Echidna and Ornithorhynchus, also known as the platypus, are two unique and fascinating creatures that belong to the class Mammalia. They are often referred to as "connecting links" because they possess characteristics that are intermediate between reptiles and mammals. Let's explore why these animals are considered connecting links between reptiles and mammals.

1. Echidna:
Echidna, also known as spiny anteaters, are small mammals native to Australia and New Guinea. They possess several reptilian features, making them an important link between reptiles and mammals.

- Reproductive System: Echidnas lay eggs, which is a characteristic typically associated with reptiles rather than mammals. This is one of the reasons they are considered a connecting link between the two groups.
- Mammary Glands: Echidnas possess mammary glands that produce milk to nourish their young, a characteristic of mammals.
- Egg-Laying Mammals: Echidnas, along with the platypus, are the only mammals that lay eggs.
- Beak-like Snout: Echidnas have a beak-like snout similar to that of reptiles. This feature is not commonly found in other mammals.

2. Ornithorhynchus (Platypus):
The platypus is another extraordinary mammal found in Australia. It also possesses unique characteristics that bridge the gap between reptiles and mammals.

- Egg-Laying Mammals: Like echidnas, platypus is one of the few mammals that lay eggs. This is considered a reptilian characteristic.
- Venomous Spurs: Male platypus have venomous spurs on their hind legs, a reptilian feature not found in most mammals.
- Bill or Beak: The platypus has a bill or beak, similar to that of a duck, which is a reptilian trait.
- Webbed Feet: The platypus has webbed feet, a characteristic of many reptiles and amphibians.

These unique features and characteristics of echidnas and platypus make them important connecting links between reptiles and mammals. They possess a combination of reptilian and mammalian traits, highlighting the evolutionary transition from reptiles to mammals.

In conclusion, echidnas and platypus are considered connecting links between reptiles and mammals due to their possession of both reptilian and mammalian features. Their ability to lay eggs, possess venomous spurs, and have beak-like snouts are examples of traits that bridge the gap between these two groups of animals.

Echidna (spiny anteater) and Ornithorhynchus (platypus) are the connecting links between reptiles and mammals. They are the only egg-laying mammals, known as monotremes.