Body cavity or coelom

-:Body cavity or Coelom:-

The body of Nereis is transverse section looks like a tube within a tube . The wall of the outer large tube is body wall and the wall of the inner small tube is the gut wall . In between the two walls,lies a spacious cavity called coelom or body cavity, which is filled up with afluid .
The coelom is thus lined externally by parietal epithelium and internally by visceral or splanchnic epithelium. The coelom is devided into compartments by transverse partitions,called septa . The wall of each septum is perforated through which coelomic fluid communicates from one chamber to the other.
Coelom of Nereis is schizocoelic and filled with coelomic fluid having amoeboid corpuscles . It act as hydrostatic skeleton and help in transportation of digested food,gasses and waste materials.

-:Locomotion:-

In Nereis Locomotion takes place by parapodia, musculature and coelomic fluid . Nereis shows three types of locomotion -
Slow creeping:-
This type of locomotion is carried by parapodia . During locomotion each parapodia performs two strokes-
>Effective or back strok
> Recovery of forward strok.
Fast crowling :-
This type of locomotion is done by contraction of muscle
Swimming:-
Sometimes Nereis can swim on the water surface .

Digestive system of Nereis

Alimentary canal :-
The alimentary canal is complete and diffrenciated into following parts:-
I - Foregut* or stomodium :-
i. Mouth :-
   It is present on the ventral side of the perisomium as a transverse aperture and opens to the buccal cavity.
ii. Buccal cavity:-
          It is a broad chamber with cuticular linning. The cuticles have been thickened to form teeth or denticles or paragnaths . The buccal cavity leads into the pharynx.
iii. Pharynx:-
       It extends up to fourth segment and is also lined internally by cuticle. One pair of cuticular teeth is enlarged to form jaws at the posterior end of pharynx. The jaws are extended along the longitudinal axis of the body and are round at the base and pointed at the apex. The pharynx leads to the oesophagus.
II - Midgut or Mesenteron:-
iv. Oesophagus:-
It traverses through next five segment and receives a pair of large glandular caceca. The oesophagus communicates with the intestine.
V. Stomach and Intestine:-


A distinct stomach is absent. The stomach-Itestine is the principal site of digestion and absorption. It is more or less straight tube which is constricted at each segment. The constriction are intense at the posterior end.
III - Hindgut or proctodeum:-
Vi. Rectum:-
        In the last segment intestine continues as rectum. It is lined internally with cuticle and opens ti the exterior through an aperture, called anus.
Developmentally,the buccal cavity, pharynx and rectum originate from the ectoderm which also forms the outer covering. And for the reason ,like the outer wall,these are also lined by cuticle.
Food :-
    Nereis is carnivorous and feed upon small animals like- Molluscs, Sponges etc.
Feeding :-
        Nereis shows two kinds of feeding -
i. Filter feeding:-
       When Nereis live in U-shaped burrow,it feeds by ejection out a cone of mucus in front of the mouth and creat a water current by beating of Parapodia. The food particles are held in mucus cone and ingested.
ii. Raptorial feeding:-
       When the Nereis is on surface,it feeds by putting buccopharyngeal region of food particle.

-:Digestion Absorption and Egestion :-

            The engested food is digested by peristaltic action of gut wall and enzyme.
Digestion is extracellular. Digestion and absorption takes place into stomach- intestine. The undigested food material is engested out side through anus.

-:Respiratory system:-

     Gills are absent in Nereis. The function of respiration is taken over by the lobes of parapodia and dorsal integument. For this reason these regions are richly supplied with blood vessels.
                   The physiology of respiration is known in Nereis virens. It lives at a depth of 20-30 cm and almost oxygen- free sand. The animals draw water by producing water current while gaseous exchange takes place through the vascularized of regions of the parapodia.
                       When the oxygen pressure in surrounding water is equal to the Oxygen pressure of blood, gaseous exchange creases. This arrest of respiratory exchange is possible by resticting the blood flow only to the dorsal and ventral vessels. Nereis draws nearly 75%of the oxygen from water.

#shashankmishragonda

#shashank_mishra_gonda