1 Phyla Nematoda (Roundworms) and Arthropoda (Insects, Crabs, Spiders, Pillbugs, etc.) Objectives: 1. Describe the general morphology in Phyla Nematoda and Arthropoda 2. Identify the major classes of nematodes and arthropods 3. Recognize characteristics newly derived from the ancestral phyla and know characteristics shared with other phyla Phylum Nematoda Nematodes have only longitudinal muscle bands running the length of their body wall. Thus they may only produce alternating side-to-side lengthwise contractions to create sinusoidal waves for locomotion. The phylum Nematoda is part of a major group of animals called Ecdysozoa that lack any ciliated tissues. Since nematodes are pseudocoelomates, they cannot develop any muscle around their digestive tract posterior to the pharynx (Why?) and therefore lack gut motility. The intestine is essentially one tissue layer. No accessory digestive organs form, because, the intestine does not involve both endoderm and mesoderm coming together. Nematodes have no circulatory systems. Where would it form? Nematodes (roundworms) occur in great numbers. A single decomposing apple may contain 1,000,000 nematodes. Many estimate that there are over one million nematode species world-wide. Many nematodes can cause disease in both plants and animals. Filaria can clog the lymphatic system causing elephantiasis. Worms are spread by blood feeding black flies and mosquitos. Loa loa is a parasitic eye worm spread by horsefly bites in India and Africa. The cuticle protects nematodes, especially those that are endoparasis. Ascaris lumbricoides can inhabit the intestines of humans and pigs. Infection is caused by eating plant material containing worm eggs.
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Phyla Nematoda (Roundworms) and Arthropoda
(Insects, Crabs, Spiders, Pillbugs, etc.)
Objectives: 1. Describe the general morphology in Phyla Nematoda and Arthropoda
2. Identify the major classes of nematodes and arthropods
3. Recognize characteristics newly derived from the ancestral phyla and know characteristics
shared with other phyla
Phylum Nematoda
Nematodes have only longitudinal muscle bands running the length of their body wall. Thus they may only
produce alternating side-to-side lengthwise contractions to create sinusoidal waves for locomotion. The phylum
Nematoda is part of a major group of animals called Ecdysozoa that lack any ciliated tissues. Since nematodes
are pseudocoelomates, they cannot develop any muscle around their digestive tract posterior to the pharynx
(Why?) and therefore lack gut motility. The intestine is essentially one tissue layer. No accessory digestive
organs form, because, the intestine does not involve both endoderm and mesoderm coming together.
Nematodes have no circulatory systems. Where would it form?
Nematodes (roundworms) occur in great numbers. A single decomposing apple may contain 1,000,000
nematodes. Many estimate that there are over one million nematode species world-wide.
Many nematodes can cause disease in both plants and animals. Filaria can clog the lymphatic system causing
elephantiasis. Worms are spread by blood feeding black flies and mosquitos. Loa loa is a parasitic eye worm
spread by horsefly bites in India and Africa. The cuticle protects nematodes, especially those that are
endoparasis. Ascaris lumbricoides can inhabit the intestines of humans and pigs. Infection is caused by eating
plant material containing worm eggs.
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Figure 1. Examples of nematodes that cause disease; Loa loa (an eye parasite), Filaria (elephantiasis), Ascaris
(human and pig intestines.
Phylum Arthropoda
Arthropods are called protostomes (“before-mouth”) because the blastopore turns into the mouth and
the anus forms later. Arthropods similar to annelids in having segmented bodies, with each segment
having a coelomic cavity. The Phylum Arthropoda is extremely successful taxon is characterized by
cephalized, bilateral symmetry; a segmented body typically with a pair of specialized jointed
appendages on each somite (segment); fusion of multiple somites into a functional body region called
a tagma; and a chitinous true exoskeleton that provides both protection of the internal organs and
attachment of muscle bands. Whereas the arthropod class Crustacea dominate the marine
environments — and a few crustacean groups such as the pillbug isopods do inhabit the land — the
terrestrial world has been thoroughly colonized by three other classes of this phylum:
Specialized segments. In annelids, all the segments are more or less the same. Arthropods are clearly
segmented, but the different segments are very different from one another in form and function.
Also, arthropod bodies are made of several groups of fused segments; the fused segments are called
tagmata, and they act like individual super-segments.
Jointed appendages. Annelids have setae, which are like small appendages on each segment. Setae
are not jointed, however, so they are limited in the variety of functions they can serve. Annelids don’t
walk on their setae; they just use them to push through the soil. The jointed appendages of
arthropods are much more versatile, functioning as legs, wings, antennae, mouthparts, and other
body parts.
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Exoskeleton. Arthropods have exoskeletons made of chitin (the same complex carbohydrate found in
fungal cell walls) and protein. In some arthropods, the exoskeleton is made more rigid with calcium
deposition. The first arthropods lived in the ocean; their exoskeletons protected them from attack and
provided places for their muscles to attach. When later groups of arthropods moved onto the land, it
turned out that the exoskeleton happened to be very functional in preventing the body from drying
out. This is a good example of how a characteristic that evolved in one situation can become
important in organisms adapting to another situation. Having an exoskeleton also affects a couple of
other important aspects of life:
Gas Exchange & Osmoregulation. Confined in their exoskeletons, arthropods need special structures
for gas exchange, osmoregulation, and excretion. These specialized structures also seem to create
opportunities for some arthropods: with their tracheal system for gas exchange and their Malphigian
tubules for osmoregulation, the insects are able to live in dry conditions that would kill most
invertebrates.
Metamorphosis. Many animals can simply grow continuously throughout their lives. Arthropods,
however, are confined in their rigid exoskeletons. In order to grow, they must molt, crawling out of
the old exoskeleton. Then they quickly grow bigger by absorbing water before they form a new hard
exoskeleton. Their development doesn’t always proceed in a series of gradual changes; instead, they
often go through a distinct metamorphosis, in which they change their body form dramatically as
they molt and form a new exoskeleton. The classic example of this is the metamorphosis of a
caterpillar into a butterfly.
There are a lot of arthropods, both in terms of numbers of species and numbers of individuals.
Almost a million species have so far been identified, and it has been estimated that there are 200
million individual insects for every person on earth! In this lab, we’ll focus on a few classes of
arthropods:
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Figure 2. Examples of animals in subphylum Crustacea; crab, shrimp, and pillbug.
Key Points
Tissues: • Three well-defined tissue layers in embryo
Symmetry: • Bilateral, with cephalization
Body cavity: • Coelom
Proto/deuterostome: • Protostome (Ecdysozoa)
Digestive tract: • Complete digestive tract.
• Segmented body. Segments are usually very different from one
another.
• Exoskeleton made of chitin.
• Jointed appendages (“arthropod” means “jointed leg”).