Page 181 - Biology_F5

Biology for Advanced Level Secondary Schools

of coelomic fluid that moves within the female reproductive organs. Has an
fluid-filled coelom. The circulatory system opening of spermotheca, which is found
is simple and closed. The worm has a in segments 9, 10, and 11. Seminal fluid
central and peripheral nervous systems. from another worm is passed through
The central nervous system consists of two the opening during copulation. Ventrally,
ganglia above the mouth, one on either side, on segment 14, there are very small
FOR ONLINE READING ONLY
connected to a nerve cord running back along slits, which are openings of the oviducts
its length to motor neurones and sensory (female opening) and on segment 15; there
cells in each segment. A large number of are openings of the vasa differentia (male
chemoreceptors are concentrated near its opening). Additionally, earthworm has
mouth. Circumferential and longitudinal clitella (singular clitellum) which secret
muscles on the periphery of each segment a cocoon in which eggs are deposited.
enable the worm to move. Similar sets of The mouth is overhang by a flap called a
muscles line the gut and their actions move prostomium. Movement is by means of
the food towards the worm’s anus. paired chaetae.
Earthworm is a hermaphrodite, meaning
that, the organism has both male and

Segments

Head Anus

Clitellum
(saddle)

Figure 3.46: Structure of the earthworm
Adaptations of earthworms to their (d) They secrete mucus which helps to
mode of life bind the walls of burrows and helps the
(a) They have ideal shapes with chaetae worm to pass through smoothly.
for burrowing. (e) They can thrust the sediments aside
(b) They have gizzard for breaking up when it is loose and consume it when it
plant tissues. is tight. This ability ensures burrowing
(c) They have clitellum, which holds under both conditions.
together earthworms during copulation (f) They are detritivores; as they feed on
and forms an egg depositer structure decaying organic matter such as dead
called cocoon to ensure efficient plant material, animals, fungi and bacteria
provision of offspring during in the soil. This feeding habit ensures
unfavourable conditions. sustainable food supply.

Form Five Student’s Book
174