Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/39

Rh from their being found in infusions. One of the most common of these forms is known as Paramoecium; and its structure will serve to illustrate this group. The Paramoecium (Fig. 24) is often compared to a slipper-shaped body of semi-fluid consistency (central substance), inclosed in a rind (cortical layer); the rind running insensibly into the semi-fluid substance. This rind is coated on its outside with a delicate layer (cuticula), bearing on certain parts hairs. (Fig. 24, h.) If the animal remain quiet, we can see a depression in the middle of the body, which leads into the so-called mouth; this opens into a kind of gullet. (Fig. 24, g.), This is all the digestive system the Paramoecium possesses. In certain parts of the body one can observe spaces opening and shutting (Fig. 24, c), and through these spaces certain canals are said to be visible, filled most likely with water. It is said these canals or vessels communicate with the exterior by means of holes in the layers forming the walls of the body. If such a system of vessels have really been found in the Infusoria (and many competent observers are confident that they do exist), they furnish an important proof of the derivation of the Worms from the Infusoria, as this rudimentary water-vessel system is much developed in the Worms. (See Aspidogaster.) The hairs on the outer layer of the Paramoecium serve as organs of movement, and, in making currents of water, drag small particles of food, etc. into the body of the animal. These hairs (Fig. 24, h) are called cilia, and their movement ciliary action. The Infusoria have been divided according to the presence or absence of these hairs into Ciliata and Acinetae (sucking); but the transition between the two seems to be furnished by the bell-shaped Vorticellae, which are said to produce Acinetae, while the Acinetae produce Vorticellae. If this be correct, the Acinetae are only a transition stage of Vorticellae, and all Infusoria are Ciliata or haired.