The exterior layer of the gastrointestinal tract is the adventitia where the tract goes through the body wall, but it is the serosa where the tract goes through body cavities. If the layer attaches to surrounding tissue, it is adventitia, but otherwise, it's serosa, states the Southern Illinois University School of Medicine.
Adventitia consists of ordinary connective tissue in the shape of fibers that rests around the organ that the tissue supports. The serosa is also connective tissue, but its surface consists of mesothelium. It makes up the visceral peritoneum and extends over the abdominal wall, forming the parietal peritoneum. The serosa has the same composition as mesentery, which is connective tissue connecting the various loops of the gastrointestinal tract together, according to the SIU School of Medicine.
Moving inward from the adventitia/serosa layer, the GI tract has three other layers. The next layer inside the adventitia/serosa is the muscularis externa, or the muscular wall. The submucosa is connective tissue that supports the inside layer, known as the mucosa. This layer has a squishy, soft texture, and it is made of lamina propria (additional connective tissue), epithelium and muscularis mucosae (thin layers of smooth muscle), as stated by the SIU School of Medicine.