The primary function of the clitellum is to create a cocoon for the developing young of the annelids that possess them. The most well-known species which have a clitellum are earthworms and leeches, on which the clitellum appears as a pale saddle-like structure a few segments down from their mouths. While mating, the worms build the cocoon around them, leaving the fertilizing gametes behind as they withdraw and seal it.
According to Encyclopaedia Brittanica, the clitellum is solely a feature of adult worms, and it develops only as the worms reach sexual maturity. Each species of worm has a somewhat different clitellum, varying by relative size, shape and color. In all species, the structure tends to be paler than the rest of the worm. They can be saddle-shaped or more flattened, and either flared or non-flared.
The clitellum is very obvious on earthworms, overlaying their otherwise very evident segments. In earthworms, the clitellum sits about a third of the way down their bodies from their mouths, and this is an easy way of identifying which side is the mouth side. This can be helpful since earthworms don't have distinct heads, and they lack eyes, ears and noses. Many earthworms do have a sort of tongue, however, and find their way through their underground environment via a chemical sensor and touch.Learn More
The zona pellucida is the transparent, thick outer layer of the ovum in mammals that protects the cell as it travels from the ovary to the uterus. The National Institutes of Health states the layer regulates interactions between ovulated eggs and free-floating sperm as the egg is fertilized. In order to fertilize the egg, sperm must penetrate this outer layer as it thins on the way to the uterus.Full Answer >
Human skin cells contain 46 chromosomes, or 23 chromosome pairs, and after mitosis, the two daughter cells also have 46 chromosomes. Mitosis is a cellular process that creates two identical daughter cells from one original cell.Full Answer >
The consensus among biochemists is that virtually every cell in the human body can break down sugar, usually in the form of glucose, to use as energy. According to the authors of the 5th edition of "Biochemistry," the brain and the kidneys prefer to run on glucose. In fact, the brain favors glucose to such an extent that it only metabolizes other fuel sources after several days of starvation.Full Answer >
The Golgi apparatus is a cellular organelle that is found in most eukaryotic cells, including human cells. It was first identified in 1897 by Camillo Golgi and was named after the physicist the following year.Full Answer >