Though this question seems to be trivial, but in fact it is not. And if we want to introduce animal welfare legislation, we need to establish what organisms are animals.
In modern taxonomy animals (Animalia) are known as Metazoa. Animals are multicellular eukaryotes (i.e. animal cells have a nucleus), are heterotrophic (i.e. they don’t do photosynthesis, and chemical energy in the form of organic molecules to survive), and their cells don’t have a rigid cell walls (unlike plants, fungi and bacteria).
Taxonomists have divided the animal kingdom in three main groups: Eumetazoa, Mesozoa and Parazoa. The last subkingdom consists of multicellular animals who (unlike other animals) do not have tissues or organs. Currently only sponges belong to the Parazoans. The second group, Mesozoa, contains only worm-like parasites, and its actual status is subject of scientific dispute.
The first subkingdom, Eumetazoa, is by far the most interesting one, since it contains all other animals. Eumetazoans are animals with differentiated tissues and organs. Most aminals of this group of a symmetric body to a certain degree. Since only Eumetazoans are known to have nerve systems, and hence capable of suffering, it might be an idea to restrict animal welfare legislation to Eumetazoans rather to all Metazoans.
Many people have learned at school that life is divided into four kingdoms: bacteria, fungi, plants and animals. Only this system is now outdated due to new scientific (genetic!) research. There are two superkingdoms: Prokaryotes and Eukaryotes. The former is divided into Bacteria and Archaea. The latter is divided into: Unikonta and Bikonta. The latter contains plants, algae and similar organisms. Unikonta contains amoebozoa and opisthokonts.
Opisthokonts are further divided into main groups: Holomycota (includes among others fungi) and Halozoa. The latter group is then further divided into Mesomycetoea and Filozoa. Filozoans are divided into Filasterea, Choanoflagellata and Animalia. Choanoflagellata are a group of unicellular organisms, and they are the closest relatives of animals (most recent common ancestor living about 600 million years ago).