Invertebrate Zoology Lecture 12: Phylum Nematoda (Nemata) The Nematodes
Invertebrate Zoology
Lecture 12: Phylum Nematoda (Nemata)
The Nematodes
Lecture outline Phylum Nematoda (Nemata)
DiversityPhylogeny (briefly)Bauplan BasicsFeedingCirculation/Gas ExchangeOsmoregulation/ExcretionNervous SystemMovementReproduction
Diversity More individual nematodes than
members of any other phyla? Free-living forms found in nearly
every environmentFree-living marine & freshwaterBetween grains of beach sandKey soil dwellers (nutrient
processing)Polar ice fields
Key plant & animal parasites
Phylogeny: Hypothesis 1Based on body plan & development
Hypothesis 1:
Nematoda cross-section:Note muscles, pseudocoelom & cuticle
Detour: Other Blastocoelomates
Rotifera, Gastrotricha, Kinorhyncha, Nematomorpha, Priapula
Phylogeny: Hypothesis 2Based on molecular data, cuticle features
Bauplan basics Protostome features include
Triploblastic (distinct mesoderm muscles)Bilateral symmetry & cephalizationCell fate is determinate
No spiral cleavageCleavage pattern unique to Nematoda
Blastocoel forms blastocoelom (=pseudocoelom)Feature shared by several phyla
Bauplan basics Cuticle
In common with Arthropod cuticle: Contains chitin Secreted by epidermis Several layers Molts with growth
Ecdysone is key hormone
Fibers form flexible coil elastic qualities for movement
Varies in appearance
Feeding/Digestion
Simple mouth, or with stylet, teeth, jaws, etc…
Muscular pharynx lined with cuticleUse of suction
Digestive system usually simple, not highly branchedIntestinal microvilli increase
surface area Some with bacterial
symbionts & reduced digestive system
Feeding/Digestion
Gas exchange Body surface Pseudocoelomic (=blastocoelomic)
cavity Exchange between pseudocoelomic
(=blastocoelomic) fluid and organs
No circulatory system
Osmoregulation/excretion System of lateral cords
connected to a large cell ("renette")
Associated with digestive system
Not well-studied Empties via excretory
pore Excretion also via the
digestive system
Nervous system Organization
Nerve ring Plus associated ganglia Sensory nerve input (from head)
Four major nerve cords: 1 dorsal, 1 ventral, 2 lateral Ventral cord may be fused & with ganglia (as in
Annelida and Arthropoda) Connection of longitudinal muscles is
unique Muscle arms extend to nerves!
Nervous system
Muscle arms attach to dorsal and ventral nerve cords
Sensory structures Anterior
Amphids (chemosensory?) Narrow canal leads to cell
Modified cilia: non-motile! Non motile
Ocelli (some) Additional sensory
structures on entire body Cuticular bristles and
papillae Caudal (on parasites)
Phasmids (chemosensory?)
Movement
MusclesLongitudinal muscles
only Several along body
length
No peristalsis!Must push against solid
surface to move forwardElasticity of cuticle may
aid in movement
Movement
Note antagonistic muscle actionRight contracted
left relaxed
Reproduction Sexual reproduction
Sexes separate Single pair of highly coiled gonads internal fertilization
Male has cuticular spicules
sperm lack flagella (amoeboid) Oviparous or ovoviviparous High incidence of parthenogenesis
In some, mating occurs but sperm nucleus not used.
Some hermaphrodites
Parasitic life cycles
Next time!