nephridia
• Nephridia are segmentally arranged coiled tubes of ectodermal origin developed as invaginations from ectoderm into
coelom.
• They communicate with the exterior through laterally placed small apertures called nephridiopores.
• Nephridia are of two types, protonephridia and metanephridia.
• Protonephridia are closed nephridia as they terminate in the coelom as blind tubes. The closed end of protonephridium is
provided with peculiar specialised excretory tube cells or solenocytes. These are similar to flame cells of platyhelminthes.
• Protonephridia is found in all larval polychaetes and some adult polychaetes such as Vanadis, Phyllodoce, etc.
• Metanephridia are open nephridia, as instead of solenocyte, the inner end of metanephridium opens into coelom by a ciliated
funnel or nephrostome.
• Metanephridia are more advanced and found in the majority of polychaetes, all oligochaetes and leeches.
• Nephridia may be micronephridia or meganephridia on the basis of their size and number. Micronephridia or
meronephridia are smaller in size and found in oligochaetes. Meganephridia or holonephridia are larger in size and
found in polychaetes and leeches.
• Nephridia are termed exonephric or ectonephric when they directly open to the exterior through nephridiopores, such as
meganephridia of Nereis and Hirudinaria and integumentary micronephridia of Pheretima.
• Nephridia are termed enteronephric when they lack nephridiopores and open into the excretory canals or alimentary canal,
such as, septal or pharyngeal nephridia of Pheretima.
• Nephridia are primarily excretory in function but may secondarily serve to convey the genital products to the exterior.
metamerism
• When the segmentation in bilateral animals, such as annelids, involves a longitudinal division of the body into a linear series of
similar sections or parts, it is termed as metameric segmentation or metamerism. Each section or part is called a segment,
somite or metamere. Each metamere typically repeats some or all of the various organ units. The term metamerism is applied
only when organs of mesodermal origin are so arranged.
• Metameric segmentation of the body, encountered for the first time in Annelida, is of considerable interest because the most
successful groups of animal kingdom, i.e., Arthropoda and Vertebrata, also have their parts metamerically repeated.
• True metamerism, as shown by annelids, must not be confused with the pseudometamerism or strobilisation of the
tapeworms, which refers to superficial segmentation and could be termed body annulation.unique features
• They are first animals to have metamerically segmented
body.
• They are first animals to have true coelom.
• They are first animals to have a closed vascular system.
• Presence of haemoglobin or erythrocruorin
in plasma.
• They have nephridia for
excretion and osmoregulation.
• Presence of setae and
parapodia for locomotion.
• Morphologically and
anatomically very similar to
arthropods but embryologically
most similar to molluscs.
advanCement over round worms
• Presence of metameric segmentation.
• Presence of true coelom.
• Presence of circular and longitudinal muscles in both the
body wall and the wall of alimentary canal.
• Presence of locomotory organs such as setae
and parapodia.
• Schizocoelomic fluid as
hydroskeleton.
• Presence of cutaneous and
branchial respiration.
• Presence of circulatory
system.
• Nervous system consists of nerve
ring, nerve cord and ganglia.