Importance of considering internal features like ultrastructure in classification:
Internal features like ultrastructure are essential in classification for the following reasons:

1. Determining natural affinities:
- Internal features provide crucial information about the evolutionary relationships between different organisms.
- Ultrastructural features such as cell organelles, cell membranes, and cytoskeletal elements can reveal similarities and differences that might not be apparent from external morphology alone.
- By examining internal structures, taxonomists can determine the natural affinities and evolutionary history of organisms, leading to more accurate classification.

2. Providing insights into evolutionary relationships:
- Ultrastructural features can provide insights into the evolutionary history of organisms and help in tracing their phylogenetic relationships.
- For example, similarities in the ultrastructure of certain organelles like mitochondria or chloroplasts can indicate a shared evolutionary history between different groups of organisms.

3. Complementing external features:
- Internal features complement external features in classification by providing a more comprehensive understanding of an organism's biology.
- While external features like body shape, color, or size are important for initial identification, internal features offer a deeper insight into the organism's genetic makeup and evolutionary relationships.
In conclusion, considering internal features like ultrastructure is crucial in classification as they help determine natural affinities, provide insights into evolutionary relationships, and complement external features. By incorporating internal features into classification schemes, taxonomists can create more accurate and robust classification systems that reflect the true evolutionary history of organisms.