Question

Muscles must have something rigid to pull against if they are to cause movement. In the case of humans, the rigidity is supplied by ____; in the case of earthworms, the rigidity comes from???.
A. A hydrostatic skeleton; an endoskeleton.
B. Opposing muscles; an exoskeleton.
C. An exoskeleton; an endoskeleton;
D. An endoskeleton; a hydrostatic skeleton.

Answer 1

Hint: Animals, especially vertebrates can have two types of structural frameworks for the purpose of protection, rigidity and a defined shape of the body: Exoskeleton and Endoskeleton. Some animals tend to have both types. Some Invertebrates have no skeleton or a semi-skeleton.

Complete Step by Step Answer:
In the case of humans, an endoskeleton is present composed of 206 bones and cartilage. These bones are joined to ligaments for junctions between bones; tendons for connections between muscles and bones; cartilage, muscles. The points between two bones are called joints that help in performing various types of movements in the body. It is called an endoskeleton because it is present as an internal structure in the body.

In the case of earthworms, they have a segmented hydrostatic skeleton. One continuous hole like process runs throughout the body, called a coelom. This coelom is filled with a fluid. This puts pressure on the walls of the body and also allows flexible movements. This fluid is called haemocoel.
Their movement is not by joints but through a particular movement called peristalsis. It moves with contractions and stretching of muscles.

Thus, the correct answer is D: An endoskeleton; a hydrostatic skeleton.

Note: Bones are composed mainly of inorganic matter like calcium hydroxyapatite, phosphorus hydroxyapatite, some organic matter which is constituted by different proteins, water and lipids.
Exoskeleton is a hard covering that blankets the external surface of the body. Example: Chitin exoskeleton of insects, shells of turtles.