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!