28 



Annelias — Pioneers in 

 Segmentation 



Annelids are extremists. The outside of an earthworm is monotonously 

 austere; there are no decorations. But many among the marine worms bear 

 plumy gills; those of the peacock and feather duster tribe are Hke miniature 

 fountains shifting with iridescence (Fig. 28.1). 



Annelids were pioneers in segmentation, the plan in which similar parts 

 of the body are repeated over and over. It is conspicuous in only two groups 

 of animals, the Phyla Annelida and Arthropoda, the latter known to everyone 

 through the lobsters, flies, and grasshoppers. Although it is less obvious, seg- 

 mentation is present in all higher animals, especially in the embryos but 

 clearly traceable in their later life. The rings of an earthworm's body and the 

 human vertebrae are evidences of segmentation. Both owe their origin to the 

 segmentation established in the ancestors of annelid worms some 550 mil- 

 lion and more years ago. 



Annelids exist in variety — earthworms in sober colors and streamlined 

 form, leeches with the parasite's appetite, marine worms of flowerlike beauty, 

 delicacy and diversity. They are as significant in the economy of the sea and 

 land as they have been in the evolution of the animal body. They are respon- 

 sive to their environments to an extent and precision, ordinarily little credited 

 to "worms." Examples of it are in: the burrowing habits of earthworms; their 

 responses to the chemical and physical nature of the soil and their age-old 

 plowing of the earth; the swarming of clamworms and the famous Palolo 

 worms. Out of all the days of the year spent on the sea bottom, Palolo worms 

 come to the surface only a few hours on nights appointed by the moon and 

 tides, and by forces beyond our solar system. They answer to an environment 

 that extends very far away. 



Ecology. Habitats of Annelids. AnneUds are numerous, biologically suc- 



552 



