The gut of a vertebrate is often described as an elongated tube consisting of four layers of different cell types.
which of the following represents the correct order of those layers, from innermost (lumen-facing) to outermost?

a. submucosa, cartilage, mucosa, endoplasmic reticulum
b. smooth muscle layers, submucosa, mucosa, cartilage
c. cartilage, smooth muscle layers, mucosa, circular muscle
d. mucosa, submucosa, circular muscle, longitudinal muscle
e. cartilage, mucosa, endoplasmic reticulum, submucosa

The correct answer is C. "Cartilage, smooth muscle layers, mucosa, circular muscle".

Explanation:

The intestine wall is conformed of 4 layers: mucosa, submucosa, muscular and serous.

The mucosa layer is composed of cylindrical simple epithelium that shapes tubular glands named Lieberkühn crypts and villus, these las one in charge of absorption. The crypts are invaginations of the epithelial surface and they have the stem cells. The epithelial cells renew constantly, so cells divide in the base of the crypts and then they move to the superior portion of the digestive tube, where they die. This layer also has a basal sheet and a thin muscular sheet. The submucosa layer is composed of dense connective tissue, with adipose tissue located in some specific areas and large-caliber blood vessels. The muscular layer is composed of two bands disposed perpendicularly between them, the internal thick circular muscle and the external thin longitudinal muscle layer.The serouse layer is composed of a very thin connective tissue that in some points continues with the peritoneum.

In the air and in the lower fre ah water