Lecture 12: Phylum Nematoda (Nemata) The Nematodes 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

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 No spiral cleavage 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 No peristalsis! 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!