402 GENERAL ZOOLOGY. 



protected it may be in the adult, the central nervous 

 system arises from the outer surface (ectoderm, p. 155) and 

 secondarily attains its permanent position. 



Since most animals must search for their food, we find 

 that except in the lower forms, one end becomes adapted 

 for always going in front, and in this way a head has come 

 into existence, and here are situated the brain and the 

 most important sensory organs, as well as the mouth, 

 since this part of the body first comes into the neighborhood 

 of substances useful as food or likely to be injurious to the 

 animal. Further, it is probable that this same loco- 

 motion has resulted in bilaterality of the body, which is so 

 marked in all animals except the Ccelenterata and sponges. 



Locomotion implies motion of the parts, and when 

 resolved to its ultimate all motion is to be referred back 

 to the contractility of protoplasm (p. 139). In the lower 

 Protozoa (Amoeba, p. 148) all parts of the protoplasm 

 (cell) are equally contractile, but from this point onward 

 specialization sets in. In some there occurs the develop- 

 ment of special vibratile organs cilia and flagella 

 moved by contraction of the protoplasm in them or at 

 their bases. Cilia and flagella occur in most groups 

 where a slow motion is needed. Cilia occurs even in 

 man in the trachea, and doubtfully in the tubules of the 

 kidney, while the tail of the spermatozoan is but a flagellum. 

 Another type of modification is the conversion of a part 

 of the protoplasm of the cell into muscular substance 

 which contracts under stimulus. Traces of this appear 

 even in the Protozoa (Stentor, p. 146), and it is abundant 

 in all other groups except sponges. Cilia are apparently 

 automatic in action and while in a few instances they may 

 be regulated by something like nerves, they are not stimu- 

 lated by them. Muscles, however, are quiescent unless 



