Transcript PowerPoint 12: Nematoda 1

Invertebrate Zoology
Lecture 12: Phylum Nematoda (Nemata)
The Nematodes
Lecture outline
 Phylum Nematoda (Nemata)
Diversity
Phylogeny (briefly)
Bauplan Basics
Feeding
Circulation/Gas Exchange
Osmoregulation/Excretion
Nervous System
Movement
Reproduction
Diversity
 More individual nematodes than
members of any other phyla?
 Free-living forms found in nearly
every environment
Free-living marine & freshwater
Between grains of beach sand
Key soil dwellers (nutrient
processing)
Polar ice fields
 Key plant & animal parasites
Phylogeny: Hypothesis 1
Based on body plan & development
 Hypothesis 1:
Nematoda cross-section:
Note muscles, pseudocoelom & cuticle
Detour: Other Blastocoelomates
 Rotifera, Gastrotricha, Kinorhyncha,
Nematomorpha, Priapula
Phylogeny: Hypothesis 2
Based on molecular data, cuticle features
Bauplan basics
 Protostome features include
Triploblastic (distinct mesoderm  muscles)
Bilateral symmetry & cephalization
Cell fate is determinate
 No spiral cleavage
Cleavage 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 cuticle
Use of suction
 Digestive system usually
simple, not highly
branched
Intestinal 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
 Muscles
Longitudinal muscles
only
 Several along body
length
No peristalsis!
Must push against solid
surface to move forward
Elasticity of cuticle may
aid in movement
Movement
 Note antagonistic
muscle action
Right 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!