PRINCIPLES OF PHYSIOLOGY 99 



toplasmic gel is converted to sol to flow forward. At the front end, the 

 streaming protoplasm bulges out in a projection known as a pseudopod 

 (false foot) and changes from a sol to a gel. The push for the move- 

 ment of the protoplasmic sol is believed to come from the contraction 

 of the gel protoplasm comprising the layer near the surface of the cell. 

 The tip of the pseudopod is covered with a thinner gel layer than that 

 elsewhere in the cell and hence is the part to bulge when contraction 

 occurs. By regulating the thickness of the local areas in the cell wall, 

 the ameba can determine where a pseudopod will form and hence 

 which direction he will move. The animal has no permanent front and 

 rear ends. Ameboid motion is a crawling motion, not a swimming one; 

 the cell must be attached to some physical substrate in order to move. 

 Another type of motion is seen in the movable, slender, protoplasmic 

 processes which project from certain cells. These projecting hairs are 

 termed flagella if each cell has one or a few long, whip-like processes 

 and cilia if each cell has many short processes. Flagella are found on 

 certain protozoa (the flagellates), on the collar cells of sponges and on 

 certain cells lining the gastrovascular cavity of coelenterates. Cells 

 equipped with cilia occur very widely: in certain protozoa (called cili- 

 ates), on the body surfaces of ctenophores, flatworms and rotifers, on 

 the tentacles of bryozoa, certain Avorms and coelenterates, on the gills 

 of clams and oysters, and lining certain ducts in the vertebrate body 

 such as the bronchi and oviducts. The paramecium is an example of a 

 ciliate, with some 2500 short cilia covering each single-celled animal. 

 The protoplasmic extensions beat in a coordinated rhythm, not simul- 

 taneously but one after another, so that waves of movement pass along 

 the body surface. The eftect of the combined effort of the cilia beating 

 backward is to move the animal forward. The cilia beat somewhat 

 obliquely so that the animal revolves on its long axis and moves in a 

 spiral path. The beating of the cilia is under the control of the animal, 

 and by reversing the ciliary beat it can back up and turn around. The 

 beating of cilia and flagella is believed to result from the contraction 

 of the iMoto}ilasm in these projections but the details of the process are 

 quite unknown. Cilia beat quite rapidly, up to 40 beats per second. In 

 the electron microscope a system of fibers is visible extending down the 

 long axis of the flagellum or cilium and this undoubtedly plays some 

 role in its beating. 



Muscles. Motion in most animals is a function of the contraction 

 of specialized cells, the muscle cells. The contractile mateiial, actomyo- 

 sin, is fundamentally similar in smooth, striated and cardiac muscles of 

 vertebrates and in the muscles of invertebrates as well. Muscles that 

 contract rapidly and briefly, such as the skeletal muscles of mammals, 

 are striated, whereas those that contract slowly and remain contracted 

 for a long time, such as those in the walls of the digestive tract or urinary 

 bladder, are unstriated. This basic physiologic and histologic correla- 

 tion is evident in the contractile cells of coelenterates, for those of jelly- 

 fish, which contract in twitches, have microscopic cross striations and 

 those of sea anemones, which contract very slowly, are unstriated. 



The coelenterate contractile cells in the ectoderm are arranged at 



