Coelom and Nephridia in annelida

Coelom in Annelida:

coelom refer to the main body in the multicellular animals and is positioned inside the body to the surrounded and contain the digestive tract and other organs.In developed animals, it is lined with a mesodermal epithelium.In other animals, such as molluscs, it remains undifferentiated.

Structure:

Development:Coelom formation begins in the gastrula stage.The developing digestive tube of an embryo forms as a blind pouch called the archenteron.

In protostomes, the coelom forms by a process known as schizocoelyThe archenteron initially form, and the mesoderm splits into two layers:. the first attaches to the body wall or ectoderm forming the parietal layer and second surround the endoderm or alimentary canal forming the visceral layer. The space between the parietal layer and the visceral layer is known as coelom or body cavity.

In Deuterostomes, the coelom forms by enterocoele : mesoderm buds from walls of the archenteron and hollow become the coelomic cavities.

Origins:

The evolutionary origin of the coelom is uncertain. The oldest known animal to have a body cavity was the Vernanimalcula.Current hypothesis include:

• The acoelomate theory, which states that coelom,evolved from an acoelomate ancestor.
• The enterocoel theory, which states that coelom evolved from gastric pouches of cnidarian ancestors. The supported by research on flatworms and small worms recently discovered in marine fauna.

Function:

A coelom can absorb shock or provide a hydrostatic skeleton. It can support an immune system in the form of coelomocytes that may either be attached to the wall of the coelom or may float about in it freely. The coelom allows muscles to grow independently of the body wall -this feature can be seen in the digestive tract of tardigrades (also known as water bears) which is suspended within the body in the mesentery derived from a mesoderm - lined coelom.

Nephridia in Annelida:

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 nephridiopore. Internally they may lead blindly into the coelom (protonephridia) or may open into the coelom by a ciliated funnel or nephrostomes (metanephridia).Nephrostome may open into coelom either in the same segment in which it lies or in the segment just in front. Nephridia are primarily excretory in function but may secondarily serve to carry genital products to the exterior.Polychaeta excretory organs are either proto-nephridia or metanephridia.

• Protonephridia:

The 'closed ' or protonephridium is the primitive type.It terminates in the coelom as a blind tube,.Protonephridea, made of a few syncytial cells with an intercellular blind ending tubule, always develop in the larval polychaetes,The blind end is fringed with splenocytes. They are similar to the flame cell of platyhelminths and Rotifera.The splenocytes are round ciliated cells connected to the nephridium by a narrow tube.The lumen of a tube encloses a long vibrate flagellum. The excretory fluid enters through the walls of nephridium tubules which are internally ciliated. This fluid is then driven into the lumen of nephridium by flagellum and forced to the exterior through nephridiopore.Protonephridia are found in some adult Polychaeta such as Vanadis, Phyllodoce, Glycerin, Nephthys.

• Metanephridia:

The 'opened' or metanephridia are more, far advanced . They are found in the majority of polychaetes (Neanthes), all the oligochaetes (Lumbricus) and leeches. A meta- nephridia- is opened at the both ends. It opens into the coelom by a ciliated funnel or nephrostomy. The other end of it opens to the exterior through the nephridiopore.Principal nitrogenous product in Annelida is NH3.The excretory products diffuse from the coelomic fluid or blood into the lumen of .nephridia tubule. It is discharged to the outside through nephridiopore

Micro and Meganephridia:-

on the basis of their size and no. of nephridia are divided into two groups.

Micronephridea:-

These are smaller in size, sometimes microscopic, and are numerous in each segment. They are a network of fine tubes lying on the body wall and septa in each segment. All the nephridia of peritomy are micro nephridia.

Metanephridia/Holonephridia:-

These are larger in size. and generally one pair per segments.They usually extend over the 2 segments and their nephrostomes open into the segments next in front.They are found in polychaete and Hirudinea.

Exo and enter- nephridia:

Nephridia is termed as enteronephric when they lack nephridiopore and open into the excretory or alimentary canal, as septal and pharyngeal nephridia of peritomy.

Nephridia are termed as exo-nephric or too epic when they directly open to the exterior through nephridiopore.

Such as metanephridia of Nereis, Hirudinaria, Lumbricus and integumentary micro nephridia of peritomy.

Reference:

https://en.m.wikipedia.org/wiki/Coelom

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

https://web.stanford.edu/class/

https://en.m.wikipedia.org/wiki/Nephridium

http://ursadhip.blogspot.com/2011/09/nephridia-in-annelida.html