OF THE ANNELIDES. 161 



and rivers. In the second family, or Dorsibranchiata, 

 external breathing organs, or gills, often resembling 

 beautiful feathery tufts, are attached in pairs either to 

 every segment of the body, or to a certain number of 

 the middle segments. These organs sometimes dis- 

 play the most elegant varieties of form and the richest 

 colours, and afford, by their minor variations, excel- 

 lent characters for classifying the smaller groups or 

 genera. To this order belong the majority of the marine 

 Annelides ; and among the rest, the Arenicola piscato- 

 rum (Lug Worm), so commonly used as bait by fisher- 

 men. Lastly, there is the family called Tubicola, which , 

 differs from the two preceding groups in being com- 

 posed of sedentary animals. In both the former orders 

 the animals possess considerable activity : as the Earth- 

 Worm, which pushes its way through the soil, in which 

 it excavates extensive galleries ; or as the Leech, which 

 progresses by successive steps by means of the suckers 

 attached to its head and tail ; or as the individuals of 

 the Dorsibranchiate order, which creep along by means 

 of the bristly oars attached to each joint of the body, 

 or swim through the water by the help of the same 

 organs. But in the Tubicola we find a set of animals 

 which have partly the aspect of Earth- Worms, partly 

 that of Dorsibranchiate Worms, yet which differ from 

 both in having the greater part of their body enclosed 

 in a more or less perfectly formed tube or shell, which is 

 permanently attached to some extraneous object. When 

 the animal has once taken up its position, it remains 

 fixed throughout its life. The greater part of the body 

 being enclosed in the sheath, we do not find the gills or 



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