The vertebrate endoskeleton is internal while an arthropod's exoskeleton is not
The tick attaching itself to the skin of the dog is the parasitic relationship because the tick is relying on the dog as a host in order to live.
The study of the relationship between structures and functions is at the very heart of biology. This relationship is expressed in living beings by the adaptation of the first to the second and poses a series of absolutely fundamental problems, such as the relationship between causality and finality, analogy and homology, structural improvement and evolutionary level, etc.
For example, the structure of the hand is related to its function, thanks to the fingers and the thumb it is easy to wear and store objects.
The zigzag structure of the intestine is related to its absorption function, as this makes it possible to increase the contact surface between the alimentary bolus and the intestinal wall, and thus to increase the absorption.
The homology of animals is based on scientists current understanding on the evolutionary history of living species.
A. Hormones
secreted directly into the blood