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Biology for Advanced Level Secondary Schools

(f) It respires anaerobically, and have low 3.7.3 Phylum Annelida
metabolic rate; hence, they are able to Members of this phylum are known as
live inside the host’s intestine. ringed or segmented worms. This is a large
(g) It has high reproductive potential; thus, phylum comprising lugworms, ragworms,
by producing large number of eggs, it earthworms, and leeches. The species
ensures its survival and existence. are adapted to various habitats; some
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(h) The pharynx has valves, which prevent members are aquatic, living in marine
regurgitation of the ingested food. and freshwater, and others live in moist

Economic importance of phylum terrestrial environments.
Nematoda
Members of the phylum Nematoda are Characteristics of phylum Annelida
associated with both advantages and (a) They are triploblastic, coelomate
disadvantages as follows: organisms.
(b) They are bilaterally symmetrical.
Advantages of phylum Nematoda (c) They are metamerically segmented.
(a) Nematodes, such as Ascaris are used (d) They have a definite outer covering
in biological studies in different ways; called cuticle.
for instance, in laboratory practical
studies. (e) They have chitinous hair-like structures
called chaetae (except in leeches), the
(b) Some nematodes are used as biological chaetae of polychaetes are found on
control agents for insect pests. They structures known as parapodia.
attack insects and kill or hinder their (f) They have a lip-like extension on
development. the first segment above the mouth
(prostomium).
Disadvantages of the phylum Nematoda (g) They have nephridia as excretory
Some Nematoda are parasites and cause organs.
diseases in plants and animals. Examples (h) They have central nervous system,
include: with paired cerebral ganglia in which
(a) Wuchereria bancrofti causes elephantiasis. paired commissures around the gut
This disease infects the human lymphatic lead to a double-ventral nerve cord,

system by blocking it; resulting in the expanded to form a ganglion in every
accumulation of the lymph in legs, hands, segment.
and or scrotal sacs. Body plan of coelomates
(b) Ascaris lumbricoides causes ascariasis In annelids and other coelomates, there is
in humans that may lead to obstruction an extensive internal space or body cavity
in the gut and anaemia. called a coelom. This is the space between
(c) Meloidogyne incognita causes the body wall and the alimentary canal

root knot galls in plants that drain (Figure 3.45).
photosynthetic products; hence, affect Functions of the coelom
plant growth. The coelom has the following biological

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