LIFE HISTORY OF NEREIS/NEANTHES

- Nereis is a unisexual polychaet. Their ( and ovaries) appear only during breeding season (Summers) so there are no regular ducts to carry gametes to the exterior. Their gametes are discharged through temporary aperture formed in the dorsal body wall in gonad bearing segment or through nephridiopore.

- Nereis undergo remarkable changes in external form and behaviour during breeding season called swarming e.g., eyes enlarge, parapodia of posterior half get modified into swimming paddles, older setae are replaced by large, numerous setae. The posterior half of the body is called epitoke and anterior half with normal parapodia is atoke. This shape of Nereis is called Heteronereis.

Life History of Nereis Show Following Stages :

- The newly discharged eggs are rich in oil droplets and yolk spherules. They are covered by zona radiata.

- Fertilization takes place in sea water and results in disappearance of zona radiata. -

- Cleavage in Nereis is spiral. The first two cleavages are vertical and third is horizontal much above equatorial plane of blastomeres. The next three divisions are also horizontal and separate three more quartettes form macromeres. One micromere in second and one in the fourth quartette is larger than sister micromeres, called Ist and 2nd somatoblast respectively.

- Micromeres will give rise to ectoderm and macromeres form endoderm. The second somatoblast make the entire mesoderm. 

- Now micromeres multiply to cover the macromeres completely and the embryo turned into two layered gastrula (epibolic process). - Now gastrula changes to a small, transparent, ciliated larva of oval shape. It lacks the traces of segmentation and coelom. This is a typical trochophore larva. In Nereis, fate of every cell is predetermined.

- Trochophore larva swim freely for few days and feeds on micro-organisms. It undergoes metamorphosis. It develops setigerous sacs on its lateral side which give rise to larval setae. Third pair of setigerous sac with projecting setae is soon added. Constriction appears between setigerous sacs marks the first three segments. Body elongates and new segments are added along with setigerous sacs. Later on larval setae and larval nephridia are replaced by permanent setae and nephridia and also new internal organs formation is completed.