124 MOVEMENT IX ANIMALS 



(4) The Motions of Play. — Finally, we must not 

 omit to make mention of the motions which are made in 

 play. These are particularly characteristic of the young, 

 but are by no means missing in adults, either among the 

 higher animals or man. A fact that is worthy of con- 

 sideration is that while the motions of the first three 

 groups are practically universal among animals, it is ex- 

 tremely difficult to detect with certainty any motions of 

 play except in the higher and more intelligent forms. 

 Most kinds of mammals play, but it would be hard to say 

 certainly whether fish ever play, and one would certainly 

 be justified in doubting the occurrence of play among 

 either worms or starfishes. 



The Regulatiox of Motion, (a.) Receptors. — It 

 is quite evident that the mere power to make motions has 

 very little biological importance. In order that motions 

 shall be effective they must be adjusted to the require- 

 ments of the situation. This means that the organism 

 must have knowledge of its environment. Motions can- 

 not be directed toward securing food unless the posi- 

 tion of the food can in some way impress itself as a guide 

 for the motion. In general terms, the situation or envi- 

 ronment must have a means of influencing the organism. 

 This consists in> all animals, except the lowest, of special 

 organs of one kind or another which are known as the 

 sense organs. As a convenient single word, applied to 

 all the various kinds of sense organs that exist, the term 

 receptor has been adopted. 



(6.) Adjustors. The reception of impressions from the 

 surroundings will not by itself bring about effective mus- 

 cular motion. An additional step is necessary, namely, 

 the establishment of connections between the receptors 

 anrl the organs that are to carry out the movements. In 

 other words there must be a means of controlling the 

 motions in such a way as to produce a suitable and effec- 

 tive muscular response as a result of the impression made 

 upon the receptors. The apparatus for exercising this 



