frozonecom said:
@Darwin123
Thanks for the answer. Just wanted to clarify something. You said this:
Wiki calls the coelum a hydrostatic skeleton. However, this is a misrepresentation of the word. A hydrostatic skeleton is not a skeleton. The word skeleton is being used by Wiki in a completely different way. A hydrostatic skeleton isn't a skeleton any more than a balloon animal is a vertebrate
We were taught in university that there are 2 types of skeleton. Rigid and Hydrostatic. I don't know why my lecturer(teacher) said that, because I can see your point. So, hydrostatic skeleton isn't really a skeleton?
I guess it is a matter of semantics. Aesthetically, I can't see a hydrostatic skeleton as being analogous to a vertebrates skeleton. I will play Devils' advocate by bringing up an odd borderline example.
The notochord of chordates is not made of true bone. It is made of cartilage, which is a type of flexible connective tissue. The notochord is stiff only because of hydrostatic pressure, not because the cartilage is stiff. In this way, it is like the cuticle of a nematode.
In some primitive chordates (amphioxus, tunicates, hagfish), the notochord is used by the adult for propulsion just as you described the cuticle is used. The notochord is highly elastic. The muscles of the chordate adult bends its body. When the muscles relax, the notochord springs back. Thus, the notochord provides an efficient form of propulsion. The muscles couldn't provide propulsion without it. This is probably the primitive function of the notochord. So the notochord of chordates can be considered analogous to the cuticle of nematodes.
Since it is stiff due to hydrostatic pressure, it can be considered a hydrostatic skeleton. In protochordates, it serves the exact same function as the cuticle of a nematode.
The notochord is not used for propulsion by vertebrates, which are the specialized chordates that are common now. Bone forms around the notochord, producing the vertebrae. The notochord withers away. The remains of the notochord become the discs between the vertebrae that serve mainly to lubricate the motion of the backbone.
Cartilage is often a precursor to bone in vertebrates. In the case of the notochord, the cartilage remains as part of the backbone (i.e., the discs). So the notochord can be also considered part of the rigid skeleton.
Strangely, the notochord is both a hydrostatic skeleton and part of a rigid skeleton at different stages of development. It is both.
Maybe the instructor has organized the lecture so that the notochord will be discussed later in the class. He threw the hydrostatic skeletons and the rigid skeletons together into the same classification just to avoid confusion when he starts to talk about notochords. In other words, he cooked the lecture so that the notochord is not an ambiguous case. Now, e can simply get up and call the notochord a skeleton without anyone asking embarrassing questions.
The notochord is an unusual case of a part that is both a hydrostatic skeleton and a rigid skeleton at different points of development. In the embryo, it is a hydrostatic skeleton. In the adult, it is part of a rigid skeleton.
Organizing lectures is hard, so I can appreciate this point. The fewer borderlands, the easier the journey. However, I still think there were better ways to deal with the notochord than throwing two concepts together that have very little overlap. The lecturer should not dump hydrostatic skeletons in the same brew with rigid skeletons.