Question

1.What characteristics classify organisms as animals?

2. What new structural adaptations appear in the fossils of the Cambrian explosion that were not present in the Precambrium period?

Answer 1

Ans.1 : Characterstics which classify organisms as animals are as follows :-

1. Body form is regular.
2. Growth is definite. Well defined growing points are absent.
3. Response to stimuli is quick.
4. A cell wall is absent.
5. They have muscle cells for their mobility and nerve cell for the conduction of impulses. They are, however, absent in sponges.
6. Reproduction is mostly sexual.
7. Embryo stage is present and number of tissue layers formed during development.
8. Ecologically animals are consumers.

Ans.2 : New structural adaptations which appear in the fossils of the Cambrian explosion that were not present in the Precambrium period are :-

1. Presence of exoskeleton.
2. Presence of joint appendages.
3. Presence of distinct head with eyes.