Download Platyhelminthes: The Flatworms

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The Nematodes
I. Introduction
A. Another example of where phylogeny and taxonomy are in dispute
B. Nematodes our first example of pseudocoelomate animal - although there is now
some dispute as to whether all nematodes are pseudocoelomate
C. Unsegmented
D. Diversity and abundance: Incredibly ubiquitous – the most abundant multicellular
animal
1. Can reach concentrations of up to 4 million / square meter of some habitats.
2. They are successful in every type of environment –
a) Both freeliving and parasitic
(1) Predators, herbivores, detritivores
(a) One of the planets’ primary decomposers
b) From the tropics to the poles
c) Marine, freshwater and terrestrial
d) If took away all of the planet’s biomass except nematodes, there would
remain an outline of everything in nematodes
3. Thus far 20,000 species described but there are likely up to 50 times that
number – but difficult to distinguish different species because of morphological
similarity
E. For a long time placed in a phylum of Aschelminths – with several other types of
morphologically dissimilar animals including rotifers – which we will cover on
Wed.
1. Features common to aschelminths include
a) Small
b) Pseudocoelomate
c) Syncytial epidermis - cells not separated by cell membranes
d) Eutely – constancy of cell numbers
(1) Once organogenesis is complete, mitosis ceases and animal grows only
by increasing cell size – not cell number
e) Cryptobiosis – suspended animation where animal can go into a dormant
state to resist dessication
f) Interstitial animals – animals adapted to living in between sand grains or
particles of soil
2. Now evidence that Nematodes are very closely aligned with arthropods – in fact
this lecture should perhaps come later in the quarter
a) centers around act of molting - now thought to have arisen only once in
animal evolution
b) also arthropods and nematodes have similarities in striated muscles
F. Very important study organism for developmental biologists – and we’ll develop
this as we go through the lecture
II. Morphology
A. Typical freeliving nematode is only 1 – 2 mm long with no segmentation tapered at
both ends
B. Covered in a thick multilayered cuticle of collagen that is secreted by the epidermis
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1. show fig 16.2
2. Anchored to epidermis 3. Gas exchange takes place across cuticle and epidermis – no specialized gills
4. Cuticle not resistant to dehydration – so animals restricted to aquatic
environments or moist terrestrial ones.
5. Molted 4 times during development – then grow another
C. Musculature
1. Oddly nematodes lack circular muscles
2. Also lack cilia on epidermis
3. So limits on types of locomotion can do with just longitudinal muscles
4. Also oddly – muscles are not ennervated in a typical fashion – rather there are
processes extending from muscles cells that insert into nerve cords
a) Show Fig 16.4
D. Locomotion
1. Because lack circular muscles – can’t use peristaltic motion like an earthworm –
that we’ll learn about later. Also can’t use ciliary motion – like flatworms
2. Most move by sinusoidal contraction of alternate sides of longitudinal muscle
a) This motion aided by the high hydrostatic pressure present in the pseudocoel
– good use of hydrostatic skeleton – animal springs back after contraction
b) This is quite effective motion in their interstitial environments
3. Fig 16.3
E. Nervous system
1. Some cephalization – simple nerve ring and ganglia at the anterior
2. Four nerve cords – dorsal ventral and lateral nerve cords
3. Anterior end also holds some sophisticated chemosensory organs
a) Sensillae – body bears numerous projections of the cuticle that serve as
mechanoreceptors See web
b) Amphids – pits at the anterior tip that have specialized chemosensory cilia
nematodes excellent at chemoreception – detecting food in their
environment. See web
F. Presence of pseudocoel where internal organs are housed
1. Fluid is pressurized
2. Pseudocoel fluid can serves as circulatory fluid and often contains hemoglobin
a) Hemoglobin binds O2 for transport
b) No other form of circulatory system
G. Digestion
1. Our first example of a complete digestive tract – mouth to anus
2. Pharynx very muscular,
a) Because of high pressure in pseudocoel, food has to be pumped through gut
by drinking and pumping water through gut
b) Also presence of a buccal cavity that is lined with cuticle
c) mouthparts varied depending on food type See web
3. Non-ciliated – instead many microvilli
4. Digestion is extracellular – like ours and unlike animals up until now
a) Nutrients absorbed across gut lining
III. Reproduction and development
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A. Most nematodes are dioecious
B. Reproductive organs are well developed and animals undergo copulation
C. Male has testis that reaches outside via a cloaca – also for gut waste
1. Has a copulatory spicule that holds open female opening during copulation
See web
D. Female opening or vulva not at very posterior of body rather on side - ovaries open
into this
See web
E. Male grabs female and wraps body around hers
F. Inserts sperm with help of spicule
G. Interestingly sperm are not flagellated – are ameboid
H. Development – no larval stage
1. Eggs hatch into little worms
IV. Nematodes as models for the study of development
A. Special features of nematode development make them wonderful models for the
study of metazoan development
B. Caenorhabditis elegans a freeliving soil nematode is the focus of at least 60 major
labs worldwide
C. Why?
1. Eutely – as mentioned earlier means that all cells cease mitosis after
development is complete
a) Adult C. elegans have exactly 959 cells and the fate of every single cell has
been mapped
(1) E.g. there are 302 nerve cells, 172 digestive cells etc.
(2) The development of each of these cells is entirely described
(3) Can figure out exactly what happens when certain genes are turned on
through development and through the lifespan of the worm
b) This is a very powerful experimental system to work with
2. Only 6 chromosomes – so not very much genetic material to have to work with
a) Complete genome has been sequenced entirely
3. Short generation time – about 3 days in the lab make them great for genetic
studies
4. Can grow them at very high densities – about 10,000/petri dish so can work
with a lot of material easily