Question

In earthworm, dorsal wall of intestine from $26^{th}$ segment to $95^{th}$ segment forms a median internal fold called
A. Trochophore
B. Typhlosole
C. Clitellum
D. Trachea
E. Nephridium

Answer 1

Hint: The median inner fold increases intestine surface area for more efficient absorption of digested nutrients. In different earthworm families, the organ appears to have multiple origins. One of the villi becomes larger and well developed than the other and it runs mid-dorsally from $27^{th}$ to the last $25^{th}$ segment.

Complete Answer:
- Typhlosole is a dorsal flap of the intestine in earthworms and this flap runs along most of its length which then forms a tube within a tube which is called typhlosole. By forming a tube within a tube, it increases the absorption area by that of its inner surface which further helps for more effective absorption of digested nutrients.
- Presence of an inner fold of the dorsal wall of the intestine and this typhlosole is present in between the $26^{th}$ and $95^{th}$ parts, which causes increase in the area for efficient absorption of digested nutrients. In various earthworm families, the typhlosole has several origins.
- For instance, the Lumbricidae have a typhlosole which is an inner folding of all layers of the intestine wall, while in some other families (like Megascolecidae), it is an inner folding of the inner layer, and in many earthworms it is lacking.

Hence the correct answer is option B.

Additional information: The alimentary canal in the earthworm has a straight tube-like structure. It is in between the first and last segments. Mouth opens in the buccal cavity (i.e. 1- 3 segments).
- Pharynx opens in the oesophagus (i.e. 5-7 segments). Oesophagus is a small, slender tube. It further continues to a muscular gizzard (i.e. 8-9 segments). The stomach ranges from $9^{th}$ to $14^{th}$ segments.
- Decaying organic matter and leaves combine with soil. This is the food for the earthworm. The humic acid that is located in humus, is neutralized with the help of calciferous glands situated in the stomach.
- The intestine is from the 15th segment till the last segment. Upon the $26^{th}$ segment, a pair of caeca come out from the intestine. In between the $26^{th}$ and $35^{th}$ segments, the inner median fold of the dorsal wall is there in the intestine.

Note: Digestive system of earthworm consists of alimentary canal and glands along with physiology of digestion. A typhlosole is an internal fold of the intestine or intestine inner wall. Typhlosole occurs in bivalve mollusks, lampreys and some annelids and echinoderms.