144 THE STUDY OF ANIMAL LIFE CHAP. 



play an important part in food-wafting, just as in the 

 gills of bivalves which are really external organs. 



Besides amoeboid and ciliary movement, there is in 

 rare cases among multicellular animals what may be called 

 epithelial movement, Avhere covering cells exhibit non- 

 amoeboid contractions and expansions. 



In most cases, however, animals move by means of 

 muscular tissue. Of this there are two kinds unstriated 

 and striated. Unstriated or " smooth ' muscle consists 

 of long flattened spindle-shaped cells with a central 

 nucleus ; these are bound together with a little inter- 

 cellular cement into a band ; and several bands may 

 be bound together with connective tissue. Zoologically, 

 it is interesting to notice that smooth muscle occurs in 

 sluggish animals such as Ascidians and in the slowly 

 moving parts of active mammals. In man and mammals 

 it occurs notably in the walls of the food - canal, the 

 bladder, and the blood-vessels (Fig. 45). 



A piece of typical striated muscle consists of numerous 

 fine fibres, each invested in a sheath (or sarcolemma), 

 and all bound together by connective tissue. It usually 

 runs from one part of the skeleton to another, drawing 

 one piece nearer the other when it contracts. It is 

 stimulated by motor nerves and is richly supplied with 

 blood. In backboned animals the connective tissue 

 envelope around a muscle is usually continued as a tendon 

 on to a bone ; in the extremely muscular Arthropods 

 the tendons are strips of non-living chitin. 



When a muscle contracts there is the obvious change 

 of shape which we see and feel ; it becomes shorter and 

 broader. The energy expended in the work done is 

 derived from an imperfectly understood chemical ex- 

 plosion in the muscle-fibres. Some explosive material is 

 manufactured in the proteid framework of the muscle, 

 the oxidation of sugar being probably involved in making 

 the material. The muscle explosion produces heat, CO 2 , 

 and water. Lactic acid is also formed as a by-product 

 in muscle-contraction ; and with each contraction there 

 is associated a change in electric potential. A similar 

 electric change occurs when the leaf of Venus' Fly-trap 



