AQUATIC EARTHWORMS 633 



at any great distance from the sea. Further, more careful study 

 of the Ufe in brackish water estuaries and fresh-water bodies in 

 close contiguity with the ocean is likely to reveal the presence of 

 such forms in other locahties. 



Fresh-water Oligochaeta 



The Oligochaeta, including the earthworms and related aquatic 

 forms, are segmented worms which have a somewhat extensive 

 and well-defined body cavity separating the aHmentary tract from 

 the body wall. They are hermaphroditic, with the reproductive 

 organs limited to a few definite segments or somites. The bristle- 

 like setae in the body wall aid in locomotion, but such setae are 

 absent in the family Discodrihdae which are parasitic on the ex- 

 ternal surfaces of crayfishes. The majority of the species are 

 terrestrial, and the aquatic forms are nearly all confined to fresh 

 water. Seven families of aquatic Oligochaeta are found in the 

 northern hemisphere and are all abundantly represented in the 

 United States. 



Morphological Relations. The general plan of structure of the 

 aquatic forms agrees essentially with that of the earthworm. 

 External metamerism is indicated by the transverse grooves and 

 by the segmentally arranged setae, and the corresponding internal 

 metamerism is recognizable in the septa, nephridia, transverse 

 blood vessels, and in the ganglia of the ventral nerve chain. The 

 prostomium — the dorsal part of the anterior somite extending 

 anterior to the mouth — is very flexible and sensitive, and is an 

 important tactile organ which in some species is prolonged into a 

 proboscis (Fig. 984.) The somites are numbered consecutively from 

 the anterior end, and are designated in these pages by Roman nu- 

 merals in accordance with a common practice. The boimdary 

 between two somites is indicated thus: X/XI. 



The setae are usually conspicuous and are of taxonomic impor- 

 tance. They are commonly grouped into dorsal and ventral bundles, 

 the most anterior ventral pairs being always on the second somite. 

 Figure 991 illustrates some of the more usual types of setae. 



The nephridia are the segmental excretory organs, which typi- 

 cally are paired and are usually present in all somites of the body 



