Skeletal System

The skeletal system gives shape, structure and allows the organism to move. In class we learned the three types of skeletons are Hydrostatic Skeleton, Exoskeleton, and Endoskeleton. A hydrostatic skeleton mostly found in organisms with soft bodies which is cavities filled with fluids surrounding by muscles. The Exoskeleton, found on shelled fish and insects, it provides a tough outside to protect the soft insides of the organism. Endoskeletons is internal skeleton, such as the bony or cartilaginous skeleton of vertebrates.

Worm

Worms do not have bones, only a hydrostatic skeleton, that means that it can easily stretch and squeeze into places without their bones breaking. When it comes to pushing heavy things around, it is very hard without the hard bones to help out, so instead the muscles try their best to help out with the movements. Without the bones to help move, the worm has bristles on each segment called setae that help the earthworm move. 

Crayfish

The shells that the crayfish has are called exoskeleton, it protects the organs and tissue of the crayfish, but if it creaks all of the insides will be exposed. Even though the exoskeleton gives protection and support, it can also limit somethings. For example, the crayfish being cold-blooded, it has to move to different environments to maintain homeostasis, such as colder places. The other way around, if the crayfish is too cold, it must move to a more suitable  climate for themselves. The crayfish's exoskeleton must molt. Like how a snake must replace it's skin every so often. So when the crayfish just molted out of it's old shell, the crayfish is vulnerable to it's predators due to not having a shell yet. There is also a disadvantage when gills are attached to the legs, if any of the legs were torn off, it may slow the breathing rate down. That is why the crayfish have multiple legs.


Frog 

The frog's skeletal system is very different from the humans even though they both have a endoskeletons. The endoskeleton forms a supporting framework for the body, it can be well know as an appendicular skeleton and an axial skeleton. Appendicular skeleton consists of the pectoral, pelvic, and the limb bones. Axial skeleton consists of skull, vertebrae, and sternum. Frogs do not have necks and cannot turn, lift or lower their heads like humans can. Another thing that is different in the frog's skeletal system is that the frog does not have ribs. The reason for that is because when ever they jump, half of the time they land on their bellies, so if they ribs there it would crack every time. A frog's pelvis can also slide up and down its spine.

Rat

Like the frog, the rat also has an endoskeleton with 223 bones that are distributed into the axial skeleton and the appendicular skeleton. The rat is also the only organism that we have studied that has a ribcage that covers most of its body. Unlike the frog that did not have the ribcage to protect the organs, the rat has a ribcage to protect the internal organs.