C. elegans

C. elegans (Caenorhabditis elegans is a model organism that belongs to the nematode family and are transparent worms that live in neutral soil. They are usually around 1mm in length and are used extensively in research.

They are unsegmented creatures that have a bilateral symmetry. They use bacteria growing on decaying substances as a nutrient source. 

They have no female sex but are either male or hermaphrodites. This is determined by the number of sex-chromosomes to autosomes. If it is 1:1 then the worm is a hermaphrodite but if it is 1:2 it is male. C. elegans reproduce by laying eggs (laid by the hermaphroditic worms) and can lay up to 1000 eggs, although the average is ~ 300 eggs. However, if a hermaphrodite mates with a male, there can be as many as 1,400 offspring.

The great model organism features of C. elegans: 

1. homologs genes: 1/3 of the genes identified in c.elegans have homologs in the human genome so they are very useful as model oragnisms when looking at genetic manipulation.
2. The transparency of C. elegans: Another strong feature that makes the worms so useful for studying is that the adult worms are transparent and have in total 959 cells in the hermaphrodite worms, therefore it is easy to see firstly how the genetic manipulations affect the worms' development and secondly the exact cells in which the genes are expressed. Through studying biological processes occuring in the worms, 3 Nobel Prizes for Medicine have been gained.
3. Short generation time around 4 days. 
4. RNAi can be used as a method of knockdowning specific genes. C. elegans can be fed with transformed bacteria containing dsRNA of interest.