Question

please answer both questions

8. Discuss some functions expected to be needed in a multicellular organism, but not in a unicellular one. dhe Choc bran

Answer 1

8. A unicellular organism is made of a single cell. The single cell is always surrounded by the environment. All the requirements of the cell are easily absorbed by the cell membrane such as the O2, nutrients. Whatever is not needed is expelled from the cell membrane itself. In multicellular organism, there are many cells with in the body which have no access to these materials. So there has to be a mechanism to transport these substances required by the cell which are located deep inside the body. That is the reason why the multicellular organisms have a respiratory system and blood to transport respiratory gases. They also have digestive system for taking food, excretory system for removing wastes etc. That is why the functions of transport arose. Communication and control and co-ordination are important functions which we see only in multicellular organisms not in unicellular organisms. Because multicellular organisms have many cells and the functions of these cells need to controlled and co-ordinated. For this reason they have nervous system for control and co-ordination.

9. Till recently it was though that the Poriferans are the organisms which are the sister groups of all other metazoans without any doubt or controversy. But recently there are theories which state that the Ctenophorans are the sister group organisms which gave rise to the entire metozoans.

If the morphology and anatomy of the Poriferans is observed, we see that the poriferans are the simplest as they do not have tissue formation, gut, nervous system, sedentary etc. So, they are simplest and gradual evolution in these structures has given rise to other metazoans. This is what Porifera - First hypothesis believes in.

But Ctenophora first theory is gaining momentum because of the genomic studies done in the recent years. The genomic study shows number of similarities in the Ctenophorans with the metazoans, ndicating they are the sister-groups to all other metazoans. So the Ctenophora first theory is gaining momentum only because of the genomic studies. The group beliving the Ctenophoran -First hypothesis argues that many characters in the animal kingdom are repeatedly gained and lost during evolution. So the sponges may have lost some of the characters during the course of evolution.

Since Porifera are too simple compared to Ctenophora,I believe it is Poriferans which are the sister groups to all metazoans.

Too many organs and structures can't be lost and regained during evolution.