Bilaterally symmetric, triploblastic, pseudocoelomate worms with cuticle containing cuticulin
General characteristics
a complete digestive system
The firs major phylum to posses
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little external evidence
whipping motion, cannot extend and contract like a flatworm
How do they move?
Covered by a non-living cuticle containing the protein cuticulin, found in no other animals
Highly resistant to chemicals
Epidermis (of ectodermal origin) is often syncytial, with nuclei lying in subdermal cords
Only longitudinal fibers exist, accounting for the unusual manner of movement
A body cavity filled with fluid under pressure, lies between the muscle tissue and the gut
This is a pseudocoelom,
The gut lacks muscle in its walls, so pseudocoelomic pressure must force food through it
The nematode gut is complete, with a mouth at one end and an anus at the other
This allows continual processing of food, not possible in incomplete digestive tracts
Body Organization
to one of the following classes based on morphology of sensory and excretory systems:
How are they classified?
Phasmidea and Aphasmidea
What are the classes?
nematodes with phasmids and tubular excretory systems
Phasmids are glandular sensory structures believed to be associated with chemoreception
The tubular excretory system consists of lateral canals running the length of the body
Two types of nematodes are found in this class
The majority of parasitic nematodes are in this class
Most others are soil dwellers (actually live in a water film surrounding soil particles)
Aphasmidea = nematodes that lack phasmids and have glandular excretory systems
These worms have more highly developed amphids (another type of chemoreceptor)
The glandular excretory system is localized, not spreading through the entire body
The types of nematodes in this class are many and varied
Only a few parasitic forms are found in this class, and many of them parasitize plants
The free-living species in this class may be aquatic or terrestrial
Freshwater species
Marine species
Soil dwellers (like above, really live in a watery film surrounding soil particles)
The cuticle is permeable to gases, so diffusion accomplishes gas exchange
Gas Exchange
Free-living species generally eat bacteria and/or fungi
Parasitic species may consume plant or animal body fluids or cells
Parasitic species may compete with host animal’s digestive system for nutrients
Food Acquisition
Nematodes possess a complete digestive tract with very little tract specialization
Lips surround the mouth in most species
A pharynx may be present in suction feeders
The intestine is amuscular; pressure waves in pseudocoelomic fluid push food along
Digestion in the intestine is about 70% extracellular, but phagocytes do contribute
real system is necessary as cells obtain what they need by diffusion
Gut cells transfer nutrients to, and exchange gases with, the pseudocoelomic fluid
Surface cells exchange gases through the cuticle & get nutrients from pseudocoelomic fluid
Nematodes are ammonotelic, relying on dilution (or a host’s systems) to remove ammonia
Phasmidean nematodes use tubular excretory systems to maintain water balance
Aphasmidean nematodes use a glandular excretory system to maintain water balance
Waste disposal/ osmoregulation
Sensory systems include phasmids and amphids (chemoreception) and tactile papillae
The CNS consists of dorsal and ventral nerve cords periodically connected by ring ganglia
The muscle is peculiar in two respects
Muscle fibers extend to synapse with nerves, not vice-versa
Muscle fibers run only longitudinally, permitting only a whipping motion
Sexual only
dioecious- copulating, larvae must undergo four molts

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