomach. Digestive glands called the
digestive caeca covering the gizzard and
surrounding the stomach secrete digestive juices
and enzymes into the stomach so that chemical
digestion can be completed. Nutrients are
absorbed from the stomach into the blood. The
undigested food contains a great deal of water
which must be prevented from being lost. The
large intestine has evolved to be able to
reabsorb all the water from the waste, leaving it
a dry solid.