PHYLUM NEMATODA

Chapter 6

Phylum Nematoda

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• Round or Thread worms

• size 1-2 mm mostly but some may reach 60 cm or more

• Pseudocoelomates

• non-sigmented

• Free living and parasitic species

• Pointed at both ends

• Covered by a thick multilayered cuticle (non-cellular covering)

• Epidermis is syncytial (secretes cuticle)

Nematode life cycle

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• Cuticle is shed 4 times during development

Musculature

• Lack circular muscles • muscular layer (longitudinal muscles) that arrange in 4

groups separated by the dorsal, ventral and lateral hypodermal chords, each muscle cell connected to either the dorsal or ventral nerve chord by muscle cell process;

Movement and hydrostatic skeleton

• Movement is by thrashing the body into sinusoidal waves generated by alternating contraction of longitudinal muscles on each side of the body.

• The round shape of nematodes is due to the hydrostatic pressure generated by celoemic fluid and its opposing rigid cuticle.

Nervous system

• Nervous system made of brain (nerve ring and associated ganglia and at least 4 longitudinal nerves that run in the dorsal, ventral and lateral nerve chords in the hypodermis

• Sense organs include a pair of head chemoreceptive amphids (characteristic feature of all nematodes), other sense organs found in certain groups include: posteriorly located chemoreceptive phasmids, ocelli, cephalic and caudal papillae as well as mechanoceptors

Nematode Features II • Eutely: Cell number in adult tissue remain constant throughout

life so that the limited increase in size is a function of increase in cell size NOT number).

• Tubes within tubes worms, all organ systems tubular;

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Nematode Features

• Digestive system complete with mouth, muscular pharynx (esophagous), intestine and rectum;

• Excretory system made of renette glandular cells in most spp;

• No specialized gas exchange or circulatory system.

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Nematode Reproduction & Life Cycle • Worms are mostly dioecious;

• Male reproductive system tubular and made of testis, vas deferens, ejaculatory duct that opens in cloaca, many spp. have bursa and 1-2 copulatory spicules;

• Female reproductive system tubular and made of one or two ovaries, each leads into oviduct--seminal receptacle—uterus, a vulva leads to female gonopore.

• Fertilization internal following copulation or hypodermic impregnation;

• Females are oviparous, ovo-viviparous or viviparous;

• L.C. involves ecdysis of larval stages controlled by molting hormones with 4 larval stages before worms become adults; some larval stages can molt within the egg shell; Parasitic stages mostly use the third larval stage (L-3) as infective stage to the final host..

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Classification of parasitic Nematodes

• Class Chromadorea

(3 sub-classes) Sub-class: Rhabditia • Orders: 1. Rhabditida –

• 2. Strongylida - Ancylostoma (hookworm), Enterobius (pin worm)

– Sub-class: Spiruria – • Orders: 1. Spirurida - Filarioidea - filarial worms. Wuchereria

bancrofti (elephantiasis), Onchocerca volvulus (river blindness), Dirofilaria immitis (dog heartworm).

• 2. Ascaridida - intestinal parasites of vertebrates. Ascaris. Ascaris, Toxocara canis (dog ascarid)

Debilitating effect of parasitic Nematodes

• Blood suction. E.g Hook worms

– Suck about 0.6 ml blood/day

– 100 worms= 60 ml/day= 180 ml/month

• Blocking of intestine and bile ducts. E.g ascaris

– In cases of heavy infection.

• Blocking of blood or lymphatic system. E.g Filaria

– Elephantiasis as in Onchocerca volnulus

Trichuris trichura L.C.

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Hookworm L.C.

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A. Lumbricoides L. C.

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E. vermicularis L.C.

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O. volvulus L.C.

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Wuchereria bancrofti L.C.

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