Phylum Nematoda PDF

Phylum Nematoda  Phylum Nematoda Nematodes are unsegmented round worms that possess a cuticle, which is a flexible, nonliving external layer that covers their entire body. When their body increases in size, the cuticle is molted and the outer layer shed, a process c...

Phylum Nematoda  Phylum Nematoda Nematodes are unsegmented round worms that possess a cuticle, which is a flexible, nonliving external layer that covers their entire body. When their body increases in size, the cuticle is molted and the outer layer shed, a process called ecdysis. Defining characteristics: Paired sensory organs on the head - Cephalic setae and Amphids Protostomes, triploblastic, bilateral, pseudocoelomates; Pseudocoelom in which internal organs lie, function as a hydrostatic skeleton; Moulting (Ecdysozoa); Parasitic and beneficial species; Free-living and parasitic; Terrestrial, aquatic, marine, in hosts; Most are less than 5cm length, but some parasitic can be over a meter; Protostome phyla are divided between two large clades: Lophotrochozoa and Ecdysozoa. Ecdysozoa: Comprises those taxa that molt cuticle as they grow. Molting is regulated by the hormone ecdysone. General Body Plan Most are 1-2 mm long, unsegmented worms; Their body is covered by a thick, non-living cuticle —> Made of collagen Growth Patterns Nematodes can increase in size between moults! Only moult 4 times in their life; Grow via eutely; *Eutely: Development proceeds by cell division until maturity. Further growth occurs via cell enlargement only. Physiology Gas exchange by diffusion, no respiratory or circulatory systems; Complete gut, metanephridia, well-developed nervous system; Muscles only run in one direction —> longitudinal muscles Excretory system may include gland cells and canals, opens via excretory pore; Ring of nerve tissue with ganglia around pharynx, dorsal and ventral nerve cords Sensory system includes papillae and setae for touch, anterior amphids and posterior phasmids are likely chemoreceptors. Reproduction Most are dioecious - separate sexes, fertilization is internal, but sperm are ameboid, not flagellated. Parthenogenesis occurs rarely (reproduction occur without fertilization); There are 4 juvenile stages, each separated by a shedding of the cuticle. Representative Nematodes Parasites Caenorhabditis Elegans: The most famous nematode Model organism for neuronal development; First organism to have the whole genome sequenced! Ascaris Lumbricoides - Largte round worm of humans Most common worm parasite of humans Adult Ascaris live in the small intestine of their host Can produce 200,000 eggs a day; Can cause pneumonia Hookworms Most common specie is Necator Americanus (L.necator, killer); Females are larger than males; Suck blood of their hosts, and heavy infection cause anemia; Hookworm disease in children may retard mental and physical growth, and cause loss of energy; Shelled embryos leave the host body in feces, and juveniles hatch in the soil, where they live on bacteria, when human skin contacts infested soil, the Juvelina burrow through the skin to the blood, reaching the lungs. Curiosities *Parasites are not all bad... a study shown that a mild hookworm infection could be used to combat gluten intolerance. *They also can be used as natural slug killer to control pests. Nemaslug is a brand name for packaged nematode eggs sold in Europe.