NEET Zoology - MCQ Practice Questions

Arthropods have an open circulatory system, not closed. They are characterized by jointed appendages, chitinous exoskeleton, and segmented bodies.

# 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)

Platyhelminthes (flatworms) display bilateral symmetry and have no coelom (body cavity), distinguishing them from higher phyla.

Analogy refers to similar structures with different origins performing similar functions (e.g., bat wing and bird wing).

The radula is a ribbon-like feeding structure with teeth found in gastropods (snails) and many other molluscs, used for scraping food.

Amphibian skin is thin, moist, and richly supplied with blood vessels, enabling efficient cutaneous respiration.

Fish possess a two-chambered heart with one atrium and one ventricle, unlike amphibians and reptiles which have three chambers.

Most sponges are marine organisms. They lack true tissues, organs, and nervous systems but can reproduce both sexually and asexually.

Mammals are uniquely characterized by mammary glands for milk production and hair/fur for insulation, not all have placenta (monotremes lack it).

Cnidarians (hydra, jellyfish, corals) have tentacles, radial symmetry, and nematocysts, but do NOT have a segmented body. Segmentation is characteristic of Annelida and Arthropoda.

Insects and other arthropods have an open circulatory system where hemolymph bathes the organs directly. Vertebrates have closed circulatory systems.

Flatworms (Platyhelminthes) use flame cells (protonephridia) for osmoregulation and excretion. Nephridia are found in annelids, Malpighian tubules in insects.

The production of milk through mammary glands is the defining characteristic of mammals (Class Mammalia). While other features may be shared, lactation is unique to mammals.

Amphibians have a three-chambered heart with two atria and one undivided ventricle. Some mixing of oxygenated and deoxygenated blood occurs.

Frogs undergo indirect development with distinct larval stages (tadpole), while reptiles and birds show direct development without free-living larval forms.

The diaphragm is a muscular partition found only in mammals that aids in breathing. Other vertebrate classes use different mechanisms for respiration.

The muscular foot in molluscs like snails and clams is adapted for locomotion (in gastropods) and attachment to substrates (in bivalves).

Most arthropods undergo metamorphosis (complete or incomplete). They have exoskeletons, open circulatory systems, and no notochord.

Flatworms are acoelomate (no body cavity), while roundworms are pseudocoelomate (have a pseudocoelom). This is a fundamental distinction between these phyla.

In insects and many arthropods, the head contains sensory organs, the thorax bears locomotory appendages, and the abdomen contains reproductive and digestive organs.