Download Nematodes and Annelids

Difference Between Nematodes and Annelids
Both Nematodes and Annelids are invertebrates
with elongated bodies, and they prominently
feature among worms. Although they both are very
important animals for humans in many ways, the
knowledge about Nematodes and Annelids is much
low among people. Therefore, the majority of
people would not know how to distinguish these
animals from each other. Hence, the need to
understand their characteristics is high, and this
article would help a lot for that.
Nematodes
Nematodes, the members of the
Phylum: Nematoda, are also known
as Round worms. There are about one
million nematode species according
to some of the estimations, and
already 28,000 have been described.
The majority of the Nematodes
(16,000 species) are parasitic, and
that is the reason for the notoriety
about the round worms. The largest
member of the phylum is about five
centimetres long, but the average length is about 2.5 millimetres. The smallest species
cannot be observed unless there is the aid of a microscope. Nematodes have complete
digestive systems with the mouth at one end of the body while anus located at the
other end. The mouth is equipped with three lips, but sometimes the number of lips
could be six, as well. They are not segmented worms, but the anterior and posterior
ends are tapered or narrowed. However, there are few ornaments viz. warts, bristles,
rings, and other small structures. The body cavity of Nematodes is a pseudo coelom
(A fluid-filled body cavity), which is lined with mesodermal and endodermal cell
layers. The cephalisation or the formation of head to be distinctive from the other
body parts is not prominent among Nematodes, but they do have a head with nerve
centres. The parasitic species have especially developed some nerve bristles to sense
the environment that they live.
Annelids
Annelids are a large phylum
consisting segmented worms,
rag worms, earthworms, and
nuisance leeches. There are over
17,000 extant species of
Annelids presently. Usually,
they live in freshwater or
saltwater as well as around
moist terrestrial environments.
The body of an Annelid is
elongated with segmentation
has taken place externally
through ring-like constrictions.
These constrictions are called
as annuli, and they are
internally
segmented
or
partitioned via septa at the
same places as the annuli.
Their segmentation could be
regarded as the first sign of differentiation of body parts into different functions.
Annelids secrete their cuticle ( a kind of exo-skeleton) from their skin cells, and the
cuticle is made up of collagen but it is not as hard as in many other invertebrates.
Many scientists believe that they have capillaries to take blood through organs, and it
is a closed circulatory system in Annelids. They usually do not moult their cuticle, but
some species shed their skin (leeches) or jaws (polychaetes). Their body cavity is a
coelom but some Annelid species do not have a coelom and some have it in a very
small place. Their true coelom is lined with mesodermal tissues, and this is the first
instance in the evolutionary sequence where a true body coelom is found.
What is the difference between Nematodes and Annelids?
• Nematodes have been more diversified than Annelids.
• Annelids have larger bodies compared to Nematodes.
• Annelids have segmented bodies without
tapered ends, whereas the bodies of
Nematodes are tapered at both ends but
not segmented.
• Majority of Nematodes are parasitic but
not the Annelids.
• Body cavity of Annelids is a true
coelom while it is a pseudo coelom in
Nematodes.
• Annelids have parapodia to walk but
not the Nematodes.