The animal kingdom is divided into two main parts, the invertebrates and the vertebrates, according as they lack or possess a backbone. Vertebrates include fish, amphibians, reptiles, birds and mammals. Special sense organs tend to con­centrate in the head along with the development of the brain, which is enclosed by a skull. The process of respiration is carried on by organs developed from the pharynx; and the closed circulatory system, carrying red blood, is composed of blood vessels and a muscular heart of at least two chambers. Reproduction in vertebrates is normally carried by the separate sexes. The fertilized eggs develop externally in lower ver­tebrates, while in higher forms they develop in the body of the mother. With but few exceptions, there are only two pairs of appendages which are variously modified as organs of locomotion to adapt the organism to its habitat.
   In the Vertebrata there are two classes of primitive vertebrates; the Ostracoderms, or fossil forms; and the modern Cyclostomes, including lampreys and hag fish. They are simple fishlike forms, degenerate to the extent of being blood-sucking parasites on fish.