Govt. Exams
Entrance Exams
Platyhelminthes (flatworms) display bilateral symmetry and have no coelom (body cavity), distinguishing them from higher phyla.
# Solution: Notochord Replacement in Chordates
The notochord is a flexible, rod-like structure that provides skeletal support in early chordate development. Understanding when and how it is replaced requires examining its role across different life stages and chordate groups.
## Step 1: Role of the Notochord in Early Development
The notochord appears in all chordate embryos during early development. It serves as a temporary skeletal axis and acts as an organizer for neural tissue formation. All members of phylum Chordata possess a notochord at some point in their life cycle.
## Step 2: Replacement in Fish (Adult Stage)
In fish, the notochord persists throughout their entire life. While vertebrae develop around it, the notochord is never completely replaced—it remains functional as an important structural component even in adult fish. The notochord and vertebral column coexist.
## Step 3: Replacement in Higher Vertebrates (Amphibians, Reptiles, Birds, Mammals)
In tetrapods and higher vertebrates, the notochord plays a crucial role during embryonic development but is progressively replaced by the developing vertebral column. During fetal/embryonic stages, cartilaginous and later ossified vertebrae develop around the notochord.
## Step 4: Adult Vertebrate Stage—Complete Replacement
In adult vertebrates only, the notochord is almost completely replaced by the vertebral column. Small remnants may persist as the nucleus pulposus (inner core of intervertebral discs), but the primary skeletal support function is entirely taken over by vertebrae. This replacement is complete only when the organism reaches adulthood.
Answer: The notochord is replaced by the vertebral column only in adult vertebrates, as it persists in fish and is gradually replaced during development in higher vertebrates, with complete replacement occurring only in the adult stage. (Option C)
Arthropods have an open circulatory system, not closed. They are characterized by jointed appendages, chitinous exoskeleton, and segmented bodies.
