Connecting links.

Ans. 1. Organisms referred to as ‘connecting links’ are those which are structurally intermediate between two different groups. 

2. Connecting links can be found among organisms that are still living. 

3. Eg. :(a) ‘Peripatus’ has segmental nephridia, thin cuticle and parapodia - like appendages as in Annelida. At the same time it has trachea and open circulation as in Arthropoda. 

(b) The duck-billed platypus lays eggs like reptiles and has hair and mammary glands like mammals. 

(c) Lung fish, though a fish, breathes air through its lungs. 

4. These organisms point strongly to the fact that mammals have evolved from reptiles and amphibia from fishes.