168 ESSENTIALS OF ZOOLOGY 



Movement and Locomotion 



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 certain 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 are seen in Protozoa which move from 

 place to place by means of pseudopodia, cilia, or flagella. In ciliary 

 movement the numerous small strands of protoplasm 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. 



The development of a high degree of contractility in muscle cells 

 makes possible muscular movement. A muscular locomotor system 

 consists 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) jn the reaction. During 

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

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

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

 and phosphoric acid. 



In animals without a skeleton, muscle bands are arranged in both 

 circular and longitudinal directions. The contraction of the circular 

 group tends to lengthen the body, and the shortening of the longi- 

 tudinal strands draws the body along. The pressure exerted on the 

 coelomic fluid is thought to be a factor in bringing about an even 

 extension of the body by this means. In echinoderms with the water 

 vascular system the pressure is exerted on water in a system of tubes 

 which extend to make contact with the surface over which the animal 

 is moving. 



Digestion 



The general function of digestion, which is to convert complex 

 raw food material into a soluble, absorbable form, has already been 

 pointed out. The materials commonly used for foods have large 

 molecules, and are usually colloidal in nature. Digestion then must 

 serve to break up these large molecules into smaller ones, thus 

 forming solutions of substances which will readily diffuse through 

 membranes. Digestive enzymes are responsible for placing the food 



