Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/64

46 subject of admiration on the part of naturalists. The Articulata are the most complex in structure of the Invertebrata, or animals without a backbone. The nervous system is highly developed, compound eyes are present, the digestive system has various parts, excretory glands and ducts have been discovered, respiration is carried on by the beautiful system of tracheae, and of their powers of jumping, flying, stinging, biting, and making noises every one is aware. The remaining division of the Articulata, including the Crabs, etc. (Fig. 51), though differing greatly in shape, size, etc., are all alike in their early stages. The Nauplius (Fig. 50), or primitive stage of every Crustacean, seems to be more nearly allied to the Rotatoria than any other group of animals. Some of the microscopic forms of the Crustacea, as Cypris, Daphnia, Cyclops, furnish the transitions from the Rotatoria to the Crustacea; indeed, the Rotatoria have been considered as a group of the Crustacea by many naturalists.

The most striking difference in the Mollusca, as compared with the Articulata, is seen in the entire want of that segmentation which is so apparent in the Insects or Centipedes. The body of an Oyster, a molluscous animal, is a soft mass, and, though possessed of organs, never exhibits the slightest trace of joints, as seen in the higher worms, insects, etc. The nervous system is composed of a few scattered nervous masses or ganglia, there being no distinct chain of ganglia running through the body from head to tail. Indeed, some of the Mollusca have no head, being known as the Acephala. For this reason the Acephala include the Brachiopoda and Conchifera. The Brachiopoda, Lamp Shells, or Arm-foot Mollusca, are better called Spirobranchiae, as their branchiae, or gills, are arranged in the