Which of the following statements about skeletons is true?
a. Chitin is a major component of vertebrate skeletons.
b. Loss of forelimbs in snakes involved little change in the axial skeleton.
c. Most cnidarians must shed their skeleton periodically to grow.
d. Vertebrate bones contain living cells.

Verified step by step guidance

1. Step 1: Understand the question and identify the key terms. The question is asking which statement about skeletons is true. Key terms include 'chitin,' 'vertebrate skeletons,' 'axial skeleton,' 'cnidarians,' and 'vertebrate bones.'
2. Step 2: Analyze each statement individually. For statement (a), recall that chitin is a polysaccharide found in the exoskeletons of arthropods and fungi, not in vertebrate skeletons, which are primarily composed of calcium phosphate and collagen.
3. Step 3: For statement (b), consider the anatomy of snakes. The loss of forelimbs in snakes involved significant changes in the axial skeleton, as their body structure is highly adapted for limbless locomotion. This makes the statement incorrect.
4. Step 4: For statement (c), review the biology of cnidarians. Most cnidarians, such as jellyfish and corals, do not have skeletons that they shed periodically. Instead, corals have calcium carbonate exoskeletons, and jellyfish have hydrostatic skeletons. This makes the statement incorrect.
5. Step 5: For statement (d), recall that vertebrate bones are living tissues that contain living cells, such as osteocytes, osteoblasts, and osteoclasts. This is a true statement, as bones are dynamic structures involved in growth, repair, and mineral storage.

Key Concepts

Types of Skeletons

Skeletons can be classified into two main types: exoskeletons and endoskeletons. Exoskeletons, like those found in arthropods, are external structures made of materials such as chitin. In contrast, endoskeletons, which are characteristic of vertebrates, are internal structures made primarily of bone and cartilage, providing support and protection for internal organs.

Bone Composition

Vertebrate bones are complex structures that contain living cells, including osteocytes, osteoblasts, and osteoclasts. These cells are involved in the growth, maintenance, and remodeling of bone tissue. Additionally, bones are composed of a matrix of collagen fibers and mineral deposits, primarily hydroxyapatite, which provide strength and rigidity.

Skeletal Adaptations in Evolution

The evolution of skeletal structures reflects adaptations to different environments and lifestyles. For example, the loss of forelimbs in snakes is an adaptation to a burrowing or slithering lifestyle, which has resulted in minimal changes to the axial skeleton. Understanding these adaptations helps explain the diversity of skeletal forms across different species.