Nephridia are excretory organells of earthworm. Nephridium is
coiled tubular and glandular structure. A typical nephridium consists of
nephrostome, body and terminal duct. In some nephridia nephrostome is lost. The
terminal duct may opens inside body or out side body. If terminal duct opens outside
body, nephridium is called as exonephric. If opens inside body, nephridium is
called as enteronephric.
In earthworm, three tyes of neohridia are present:- septal
nephridia, pharyngeal nephridia and integumentary nephridia.
· Septal nephridia:- These are nephridia present on the surface of septum lying behind 15th segment. They are present on both surface of septum and two rows are present on each surface. Each row contains 20-25 nephridia so about 80 to 100 nephridia are present on each septum. These are large sized nephridia and consists of nephrostome, body and terminal duct.
Nephrostome is ciliated funnel
shaped structure which opens into coelom. Its opening remains bounded by upper
lip and lower lip. The upper lip consists of a large central cell and 8-9
marginal cells. The lower lip consists of 4-5 compact cells. All these cells
are ciliated.
Body is main part and consists of straight limb and twisted loop with a
narrow apical part. The length of twisted loop is about double the length of
straight limb. The twisted loop consists of proximal and distal limb spirally
twisted with each other. The proximal limb remains connected with neck while
distal limb remains connected with terminal duct.
Terminal duct is a short tubular
structure which continues from distal limb.
Nephridial tubule:- Inside nephridium coiled nephridial tubule is present in
glandular mass. Four tubules are present in straight limb, three tubules in
proximal and distal limb, two tubules in apical part and one tubule in each of
the neck and terminal duct. The tubule remains ciliated at four places; two in
straight limb, one in neck and one in terminal duct.
These nephridia remain suspended in
coelom of each segment and their terminal ducts open into a pair of septal excretory canals and these
canals open into a pair of supra-intestinal excretory ducts situated side bu
side mid-dorsally just above the intestine from 15th segment upto
posterior end of body. These ducts open into intestine in each segment. Thus
these nephridia are enteronephric.
·
Pharyngeal nephridia:- These nephridia occure as paired
tufts on either side of pharynx and oesophagus in the 4th, 5th
and 6th segments. Each tuft consists of hundreds of coiled branched
tubules without nephrostome. The terminal ducts of all nephridia of a tuft
unite with each other to form pharyngeal nephridial duct. Thus three pairs of
pharyngeal nephridial ducts are present and they run towards anterior side. The
pharyngeal nephridial ducts of 4th and 5th segment opens
into pharynx while of 6th segment opens in to buccal chamber. Thus,
these nephridia are enteronephric.
·
Integumentary nephridia:- These are nephridia scattered on
body wall except in first two sements. About 200 to 250 nephridia are present
in each segment but in clitellum more than 2000 nephridia are present forming
"forest of nephridia". These are small v-shaped nephridia without
nephrostome. The terminal ducts of these nephridia directly open into body
surface by nephridiopores. Thus, these nephridia are exonephric.
Excretion:- The gland cells of nephridia extract excess water and
nitrogenous wastages from blood. The septal nephridia also eliminate wastage
from coelomic fluid. The integumentary nephridia directly discharge
wastage through nephridiopores. In pharyngeal and septal nephridia the
wastage is discharged into alimentary canal and passes out through anus along
with wormcastings.
The excretory material contains 40%
urea, 20% ammonia and other substances.