Nematoda Flashcards
(34 cards)
Why is Nematoda called Nematoda
Because of thread-like appearance
Body characteristics of Nematodes
- head has paired lateral sensory organs from cilia and opening to the outside via a small pore
- body is elongated and round “roundworms”
- ubiquitous, externally unsegmented, acoelomate and pseudocoelomate worms
- tapered at both ends
- covered by thick, multilayered cuticle of collagen secreted by epidermis
- small (1-2mm) but parasitic ones can reach several centimetres
Habitat of Nematodes
Terrestrial and aquatic
Aquatic: fresh or marine, abundant in deep sea, hundreds to thousands per meter below ocean surface. Could be up to 1M per square meter in freshwater and 3M in marine
Terrestrial: top soil or deep soil or gold mines
-rotten apple could hold up to 90,000
Nematodes are one of the most ________ multicellular animals still in existence
Abundant
Lifestyle of nematodes
- most are free living in soil or water
- others are parasites to vertebrates, invertebrates and plants
Why are nematodes in own phylum?
- originally placed in Aschelminths because of cuticle and thought to be monophyletic
- also thought to be placed with Arthropoda simile phylum’s because of molting
- recent molecule finding shows nematodes originated from different ancestry than aschelminths
Pseudocoel fluid when present may function as a _______ in nematodes because they lack a _______
Circulatory medium
Some contain hemoglobin
But lack closed circulatory system
Nematodes organs are never wrapped by the mesodermal lining of the coelomic cavity called a
Peritoneum
Nematodes multilayered cuticle characteristics
- epidermis is syncytial; nuclei not separated by complete cell membranes
- cuticle in some composed of highly complex network of fibres that are virtually inelastic; causing ability to bend, stretch and shorten the cuticle
- cuticle permeable to water and gases; gas exchange across entire body
- cuticle selectively permeable to ions and organic substances for maintaining a stable internal and external environment
How often do nematodes molt?
Shed their cuticle periodically (molting); 4x between juvenile and adult stages
Because juvenile stages are morphologically distinct between each judge illy stages and the adult
Eutely
Special kind of growth where an organism grows by increasing the size of individual cells, rather than in cell number.
Common in tardigrades, rotifiers and nematodes
Parasitic nematode molting
- first 2 stages are free-living, forming envelope prior to 2nd molt
- exsheathment (releasing the external surface covering of the worm known as sheath), followed by last 2 molts only occurs after ingestion by definite host
- in definite host, the sheath is replaced by a microvillar surface; facilitating nutrient absorption from the host gut
Nematode musculature
-nematode body packs circular muscles; which separates them from other vermiform (worms); places great limitations on their locomotory potential in that, for example, they cannot generate peristaltic waves of contraction
Nematode internal pressure and locomotion
- turgid pseudocoel: ability to generate a substantial hydrostatic pressure within the paeudocoel
- high pressure generated by rigidity of the cuticle and the constant partial contraction of the musculature, trying to compress an incompressible fluid
- high internal pressure gives the nematode a very circular cross section which makes it roundish
Nematode movement
- move by contracting longitudinal muscles on each side in an alternate manner creating a series of sinusoidal waves which propel them forward
- contraction of one part of the muscle causes bending and stretching on other side of the body
- muscle antagonism occurring on different part of the body caused pressure changes transmitted through hydrostatic skeleton which controls movement
Nematode digestive system
Possess a straight digestive system, with a mouth (stoma) at anterior end leading in sequence through a muscular pharynx , intestine, and rectum, and then an anus located near the body posterior end
- pharynx added lubricants and digestive enzymes to the food as well as kept the gut lumen open against the high pressure of the pseudocoel
- digestion mainly extracellular, and nutrients are absorbed by the very thin wall (only one cell thick) of the intestine. Wastes are voided from the anus at intervals, about once every one or two minutes
Nematodes lack _____ and _____ systems
Instead…
Respiratory and excretory
Metabolic wastes are apparently discharged along with other materials leaving the gut, or they diffuse across the body wall
Renette
A hypothesized excretory structure which acts as a nephritis in the nematode in eliminating metabolic wastes
Amphids
Phasmids
Anterior chemosensory organ lined with non-motile cilia called sensillae in nematodes
Posterior chemosensory organ in nematodes
Cephalic and caudal papillae
Sensory structures present at the anterior and posterior ends, respectively in nematodes, containing modified cilia for sensory role
Pheromones
Despite nematodes small size, various species are known to respond to temperature, light, mechanical stimulation and a variety of chemical cues, including those produced by other individuals of the same species
Setae in nematodes role
Mechanoreceptor
Nematode reproduction
- mostly dioecious (separate sexes) and individuals copulate so fertilization in internal
- others can reproduce by parthenogenesis, without the need for mating partners
Nematode development
- lack free-swimming larval stages, young animals emerge from their sturdy egg coverings as miniature adults
- many can enter stage of arrest called dauer larva, at the second molt. These are basically inactive and have low metabolic rates and cannot feed. May lie for many months until conditions improve which then normal development resumes and final 2 molts occur
- Eutely
