THE ORIGIN OF MAN 27 



the parent. It usually develops organs similar 

 to those of the parent and either becomes inde- 

 pendent of, or remains attached to the parent as 

 the case may be. 



The second division of phylum Echinodermata 

 includes spiny-skinned animals, a name given be- 

 cause the outer surface of these animals is usually 

 more or less studded with spines and plates. The 

 parts of the body are usually arranged on the 

 plan of five divisions, five radii and the same 

 number of spaces between them. Along each 

 radius are usually two or more rows of soft, tube- 

 like processes terminating in sucking disks ; these 

 are the tube-feet which are the chief means of 

 locomotion. However in forms that do not crawl 

 about these organs may be modified into feelers 

 and respiratory organs. The tube-feet are con- 

 nected with the water-vascular or ambulacral sys- 

 tem, a system of tubes consisting of a circular 

 canal extending around the inner side of the cen- 

 trally situated mouth, from which branch five 



