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Ms. Arness
Biology 11
PHYLUM NEMATODA
Characteristics and Advances
1. Evolution of the anus allows for a complete digestive system - tube within a tube body plan
2. A pseudocoelom between mesoderm and endoderm
The Roundworms
• A very large phylum, found in almost all habitats
• most are small to microscopic
• Have bilateral body plans
• Reproduction is mostly sexual
2 forms:
A. Free living nematodes
• Most nematodes live in water or soil
• They are important in the ecology of soil - decomposers
B. Parasitic Nematodes
Some of the worst worm parasites are members of this phylum
Ex. Ascaris sp. the Intestinal Roundworm
• Most common world wide worm parasite - over 1 billion people have it
• Lives in small intestine but travels through the body as a juvenile
Coelomates
At this point in animal evolution, a true coelom developed. So there is mesoderm - and
therefore muscle - around the skin and the gut
It also marks the point where two great lines of animal evolution diverged: the Protostomes and
the Deuterostomes
Protostomes: Contain three major phyla of animals (Annelida, Arthropoda, Mollusca)
PHYLUM ANNELIDA
The Segmented Worms
Characteristics and Advances:
1. Their true coelom results in muscle around the gut and more coordinated digestion.
Specialization of the food tube begins
2. Nervous system has a single ventral nerve cord with an anterior brain
3. Bilateral symmetry – they will have have bilateral from now on.
Ms. Arness
Biology 11
4. First phylum to have a circulatory system - frees them from diffusion of materials - can
grow larger. They also have hearts
5. Respiration through skin (land forms) by diffusion so must stay moist
6. Sexual reproduction (limited regeneration)
7. Unspecialized segmentation separated by septa
8. Mostly motile, however tube worms are sessile
9. Have specialized excretory units called Nephridia in pairs in each segment
• Used as a kidney to get rid of metabolic wastes
3 types:
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Earthworms
leeches
“sandworms”/tubeworms