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Comparative studies of natural groups of organisms

significance to an organism: (d) The coelomic fluid may function as a
(a) It contains coelomic fluid which acts circulatory medium for transportation
as a hydrostatic skeleton. of food, gases, and excretory wastes.
(b) It separates the alimentary canal from (e) It provides space for enlargement of
the body wall so that the functions of internal organs.
the two parts take place independently. (f) In some animals, it plays an
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(c) It allows the animal to grow larger. osmoregulatory role.

Ectoderm
Body Somatic
wall mesoderm
Coelom

Peritoneum
Gut Splanchnic
mesoderm Gut wall
Endoderm

Gonad or Excretory organ
Mesentery Tubes leads to
the exterior of
reproductive
system

Figure 3.45: Transverse section of a generalised coelomate

Classes of phylum Annelida
The phylum Annelida has three classes namely: Class Polychaeta, includes the marine
bristle worms with many chaetae and parapodia. Examples are Nereis (the ragworm)
and Arenicola (the lugworm). Class Oligochaeta includes the earthworms and freshwater
worms, that have few chaetae without parapodia, for example Lumbricus (the common
earthworm), Tubifex (freshwater earthworm sometimes called sludge worm, or sewage
worm). Another class of annelida is class Hirudinea which includes segmented worms
with fixed number of segments, and have neither chaetae nor parapodia; an example is
Hirudo (the leeches).

Structure of earthworm
An earthworm is an elongated, tube-shaped, metamerically segmented organism (Figure
3.46). Its digestive system runs through the length of its body. It has a thin wet skin
through which gas exchange takes place. There is a double transport system composed

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