Pseudocoelomates: Phyla Nematoda & Rotifera
1
Biological Sciences 102 – Animal Biology – Notes & Vocabulary
The pseudocoel represents an increase in complexity over the body plan of acoelomates. The
pseudocoel may be filled with fluid or contain a gelatinous substances with mesenchymal cells.
The pseudocoel is derived from the embryonic blastocoel and is not a secondary cavity formed
within the mesoderm (as seen in eucoelomates).
The pseudocoel allows for:
(each of these characteristics is not necessarily found in all pseudocoelomates)
1. greater freedom of movement
2. space for development and differentiation of internal organs
3. simple means of circulation for distribution of materials throughout the body
4. a storage place for waste products to be discharged to the outside by excretory ducts
5. a hydrostatic organ (for muscle contraction and bodily movements)
Phylum Nematoda: Roundworms
Nematodes are a member of the Superphylum Ecdysozoa
Ecdysozoan Protostomes
Any member of a clade within Protostomia whose members shed the cuticle as they grow;
includes the phyla Arthropoda, Nematoda and several smaller phyla.
Average Sizes
¾ millimeters to meters (only a few get this long)
¾ most are less than 5 cm long and the majority are microscopic
Life Span
¾ days to months (maybe years for some?)
Symmetry & Body Plan
¾ bilateral symmetry with vermiform (worm-like) shape
¾ body cavity is a pseudocoel
¾ no metamerism (segmentation)
¾ cephalized
Development & Coelom Formation
¾ true tissues
¾ psuedocoelomate
¾ juvenile/microfilariae forms are infective
¾ triploblastic body plan with endoderm, mesoderm, and ectoderm
Type of Skeleton
¾ hydrostatic skeleton
Appendage Types
¾ none
Basic Form & Function
¾ eutely is evident and common = a constant number of cells or nuclei in individuals of a
specific species
¾ tube-within-a-tube arrangement of digestive system within body wall
¾ no definitive muscle layer around digestive tract (no inner mesodermal lining)
¾ lack cilia and flagella
¾ pseudocoel adapted as a hydrostatic organ
¾ longitudinal muscles only with no real antagonistic muscle groupings for movement
¾ many species are parasites
Pseudocoelomates: Phyla Nematoda & Rotifera
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Specialized Cell Types/Organs
¾ amphids = sensory organs of the head; reduced or absent in many parasitic forms
¾ phasmids = sensory organs near the posterior end
“Organ Systems”
Integumentary System (skin)
¾ syncitial or cellular epidermis
¾ thickened, non-living cuticle that is sometimes molted
¾ cilia absent
Muscular System
¾ only longitudinal muscle fibers in body wall; no circular muscles
¾ no antagonistic muscles groups for movement
Circulatory System
¾ none – nutrient/waste removal by diffusion/osmosis/cell transport
(phagocytosis/pinocytosis)
Respiratory System (gas exchange)
¾ none – gas exchange by simple diffusion = cutaneous respiration
Nervous System
¾ cerebral ganglia or nerve ring connected to anterior and posterior nerves
Sensory Organs
¾ amphids and phasmids connected to ciliated pits
¾ some have eyespots
Digestive System (food/water supply)
¾ complete digestive system with mouth and anus
¾ well muscularized pharynx
¾ no definitive muscle layer around digestive tract (no inner mesodermal lining in
pseudocoelomates)
¾ free living forms feed on bacteria, yeasts, fungi and algae
¾ some nematodes are predators on rotifers, tardigrades, small annelids and other
nematodes
¾ many parasitic forms feeding on plant and animal tissues and fluids
Immune System
¾ none (?)
Excretory System (waste removal)
¾ system of canals and protonephridia in some pseduocoelomates, but not in nematodes
¾ cloaca in some pseudocoelomates = opening that receives excretory, digestive and
reproductive products present in some
Reproduction
¾ single or double set of gonads and ducts in each sex
¾ sexes nearly always separate (dioecious) with males usually smaller than females
¾ eggs may have chitinous shell
¾ eutely is evident and common = a constant number of cells or nuclei in individuals of a
specific species
¾ cleavage is mostly mosaic
Pseudocoelomates: Phyla Nematoda & Rotifera
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Mechanism/Mode of Locomotion
¾ contraction of longitudinal muscles against pseudocoel (hydrostatic organ)
Specialized Defenses/Toxin/Poisons
¾ tissue or organ damage by parasitic forms, but not necessarily due to a toxin
Ecology & Adaptive Radiation
¾ marine, freshwater, terrestrial (soil) from polar regions to the topics
¾ from high mountains to deep seas
¾ nearly every type of animal and plant can be parasitized by nematodes
Social Organization
¾ none
Communication
¾ none(?)
Thermoregulation
¾ none
Common Nematode Parasites (see table 15.1 in text)
¾ Hookworm
¾ Pinworm
¾ Intestinal roundworm
¾ Trichina worm
¾ Whipworm
Phylum Rotifera: Rotifers
Rotifers are a member of the Superphylum Lophotrochozoa
Lophotrochozoan Protostomes
Any member of a clade within Protostomia whose members generally possess either a
trochophore larva or a lophophore; includes the phyla Annelida, Mollusca and the ectoprocts
Average Sizes
¾ Microscopic; micrometers to a few millimeters
Life Span
¾ days (a few years in dessicated form)
Symmetry & Body Plan
¾ bilateral symmetry
¾ body cavity is a pseudocoel
¾ no metamerism (segmentation)
¾ cephalized
Development & Coelom Formation
¾ true tissues
¾ psuedocoelomate
¾ triploblastic body plan with endoderm, mesoderm, and ectoderm
Type of Skeleton
¾ hydrostatic skeleton
Pseudocoelomates: Phyla Nematoda & Rotifera
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Appendage Types
¾ none
Basic Form & Function
¾ can be sessile, floaters, creepers or swimmers
¾ elongated or saclike body plan with head at one end and foot at other with one to four
toes
¾ cilia form a corona around the mouth
¾ cilia used for both locomotion and feeding
¾ eutely is evident and common = a constant number of cells or nuclei in individuals of a
specific species; most structures in rotifers are syncitial
Specialized Cell Types/Organs
¾ corona of cilia around the mouth
¾ mastax = muscular pharynx
¾ pedal glands of foot secrete adhesive material in both sessile and creeping forms
“Organ Systems”
Integumentary System (skin)
¾ syncitial epidermis
¾ some have a secreted cuticle; others have a fibrous layer to epidermis
Muscular System
¾ subepidermal muscles some of which are longitudinal and some circular
Circulatory System
¾ none – nutrient/waste removal by diffusion/osmosis/cell transport
(phagocytosis/pinocytosis)
Respiratory System (gas exchange)
¾ none – gas exchange by simple diffusion = cutaneous respiration
Nervous System
¾ bilobed “brain” with paired nerves running to internal organs
Sensory Organs
¾ sensory antennae on trunk
¾ eyespots on head
Digestive System (food/water supply)
¾ complete digestive system with mouth and anus
¾ well muscularized pharynx = mastax that contains trophi (jaws) for sucking and
grinding food particles
¾ ciliated corona helps sweep food particles to mouth
¾ feed on minute organic particles or algae
¾ carnivorous forms feed on protozoa and small metazoans; some hunter types have
trophy they can project to capture food; trappers have a funnel shaped area around the
mouth
Immune System
¾ none (?)
Pseudocoelomates: Phyla Nematoda & Rotifera
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Excretory System (waste removal)
¾ pair of protonephridial tubules with flame cells that empty into a common cloacal
bladder
¾ cloaca in some pseudocoelomates = opening that receives excretory, digestive and
reproductive products present in some
¾ protonephridia play an important role in osmoregulation
Reproduction
¾ single (male) or double set of gonads (female) and ducts in each sex
¾ sexes separate (dioecious)
¾ males unknown in class Bdelloidea and only occur a few weeks a years in the class
Monogononta
¾ all females are parthenogenetic in the class Bdelloidea (diploid females produce
diploid females)
¾ parthenogenesis = unisexual reproduction involving the production of young by
females that are not fertilized by males; common in rotifers, aphids, bees, ants and
wasps. Parthenogenetic eggs may be haploid or diploid.
¾ In the class Monogononta, most of the year diploid females produce thin shelled
diploid amictic eggs. These amictic eggs develop parthenogenetically into diploid
(amictic) females. When the environment in the pond or stream changes due to
crowding, diet, change in photoperiod, etc. some amictic eggs develop into diploid
mictic females that produce thin shelled haploid eggs. If these eggs are not fertilized
then they will develop into haploid males. If the eggs are fertilized, they become
mictic eggs with a thick, resistant shell and become dormant. These “winter eggs”
can survive until the environment becomes more suitable at which time they hatch and
develop into amictic females (completing the cycle). Females grow and reach maturity
in a few days while males rarely grow and are mature at birth.
Mechanism/Mode of Locomotion
¾ contraction of longitudinal and circular muscles against pseudocoel (hydrostatic organ)
Specialized Defenses/Toxin/Poisons
¾ none (?)
Ecology & Adaptive Radiation
¾ most freshwater, a few marine, some are terrestrial
¾ some are parasitic
¾ most are benthic in freshwater lakes and ponds
¾ many species can survive for long periods when desiccated (look like grains of sand) and
can tolerate extreme environmental conditions
Social Organization
¾ none
Communication
¾ none (?)
Thermoregulation
¾ none