Describe the excretory system of Pheretima.
Ans. Excretory System : Excretory organs are the nephridia. Nephridia
occur in all segments excepting the first three and the last segments. The
nephridia are small and coiled tubular structures and occur in huge numbers.
In the body of earthworm three kinds of nephridia occur - septal,
integumentary and pharyangeal, Structurally, these nephridia show basic
similarities, its classification is based on its position in the body. The types of
nephridia that occur in earthworm are :
(a) Septal Nephridia Septal nephridia remain attached to the two faces
of the septum. They occur from 15th segment backward. That means in the
first fourteen segments they are absent. Each septum bears 40-50 nephridia ir
average in its anterior and a similar number on its posterior face. Thus in each
segment there are about 80-100 nephridia.
Structure of Septal Nephridium : A typical septal nephridium consists
of a main body formed by a straight lobe and a long narrow, spirally twistec
loop, a funnel like nephrostome connected to the main body by a short neck
and a terminal nephridial duct. All the nephridial structures remain restricted
to the same segment.
Fig. A septal nephridium of earthworm (after Bhal).
The nephrostome or funnel is a rounded structure. The mouth of the funnel
which communicates with the coelom is provided with a large upper lip and a
small lower lip. The lips are ciliated. A narrow ciliated tube runs from the
funnel into the body of the nephridium and takes several turns inside it.
The main body of the nephridium is made up of a main lobe and a spirally
twisted loop. The loop is twice as long as the main lobe and consists of a
proximal limb and a distal limb twisted round each other. The straight lobe is
continued as the distal limb of the twisted loop and the proximal limb receives
the ciliated tubule from the nephrostome and also it gives off the terminal cuct
which opens at the nephridiopore. The straight lobe bears four parallel tubules,
the proximal and distal ones bear three tubules each and in the apical part there
are two tubules.
Terminal ducts of the septal nephridium open into a septal exertory canal
There are a pair of septal excretory canals one on each side of the scptum.
which runs parallel and internal to commissural vessels.
The two septal excretory canals open into a pair of supra intestinal excretory
ducts which turn on the mid-dorsal line side-by-side from 15th segment to the
posterior end. The supra-intestinal excretory ducts open into the lumen of the
intestine by single and small ducts at the level of each intersegmental septum.
(b) Integumentary Nephridia : They are smaller in size than the septal
nephridia. These ‘V'shaped structures occur on the inner surface of the
integument in all segments excepting the first two. They occur 200-250 in
each segment but in the 14th, 15th and 16th segments the number of nephridia
is much more. Structually they resemble septal nephridia but lack the
nephrostome. They open independently to the outside by nephridiopores on
the outer surface of the body wall.
(c) Pharyngeal Nephridia: They are as large as the septal nephridia and
occur in the forms of three pairs of bunches or tufts in the 4th, 5th and 6th
segments and on either side of pharynx and oesophagus. Nephrostomes are
also absent in the pharyngeal nephridia. In each bunch the terminal ducts of
the nephridia join together to form a slender duct. The slender ducts again
unite in each segment and form a thick walled duct which opens into the
alimentary tube. Thus there are three pairs of ducts, one pair each in the 4th,
5th and 6th segments. Some workers maintain that the pharyngeal nephridia
have digestive function or, in other words, they and in digestion and hence
they are sometimes referred to "peptic nephridia”.
The septal and pharyngeal nephridia open into the alimentry canal and are
called enteronephric while the integumentary nephridia open to the outside
directly and are called exonephric. The enteronephric system helps in the
conservation of water in the body because the water present in the excretory
product is again reabsorbed in the intestine.
Some of the nitrogenous excretory substances like guanin are extrimeted
from the blood stream by chloragogen cells. These cells collect and store
excretory products and on becoming heavily laden with excretory materials,
they pinch off into the coclomic fluid from where they are eliminated through
dorsal pores or by nephridiopores.
No comments: