418 GENERAL ZOOLOGY 



Filamentous external gills are present in some. The pharynx 

 of the free-swimming predaceous forms is provided with a pair 

 of sharp, pointed chitinous jaws and may be protruded as a 

 proboscis in capturing food. The sedentary forms live in 

 tubes of a slimy material, secreted by the integument and 

 usually encrusted with bits of sand and shell. As a rule, 

 polychaet annelids are dioecious. 



Examples: Nereis virens, the clamworm, a free-swimming 

 marine form about 25 cm. in length ; Nereis limnicola, a fresh- 

 water California species about 45 mm. in length ; Chaetopterus 

 pergamentaceus, a marine form, living in a U-shaped tube 

 buried in the sand. 

 CLASS III. OLIGOCHAETA. Fresh-water or terrestrial anne- 

 lids, lacking parapodia and cephalic appendages. The head 

 region consists of the prostomium, a lobe projecting dorsally 

 over the mouth, and the peristomium, containing the mouth. 

 The setae, mounted in pits in the body wall, are few in number 

 and may be absent. Aquatic forms may have gills. 

 Oligochaete worms are monoecious and development is direct, 

 there being no larval stage. Some also reproduce asexually by 

 transverse fission. 



Examples : Enchytraeus albidus, milk white in color, and about 

 25 mm. in length. Used as food for aquarium fishes. Lum- 

 bricus terrestris, an earthworm, about 10 in. in length. An 

 Australian species reaches a length of 3 to 4 ft. 



Earthworms are found in temperate and tropical countries 

 at depths down to 6 ft. They live in burrows in the soil and in 

 decaying vegetation, and obtain nourishment by passing soil 

 with organic matter through the alimentary canal. Burrows 

 are made by pushing the soil aside or by swallowing it. After 

 swallowing the earth, whether for food or burrowing, the earth- 

 worm returns to the surface and ejects the earth through the 

 anus. Charles Darwin, in his book "The Formation of 

 Vegetable Mold" (1881), calculated that the weight of the 

 castings of all earthworms present in an acre of soil during the 

 course of a year averaged between 7.56 to 18.12 tons. Earth- 

 worms are important agents in preparing the ground for the 

 growth of fibrous rooted plants and seedlings of all sorts. 

 Incidentally, earthworms play a part in the burial of stones and 

 parts of buildings. 



