THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 381 



The muscular system represents a system of cells highly special- 

 ized in contractility. The muscles are usually attached to the skele- 

 ton or occasionally to other muscles by fibrous cords called tendons. 

 Voluntary muscles are usually connected with the skeleton; those of 

 the visceral organs, e.g., intestine, are involuntary. Cardiac muscle 

 is the highly specialized involuntary muscle which makes up the wall 

 of the heart. 



Independent power of movement is almost a characteristic of 

 animal life. Contractility as a property of all protoplasm is the 

 fundamental basis for all animal movement. The adult forms of cer- 

 tain animals, such as sponges, corals, oysters, barnacles, and others, 

 are sessile ; however, they all pass through a free, active larval stage. 

 Most of them retain the power to move separate parts in adult 

 condition. 



Simpler forms of locomotion have already been seen in Protozoa 

 which move from place to place by means of pseudopodia, cilia, or 

 flagella. In ciliary movement the numerous small strands of proto- 

 plasm beat rhythmically with a stroke in one direction, so timed that 

 the beat passes in a wavelike progression from one end of the ciliated 

 area to the other (metachronous rhythm). The stroke of a eilium 

 consists of a vigorous bend in one direction and a very deliberate 

 recovery in the other. In many Protozoa the entire body is covered 

 with cilia while in Metazoa the entire body may be covered where 

 they are used for locomotion; but more often they cover only areas 

 of free surface of epithelium, particularly the linings of passages. 

 Here they serve to move materials along and keep the surface free 

 of foreign material and excess mucus. 



The development of a high degree of contractility in special cells, 

 such as muscle cells, makes possible muscular movement which is the 

 principal kind in higher animals. A muscular locomotor system con- 

 sists of sets of opposing muscles. In muscular contraction there is 

 a cycle of rapid chemicophysical rearrangement in the cells. Oxida- 

 tion and heat production are involved in the process. Carbohydrates 

 in the form of glucose are oxidized (burned) in the reaction. During 

 the shortening of the muscle there is a hydrolysis or absorption of 

 water by the protein product, creatine-phosphorie acid. By-products 

 of the process include carbon dioxide, lactic acid, urea, creatinine, 

 and phosphoric acid. 



