NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 5 



but a creeping animal encounters irregularities and obstructions. A 

 provision for body movements, therefore, becomes an advantageous 

 adjunct to the motor mechanism, and such movements can be pro- 

 duced only by an internal muscular system. Hence, the next stage 

 in evolution, recorded in both the flat worms and the annelids, was 



Fig. 2. — Hypothetical evolution of a swimming planulalike creature with an 

 open gastrocoele into a creeping wormlike animal with a simple alimentary 

 canal, a subapical ventral mouth, and a terminal anus. 



A, primitive swimming form with posterior blastopore. B, the same having 

 acquired the habit of sweeping up food particles from a solid surface. C, blas- 

 topore elongated forward on surface of contact to accommodate the feeding 

 habit. D, the same more fully adapted to subsurface feeding. E, final develop- 

 ment of alimentary canal, with ventral mouth and terminal anus, formed by 

 closure of intermediate part of blastopore, creeping habit fully established. 

 F-H, three stages of elongation and closure of the blastopore, ventral view. 



AlCnl, alimentary canal; An, anus; Bpr, blastopore; Gc, gastrocoele, or 

 archenteron; Mth, mouth. 



the development of contractile tissue that conferred the power of 

 diversified adjustive movements on the body itself. Muscles, how- 

 ever, -are not generally automatically active, as are cilia, and hence 

 the development of muscle tissue is usually accompanied by the 

 development of a mechanism for its activation. Furthermore, since 

 a muscular system is a provision for adjustment to external con- 

 ditions, the source of its stimulus must come from the environment. 

 The sponges are said to have a primitive contractile tissue that is 



