The Entozoa or “intestinal” worms claim a brief notice at our hands. The entozoa are those beings which inhabit, as parasites, the intestines and other parts of animals. Their history is still obscure, but there seems to be about twenty varieties of these creatures, and a great number of animals have their peculiar entozoa. The best known in the human subject are the “Ascaris” or thread-worm, the “Lumbricus Teres” or long-worm, and the “Tænia” or tape-worm; this last is jointed, and grows to several yards in length.
The development of these Tænia is one of the most curious performances of nature. Each of the joints shown in the illustration below (fig. 842) is a perfected and mature proglottis, containing the ova or eggs, which can only be brought to perfection when swallowed by a warm-blooded animal (not the same from which they emanated). The head within the embryo then holds to the tissues and penetrates to the alimentary canal, where only it can redevelop joints from the so-called head, which has no organs and merely pushes out immature joints which are continued, and they become more mature the farther they are pushed out by the new ones. The “measles” of the pig are produced by the ova of these worms.
Fig. 842.—Tape worm (proglottides).
Myriapoda.
The “many-footed” annulosa include the centipedes and millipedes, and may be regarded as a connecting link between the worms and the insects. The heads of these animals are distinct from the body.
The millipedes can be any day found under a large stone in a field which has not been tilled, or any place where a stone has been suffered to remain for some time undisturbed. These specimens are of the pill-millipede order, because they roll themselves up into a ball when disturbed. The myriapods of this country are not of large dimensions, but in tropical climates they attain a great size. The giant centipede has been found in South America more than a foot long, and is capable of inflicting severe wounds, its tenacity being extraordinary and equalling that of the bull-dog when once it has gripped its enemy.
The myriapods have no wings; they possess antennæ, and numerous, never less than eighteen, feet,—frequently twenty pairs, but never a thousand, much less “ten thousand” feet, as the class name indicates. They are provided with strong forceps or “foot-jaws,” which supply a poison for killing their enemies. The millipedes and centipedes are known scientifically as Iulidæ and Scolopendridæ respectively, and in most points of internal arrangement resemble insects, such as breathing by spiracles or (stomates), and trachæ or tubes. Some of the centipedes possess electric qualities, and can administer a shock to an opponent.
Insecta.
Insects inhabit the world around us in myriad forms in air and earth and water. Some exist for a very brief space in the air; others live under water, or in trees, or in the ground; some burrow and hide in chinks of rocks and under stones. The numbers are countless, and all have some function to perform as palpably as the busy honey-bee, or as mysteriously as the giddy, careless butterfly.