8o PRIMARY FACTORS OF ORGANIC EVOLUTION. 



ery, which distribute the products of digestion. ' The 

 first nervous threads appear. But there is as yet no 

 body cavity which should form a sac in which the or- 

 gans of nutrition and reproduction should be sus- 

 pended. The Porifera (sponges) appear to be a much 

 modified form of this type. 



This grade of specialization belongs to the greater 

 number of the five succeeding classes, the Echinoder- 

 mata, Vermes, Mollusca, Arthropoda, and Vertebrata. 

 Where it is absent from a few of the three lower 

 classes, it is supposed to have disappeared by degen- 

 eracy. ■ The Echinodermata come first in considera- 

 tion. In these animals the form inclines to be radiated 

 and the nervous system presents no single axis, but 

 consists of branches which radiate from a ring sur- 

 rounding the oral extremity of the digestive canal. In 

 the second type, the general form is longitudinal and 

 may be segmented, and a nervous axis may extend 

 longitudinally from one or more points on the oeso- 

 phageal ring. ' These are the Vermes (true worms). 

 Embryology indicates clearly the common origin of 

 the Echinodermata and the Vermes. No line of de- 

 scent can be traced from the former, but from the lat- 

 ter we have traced the remaining branches of animals, 

 three in number. Lowest of these is the Mollusca.^ 

 The form of the body is sac-like, and the nervous sys- 

 tem displays, typically, besides the oesophageal ring, 

 a second ring, consisting of lateral commissures, mak- 

 ing a single median ganglion of the foot. The body 

 is not segmented, and there are no jointed limbs. In 

 the second branch, that of the Arthropoda, the body 

 is longitudinal and segmented, and segmented limbs 

 are present. There is a median nervous axis, proceed- 

 ing from the oesophageal ring along the inferior axis 



