Class 3.—Marsipobranchii. Worm-like vertebrates having round mouths without distinctly developed jaws; no jaw-bones. Without scales and without paired limbs (fins). Six or seven gill pouches. Skeleton consists of persistent, unconstricted, cartilaginous notochord. No sympathetic nervous system. Single nasal sac. No conus arteriosus, no spleen or pancreas. Undivided segmental duct.

Class 4.—Pisces.[26] Aquatic, cold-blooded vertebrates, with a movable lower jaw. Cartilaginous or osseous skeleton, with paired pectoral and pelvic fins, supported by fin rays (radials diverging from several basal pieces); also unpaired fins. No sternum. Exoskeleton of bony plates or scales. Skull has usually one occipital condyle. Breathing by permanent gills attached to gristly or bony arches on the sides of the gill clefts. In most fishes the digestive cavity gives origin to an air bladder which may or may not remain permanently connected with the digestive cavity. It mostly serves a hydrostatic purpose, but in some of the more primitive fishes (Amia, Lepidosteus) it is occasionally of some slight use in respiration. Heart consists of single auricle and single ventricle, and contains only venous blood; there is a sinus venosus and often a conus arteriosus. No inferior vena cava. No allantois. Most fishes lay eggs which are fertilized in the water.

Class 5.—Amphibia. In the evolution of vertebrates the Amphibians represent those forms which made the transition from aquatic to terrestrial life, but have lingered near the water. Certain acquisitions gained by the Dipnoi, such as a three-chambered heart and lungs, have been accentuated and firmly established by the Amphibians.

Amphibia are cold-blooded vertebrates whose larval forms always have gill arches bearing gills. In some forms the gills are retained throughout life, though the adults always possess functional lungs. Nasal sacs open posteriorly into the mouth. In existing forms there is rarely any exoskeleton. There are two occipital condyles. Unpaired fins are frequently present, though they possess no fin rays. When limbs are present they possess distinct digits and conform to the same type as those of the higher vertebrates. Heart has two auricles and one ventricle. There is an inferior vena cava. The intestine ends in a cloacal chamber, as do also the urinogenital ducts. Eggs almost always laid in the water. Often a marked metamorphosis.

Class 6.—Reptilia. Cold-blooded, air-breathing vertebrates. Limbs usually ending in claws; limbs sometimes absent; exoskeleton of scales; ribs well developed; incomplete double circulation; heart is four-chambered in the highest forms; oviparous; no metamorphosis; embryo with an amnion and allantois.

The following are some of the orders that have become extinct, viz.: