For the eighth observation (i=8), identify the animal phylum, xi, and yi.

The correct answer is "Phylum Urochordata: xi = 25; yi = 77".
The table presented in this exercise can be used as a tool to examine any connection between the microRNA (miRNA) diversity and the morphological complexity in a phylum. The Xi column represents the average number of miRNAs in a phylum, while the Yi column represents the average number of cell types in that phylum. The eight observation of this table refers to the data obtained from the phylum Urochordata.
MicroRNAs are small non-coding RNAs which regulate gene expression post-transcriptionally. They are used as phylogenetic markers as they have a very low rate of evolution. Also, they are very useful because it has been shown that phyla with very high levels of morphological innovation have high numbers of miRNA concentration.