The term “tube within a tube” is frequently used in worm anatomy discussions. This is a well-recognized part of annelid, or segmented worm, body structure. In fact, on a broader scale, it could be used to describe animal and human systems as well.
The tube within a tube “plan,” in its most basic form, means that food goes in one end and exits at the other end. The tube portion means that as the food travels along its tract, it’s contained in a very specialized section of the anatomy. It may pass through several designated chambers as it is digested and broken down. This is opposite of the “sac system,” in which animals eat food, digest it, then excrete it through the same opening.
For annelids, however, the tube within a tube term refers to the fact that their outer body structure is also specifically tube-like. (This is not the same reference made to “tubeworms,” which create tubes around their own bodies.) The “coelom” is the area that separates the inner and outer tube structures.
Earthworms, for instance, have quite a well-developed tube-within-a-tube arrangement. The food goes into the mouth, then on to the pharynx. From there it passes through an esophagus to reach the crop, which is basically a storage bin. Next, food enters the gizzard where a muscular system begins to break up the pieces. These are strong contractions that can grind up anything an earthworm consumes, including dirt granules and leaf matter. Once these portions are manageable, the smaller particles then move into the intestine. A chemical process aids in the digestion once the longer intestinal portion is called into action. Nutrients and other essentials then enter the bloodstream. An earthworm’s intestine is also segmented and features a folded portion of the intestinal wall known as a “typhlosole.” When stretched, the surface space increases its absorbent properties, making for a very efficient chain of events. The unusable remainder of the now-digested food is then excreted.
In the large number of annelid species, components of this tube within a tube arrangement will vary. Some are primitive, while others are often complex. This setup also allows annelids to continue eating, even with undigested food in their systems. Ultimately, it’s efficient and another of nature’s amazing mechanisms.