Within the plant-cells is found a vital, vegetable substance termed bioplasm, or protoplasm; which furnishes the same nutritive power as the tissues of the polyp and jelly fish. Many families of animals have pulpy bodies, and slight instinctive motion and sensibility, and in proportion as the nervous system is developed, both of these powers are unfolded. Plants have a low degree of sensibility, limited motion, respiratory and circulatory organs. Animals possess quicker perceptions and sensibilities, the power of voluntary motion, and, likewise a rudimental nervous system. Some articulates have no bony skeleton, their muscles being attached to the skin which constitutes a soft contracting envelope. One of the simplest forms of animal life in which a nervous system is found, is the five-rayed star-fish. In each ray there are filaments which connect with similar nerve-filaments from other rays, and form a circle around the digestive cavity. It probably has no conscious perception, and its movements do not necessarily indicate sensation or volition. In some worms a rudimentary nervous system is sparingly distributed to the cavities of the thorax and abdomen, and, as in the star-fish, the largest nerve-filament is found around the esophagus, presiding over nutrition. [Illustration: Fig. 66.] A higher grade of organization requires a more complete arrangement of nervous substance. Stimulus applied to one organ is readily communicated to, and excites activity in another. [Illustration: Fig. 67. A. Nervous system of a Crab, showing its ganglia. B. The nervous system of a Caterpillar.] The nervous system of some insects consists of two long, white cords, which run longitudinally through the abdomen, and are dilated at intervals into knots, consisting of collections of nerve-cells, called ganglia. They are really nerve-centers, which receive and transmit impulses, originate and impart nervous influence according to the nature of their organic surroundings. The ganglia situated over the esophagus of insects correspond to the medulla oblongata in man, in which originate the spinal accessory, glosso-pharyngeal, and pneumogastric nerves. The latter possess double endowments, and not only participate in the operations of deglutition, digestion, circulation, and respiration, but are also nerves of sensation and instinctive motion. The suspension of respiration produces suffocation. In insects, these ganglia are scarcely any larger than those distributed within the abdomen, with which they connect by means of minute, nervous filaments. Insects are nimble in their movements, and manifest instinct, corresponding to the perfection of their muscular and nervous systems. When we ascend to vertebrates, those animals having a backbone, the amount of the nervous substance is greater, the organic functions are more complex, and the actions begin to display intelligence.