7. STRUCTURAL
ORGANISATION IN ANIMALS
ANATOMY OF EARTHWORM
It is composed of
-
Outermost
thin non-cellular cuticle.
-
Epidermis: Made up of a single layer of columnar epithelial cells which contain
secretory gland cells.
-
Two muscle layers (circular and longitudinal).
-
An
innermost coelomic epithelium.
Digestive system
-
The
straight alimentary canal extends
from first to last segment of the body. It has
- Mouth → buccal cavity
(1-3 segments) → muscular pharynx (4th
segment) → oesophagus (5-7 segments)
→ muscular gizzard (8-9 segments) → stomach (9-14 segments)
→ Intestine (15th segment to
last) → anus.
- Gizzard
helps to grind soil particles, decaying leaves, etc.
- Calciferous glands,
present in the stomach, neutralise the humic
acid present in humus.
- A
pair of short and conical intestinal
caecae project from the intestine on the 26th segment.
- The
intestinal part between 26-35 segments has an internal median fold of dorsal wall
called typhlosole. It increases area of absorption.
- The
organic rich soil is digested in the digestive tract by digestive enzymes.
Digested nutrients are absorbed through intestinal membranes. Their faecal
deposits are known as worm castings.
Circulatory system (blood vascular
system)
- Circulatory
system is closed type (blood flows
through heart and blood vessels).
- Consists
of blood vessels, capillaries and heart.
- Contractions
keep blood circulating in one direction.
- Blood glands
are present on the 4th, 5th
& 6th segments. They produce phagocytic blood cells and haemoglobin
which is dissolved in blood plasma.
Respiratory system
- No
specialized system.
- Gas
exchange occurs through moist body surface into the blood stream.
Excretory system
Excretory organs are segmentally arranged tubules
called nephridia. They are 3 types:
a.
Septal
nephridia: Found
on both sides of intersegmental septa (segment 15 to last) that open into
intestine.
b. Integumentary nephridia:
Attached to lining of body wall (segment 3 to last). They open on body surface.
c. Pharyngeal nephridia:
Present in the 4th, 5th
& 6th segments in the form of paired tufts.
Funnel-shaped part of nephridium collects excess fluid from coelom. The funnel connects with a tubular part of nephridium which delivers the wastes into digestive tube.
Nervous system
- Includes
segmentally arranged ganglia on the
ventral paired and fused nerve cord.
- The
nerve cord in the anterior region (3rd
& 4th segments) divides and encircles the pharynx and joins
the cerebral ganglia dorsally to
form a nerve ring.
-
The
nerve ring with cerebral ganglia represents the brain.
-
Sensory system: Includes
§ Light and touch sensitive receptor cells. No eyes.
§ Chemoreceptors
(taste receptors): React
to chemical stimuli.
Reproductive system
Earthworm is hermaphrodite.
Male reproductive organs:
§ Testes: 2 pairs. Enclosed in testis sacs in 10th & 11th segments. The sperms from testes shed into testis sacs.
From where, they enter seminal
vesicles for maturation. Mature
sperms move back into testis sacs and enter spermiducal funnels which
are connected to vasa deferentia (spermatic ducts). The vasa deferentia run up to 18th
segment where they join the prostatic
duct.
The common prostate and spermatic duct open to the exterior by
a pair of male genital pores on the
ventro-lateral side of the 18th
segment.
§ Accessory
glands: 2 pairs. Found in
the 17th & 19th
segments (one pair in each segment).
Female reproductive organs:
§ Spermathecae: 4 pairs. Located in 6th-9th
segments (one pair in each
segment). They receive and store spermatozoa during copulation.
§
Paired
ovaries: Attached at the inter-segmental septum of the 12th and 13th segments.
§
Ovarian funnels: Present beneath the ovaries which continue into oviduct,
join together and open on ventral side as single median female genital pore on 14th segment.
-
During
mating, two worms exchange sperms each other. They mate juxtaposing opposite
gonadal openings exchanging spermatophores
(packets of sperms).
- Mature
sperm, ova and nutritive fluid are deposited in cocoons produced by gland cells of clitellum. Cocoon with
fertilized ova slips off and deposit in the soil. After 3 weeks, cocoon
produces 2 to 20 baby worms (no larva).
ECONOMIC IMPORTANCE
-
Earthworms
are known as ‘friends of farmers’
because they make burrows in the soil and make it porous which helps in
respiration and penetration of the plant roots. This process of increasing
fertility of soil is called vermicomposting.
-
They
are used as bait in game fishing.
