PHYLUM ANNELIDA 281 



Darwin noted the abundance of earthworms in fields and estimated 

 that there are some 64,000 earthworms per acre. He then speculated on 

 the effect that so many worms would have, and concluded that they are 

 possibly the most important organism influencing the soil. According to 

 his calculations earthworms will bring to the surface two inches of dirt 

 every ten years. This not only mixes the soil, but slowly buries rocks and 

 other large objects. While such claims are now challenged, it cannot be 

 doubted that earthworms are an important agent in the conditioning of 

 soil. Their burrows help to aerate the soil and permit water to enter 

 easily during rain. The constant mixing of soil and organic debris con- 

 tributes to the development of good humus. 



1 28. Other Annelid Worms 



One of the largest annelids (15 or more inches long) is the lugworm, 

 a polychaete that burrows in muddy sand at the level of low tide. The 

 pharynx is everted into the sand and then withdrawn with its load. 

 Organic debris in the sand serves as food which is removed as the sand 

 passes through the digestive tract. Although the body is long and thick, 

 it is composed of relatively few segments. The parapodia are variously 

 modified, and they are missing from the first two and the last several 

 segments. The notopodia and neuropodia are separated widely. The last 

 several notopodia bear feathery gills. 



The small polychaete Hydroides builds twisted calcareous tubes on 

 shells and rocks. The prostomium bears a pair of large ciliated feathery 

 "gills" that are not only respiratory, but also serve as a device for catch- 

 ing food particles. 



Some fresh-water oligochaetes have more chaetae than the earth- 

 worm, but otherwise they tend to have simplified organ systems. Tubifex 

 is a small red worm that lives in the mud beneath standing or running 

 water. Large numbers often form red patches. Each worm lives head 

 down, foraging deep for food, while the posterior end is waved cease- 

 lessly above the mud for respiration. The amount of worm projecting 

 from the mud reflects inversely the amount of oxygen dissolved in the 

 water. 



Aeolosoma is a microscopic oligochaete 1 to 5 mm. long. The body 

 wall contains numerous red, yellow and green globules that give it a 

 clownlike appearance. It clambers about on fresh-water vegetation, gath- 

 ering minute debris with its ciliated prostomium. 



A number of worms can reproduce asexually like the planarians. 

 New individuals are budded posteriorly, usually forming the head before 

 detachment. The polychaete Autolytus may have several offspring bud- 

 ding at one time. Many of the fresh-water oligochaetes, including 

 Aeolosoma, reproduce in this way. 



129. Class Hirudinea 



Bloodsuckers are annelids modified for an ectoparasitic existence. 

 The body is stout and bears a large, powerful sucker on each end for 



