Excretion in invertebrates
Excretion in
Hydra:
In hydra
nitrogenous waste is in the form of ammonia. Almost all the cells of Hydra are
in direct contact with water, so ammonia is removed by simple diffusion from
the external surface as well as internal surface into external and internal
water of gastrovascular cavity.
Excretion in
Plavaria:
Free living
flatworms like plavaria have developed a tubular excretory system. In Plavaria
the excretory system consists of two longitudinal branching tubules or
excretory canals lying on either lateral sides and extending along the entire
length of the animal. Each tubule opens to the external surface of the animal
by many excretory pores. Internally in the mesenchyme, each tubule gives rise
to numerous blind bulb like cells called flame cells or pronephridia which are
bathed in the tissue fluids. Each flame cell is hollow inside and bears a tuft
of cilia which beat in manner like flickering flame (hence called flame cell),
water along with ammonia diffuse from the tissue fluid into the lumen of flame
cells. The beating of cilia propels this solution into excretory canal where it
is to be excreted out by excretory pores.
During the
movement of excretory fluid, water is being reabsorbed if required by the
animal and the rest of the excretory fluid (urine) is passed out in the form of
hypotonic solution. Thus it seems that flame cell functions mainly in
osmoregulation and most metabolic wastes are removed from body surface or
excreted in the gut where they are removed through the mouth along with
undigested food. In some parasitic flatworms which are isotonic to the body
fluids of their hosts, the flame cell, perform excretion of nitrogenous wastes.
Excretion in
earthworms:
Earthworms have
combined excretory and osmoregulatory organs called metanephridia, which are
arranged segmentally. Each metanephridium is highly coiled tubule immersed in
coelomic fluid and surrounded by a network of capillaries. It is opened at both
the ends. Its internal opening called nephrostome, which lies in the coelom is
a ciliated funnel like structure. While the external minute opening or
nephridiopore opens outside in the skin. The coiled tubular part dilates
finally to form a bladder before opening to outside through nephridopore. Due
to beating of cilia of nephrostome, coelomic fluid is pumped into excretory
tubule. Some excretory substances are also secreted by cells of tubule. Here
selective re-absorption of useful substances also occurs which are taken back
by the blood into the circulation. Finally the excretory fluid (urine) is
emptied in the bladder which excretes it outside through nephridiopore.
Excretion in
Cockroach:
Cockroach and
other insects have excretory system which consists of long, thin blind tubules
called malpiglian tubules which arise from the junction of midgut and hindgut.
Malpiglian tubules are immersed in the haemolymph (fluid of body cavity), cells
of malpiglian tubules absorb excretory wastes along with some useful substances
present in haemolymph. In the latter part of the tubule selective re-absorption
of useful substances occurs and the uric acid is discharged into the rectum.
Rectum stores uric acid for re-absorption of salts and water, so the uric acid
becomes almost dry and pass out along with faeces.
Comments
Post a Comment