Explanation:

The innermost layer lining the lumen of the alimentary canal is the mucosa. This layer forms irregular folds (rugae) in the stomach and small finger-like foldings called villi in the small intestine. The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance. These modifications increase the surface area enormously.

In the alimentary canal of earthworm, the characteristic feature of the intestine after 26th segment except the last 23rd-25th segments is the presence of internal median fold of dorsal wall called typhlosole. This increases the effective area of absorption in the intestine.

Reference: NCERT Biology Text, Class XI, Unit II: Structural Organisation in Plants and Animals, Chapter 7: Structural Organisation in Animals, Page no: 108 & NCERT Biology Text, Class: XI, Unit: Human Physiology, Chapter: Digestion and Absorption, Page no: 260.

Explanation:
Species is the basic unit of taxonomy. It is a natural population of individuals or group of population which resemble one another in their essential morphological and reproductive characters, so that they are able to interbreed freely and produce fertile offsprings.

Reference: NCERT Biology Text, Class: XI, Unit I: Diversity in the Living World, Chapter 1: The Living World.