Annelids are
the first EUCOELOMETE animals in the evolutionary order. The coelom is SCHIZOCOELOM
and it is divided into compartments by intersegmental septa.
Some
structures in annelids are arranged segmentally and are described as SEGMENTAL
STRUCTURES. Among them there are COELOMODUCTS and NEPHRIDIA .
These structures will be useful for the passage of reproductive and excretory
products from coelom to the exterior. The segmental structures mainly divided
into
I.
COELOMODUCTS
II.
NEPHRIDIA
COELOMODUCTS: These are generally wide
tubes of MESODERMIC origin and develop as ENVAGINATIONS of
coelomic epithelium. Each coelomoduct opens into coelom by a ciliated funnel
called COELOMOSTOME which opens out by a genital aperture. Coelomoducts
primarily function as GONODUCTS. So these will be limited to the with
reproductive structures. In some forms they function as excretory ducts.
NEPHRIDIA: These are generally coiled
tubes which are formed by the INVAGINATIONS OF ECTODERM. Each nephridium
coummunicates with the outside through a NEPHRIDIOPORE. If the
nephridium opens out into the coelom through nephrostome,it is described as
OPEN. If it is absent,it is CLOSED. Nephridia without nephridiostome is called PROTONEPHRIDIUM,
and if present it is METANEPHRIDIUM. Nephrostome may communicate with
prostome of the same segment or anterior to it. Nephridia are primarily
excretory(osmoregulation)in function and secondarily serves as REPRODUCTIVE
DUCTS(gonoducts).
Nephridia without a
nephrostome are considered as PRIMITIVE. In some cases,the closed end of
the tube may have SOLENOCYTES which occur singly or in group.
Nephridia with a
nephrostome are considered as ADVANCED. In a majority of polychaetes,it
opens into coelom through CILIATED NEPHROSTOME.
Based on size, nephridia
are classified into MICRONEPHRIDIA and MEGANEPHRIDIA. Generally
micronephridia will be more in segments and meganephridia will be anepair per
segment.
If nephridium communicates
outside through nephridiophore directly, it is termed as EXONEPHRIC, whose primary function is EXCRETORY and
if the terminal ducts ultimately open into alimentary canal it is called as ENDONEPHRIC nephridia, which are basically OSMOREGULATORY
in function.
MIXONEPHRIDIA OR NEPHROMIXIUM:
Incase
of oligocheates and hirudineans the nephridia and coelomoducts are separate
structurally and functionally. Where as the nephridia of polycheates and
aschiannelids, they function as excretory and reproductive ducts and so they
are described as NEPHROMIXIA or MIXONEPHRIDIA
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