262 VERTEBRATES. 



4. Excretion is performed by the skin, the lungs, and 

 the kidneys. 



II. Reproduction. As in all other animals except pro- 

 tozoans, reproduction in vertebrates is by means of eggs. 

 The sexes are distinct in all, and the eggs are fertilized 

 either within or without the body of the parent. In the 

 mammals the eggs are retained and developed within a 

 specialized part of the oviduct, the uterus (or womb). The 

 young are born after their embryonic life is past, and are 

 nourished with milk secreted in the mammary glands of 

 the female. Throughout the group, and throughout the 

 animal kingdom, the decrease in the number of progeny 

 is proportionate with their increased security from ene- 

 mies during the period of development. 



III. Voluntary Motion. An internal skeleton, usually 

 of bone, furnishes support and points of attachment for 

 the muscles of the vertebrates. The muscles overlie, and 

 more or less completely envelop, the bones. This is a 

 radically different arrangement of skeleton and muscle 

 from that found in crustaceans and insects. The muscle 

 fibers of vertebrates are insheathed together in connective 

 tissue, and often inserted into tendons, affording them 

 greater unity and efficacy of action. 



The limbs of vertebrates are never more than two 

 pairs, and may be entirely wanting. They are usually 

 developed for locomotion : as fins for swimming ; wings for 

 flying ; legs for walking, running, leaping, etc. Locomo- 

 tion is aided in fishes by unpaired fins, and by the stout, 

 muscular tail ; and in some reptiles (snakes), by the move- 

 ment of the spinal column and ribs. Movable tactile 

 organs, such as the barbels of fishes and the "whiskers" 

 of mammals, are often present. Some of the most clumsy, 

 ungainly, armor-encumbered animals, as well as some of 

 the fleetest and most graceful on land, in air, or in water, 

 are found among the vertebrates. 



