Cetacea

aquatic mammals which depart in many important anatomical points from the other members of the cla , their structure being so modified as to render them unfit for terrestria] life. The whales, the porpoise, narwhal, ete., represent the leading divisjons of the group. The body is fisblike in form, the head pa ing gradually into the trunk, which tapers posteriorly and ends in a bil-bate caudal fin which is placed horizontally, not as in the fishes, vertically. ‘The posterior limbs are wanting, and the anterior are converted into broad paddles, within which are present representatives of all the bones te found in the fore limb. of mammals, The fish-like aspect is further increased by the presence of a dorsal fin, as in the dolphin and finner whale. The arrangement of the respiratory and circulatory systems, which enable the Cetacea to remain for some time under water, are interesting. The nostrils open directly upward on the tor of the head, and are closed by valvular folds of integument which are under the control of the animal. When the animal comes to the surface to breathe it expels the air violently, and the vapor it contains becomes condensed into a cloud. The blood-ve els, especially those of the thorax and spina! canal, break up into extensive plexuses or networks, in which 4 large amount of oxygenated blood is delayed, and thus the animal is enabled to remain under water.