Why is it difficult to spot backbones of some animals?
This question could go a lot of different ways.
Animal is a kingdom, which is 1 of the 7 steps in the classification system. Kingdom, phylum, class, order, family, genus, species is the full order.
Your question asks about it being difficult to see backbones in some animals. Easiest answer is that not all animals have backbones. Insects and arachnids for instance do not have backbones. Other animals might not have any kind of internal bone structure either. For example jellyfish and octopus.
I could specify the phylum "chordata," which has the main characteristics of a notocord, phyrangeal pouches, hollow nerve chord, and a tail. Vertebrates (animals with backbones) belong to this phylum. The notocord will turn into a backbone for most vertebrates. Lancelets and tunicates also belong to the chordate phylum, but they never develop a backbone. So kind of like a vertebrate because of the notocord, but never a backbone.
I said that the notocord turns into a backbone MOST of the time. There are exceptions within the subphylum vertebrate. While bony fish have a backbone, sharks, rays, and skates do not. In fact, they never develop a hard internal skeleton at all. It stays cartilage the entire time. So in that case, they are members of the vertebrate group (which are all supposed to have a backbone) but have no backbone. That makes it difficult to spot.