D. Gradual Modifications: In several cases, organs or tissue exhibits gradual modifications during the corse of organic evolution. For example, Heart is two chambered (one auricle and ventricle) in the fishes, 3 chambered (two auricle and ventricle) in amphibians, pseudo four chambered (two auricle and partly divided ventricle) in reptiles (snake) and 4 chambered in higher reptiles (crocodile), birds and mammals (Figure 5.6).

 

Figure 5.6 : Heart from different organism

E. Connecting Links: The living organisms exhibiting character of two different group of organisms are known as connecting links. There are few selected examples of connecting links and few selected examples are as follows:

Example 1: Euglena: Euglena has dual character of plant and animals. It can perform photosynthesis through specialized chloroplast and it can perform contractile vacuole, mouth and binary fission just like animal.

Example 2: Peripatus: it is an examples of connecting link between arthropods and annelids. It hs worm like body, unjoined legs similar to annelids whereas claws, jaws, tracheae and dorsal tubular hearts as arthropods.