Answer
The cell-to-cell attachment proteins (cadherins) of animals are descended from those of choanoflagellate ancestors or from a common ancestor of animals and choanoflagellates. However, the homologous proteins in these groups do not have the same set of domains, with animal proteins having picked up CCD domains. Thus, the genes from these proteins are descended from an ancestor but have been modified from the choanoflagellate version. This is illustrative of descent with modification in that it demonstrates traits being both passed through generations as well as changing with new adaptations.
Work Step by Step
See p. 670 for relevant information. Review the definition of descent with modification in Ch. 22 and see that p. 670 shows the descent of these proteins and their modification in all animals compared to the choanoflagellates.
