Question

Which organisms are called primitive and how are they different from the so-called advanced organisms?

Answer 1

The organisms that were formed earlier and had a simple (basic) body structure are called primitive organisms whereas organisms that are formed later after evolution and have a complex body structure are called advanced organisms.

Complete answer:
Primitive organisms are the lower organisms that were the first to be formed on earth. These organisms have a simple body structure. The simplest example of primitive organisms is Prokaryotes.
In Prokaryotes all the necessary functions are carried out by a single cell. These organisms lack a membrane-bounded nucleus and other cell organelles. They undergo asexual reproduction which is usually by binary fission.
The advanced organisms are developed organisms that have a complex body structure and internally they have specialized structures that perform the necessary body functions. Humans are considered advanced organisms. They have a well-developed organ system that comprises different organs that are specialized to carry out different functions that are necessary for their survival. Reproduction is sexual that involves the fusion of gametes.

Additional Information: Phylum Porifera commonly known as sponges is the most primitive of multicellular animals. Sponges are a unique group of organisms that lack tissue level of the organization although they are multicellular. These organisms lack vertebrae and do not have a nervous, digestive, or circulating system. They have a water transport system or the canal system through which they obtain food and water and remove waste from the body.

Note: It is not necessary that only unicellular organisms are primitive organisms. Several other multicellular organisms are considered primitive depending on the level of body organization seen in them. Animals lacking organ level of body organization can also be considered as primitive organisms.