They are considered so because are the indispensable parts of the nature cycle.Without formation,decay cannot be possible and similarly,formation cannot be possible without decay.In addition to this,THEY ARE IN A CHRONOLOGICAL MANNER SET BY NATURE and that's why cannot be seperated and so together constitute the cycle of nature.
Answer:
The correct answer is - deuterostomes.
Explanation:
Duetorstomes eggs ,During embryonic development the zygote undergoes cleavage. Cleavage results is slitting into multiple cells where ease cell has the ability to develop into a complete embryo when isolated.
Preserves animal tracks are called trace.
The answer is Euglena. Euglena is a genus of single-celled flagellate eukaryotes. It is a large genus of unicellular protists, which have both plant and animal characteristics. All are motile by means of a flagellum. Most have chloroplasts (alga and plant characteristic). Euglenids are believed to descend from an ancestor that took up green algae by secondary endosymbiosis.
