There are over 4,260 species of mammals on Earth, as well as 6,787 species or reptiles, 9703 species of birds and more that 28,000 fish species. The invertebrates outnumber all the vertebrates put together, with 80,000 molluscs and more than 1 million insects. Arachnids number is only 44,000 species. These figures are always fluctuating because scientists keep discovering new species, while others become extinct at an alarming rate.