United States or Norfolk Island ? Vote for the TOP Country of the Week !
Like the reptiles they lay tough-coated eggs and have a single outlet for the excreta, and they have a reptilian arrangement of the bones of the shoulder-girdle; like the mammals, they have a coat of hair and a four-chambered heart, and they suckle the young. Even in their mammalian features they are, as the careful research of Australian zoologists has shown, of a transitional type.
Thus arose the early Arthropods, on tough-coated, jointed, articulated animals. Some of these remained in the water, breathing by means of gills, and became the Crustacea. Some, however, migrated to the land and developed what we may almost call "lungs" little tubes entering the body at the skin and branching internally, to bring the air into contact with the blood, the tracheae.
We know that the heavier salt sea-water buoys us up more than the fresh river or pond water, but we do not feel in the same way the lift of the high barometric wave. Even the rough, tough-coated maple-trees in spring are quickly susceptible to these atmospheric changes. The farmer knows that he needs sunshine and crisp air to make maple-sugar as well as to make hay.
In others defensive armour is chiefly developed, and we get the lines of the heavy sluggish shell-fish, the Molluscs and Brachiopods, and, by a later compromise between speed and armour, the more active tough-coated Arthropods.
Word Of The Day