Kuppfer's Cell, monocytes, and lymphocytes
The Serratus anterior in man is homologous to the Serratus ventralis muscle in the cat. The Serratus anterior is a muscle that originates on the surface of the 1st to 8th ribs at the side of the chest and insects along the entire anterior length of the medial border of the scapula. The Serratus anterior acts to pull the scapula forward around the thorax. Serratus ventralis in the cats looks like fingers because it attaches to the ribs, but has nothing to attach to between the ribs.
Answer - Well to put it in short terms.
Cells such as water cells or aquatic microorganisms need and required to have a large surface area increased. Because this helps them stay afloat without spending much amount of energy to float.
Oxygen, it takes in Carbon Dioxide and releases Oxygen as a waste :)