All vertebrates are chordates but all chordates are not vertebrates, why?

Options

A. Ventral hollow nerve cord remains throughout life in some chordates
B. All chordates possess vertebral column
C. All chordates possess notochord throughout their life
D. Notochord is replaced by vertebral column in adult of some chordates

Answer

To understand why all vertebrates are chordates but not all chordates are vertebrates, we can break down the concepts step by step:

### Step 1: Define Chordates
Chordates are a diverse group of animals that belong to the phylum Chordata. They are characterized by having a notochord at some stage of their development, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

### Step 2: Define Vertebrates
Vertebrates are a subphylum of chordates known as Vertebrata. They are characterized by having a backbone or vertebral column, which replaces the notochord in adults. Vertebrates include animals such as fish, amphibians, reptiles, birds, and mammals.

### Step 3: Relationship Between Chordates and Vertebrates
- All vertebrates possess the basic characteristics of chordates, including the presence of a notochord during some stage of their life cycle (even if it is replaced by a vertebral column in adults).
- However, not all chordates develop a vertebral column. Some chordates, such as tunicates and lancelets, retain the notochord throughout their life but do not develop a backbone.

### Step 4: Conclusion
Thus, while all vertebrates are chordates because they share the fundamental characteristics of the phylum, there are chordates that do not possess a vertebral column and therefore are not classified as vertebrates. This distinction explains why the statement "all vertebrates are chordates but not all chordates are vertebrates" is true.

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