GENERAL SURVEY OF THE CLASS CHsETOPODA, 219 



forms ; of these most familiar are Titbifex rivulorum, often found 

 in the mud of brooks, and the species of Nais, remarkable for their 

 power of asexual budding. The leech-like Branchiobdella, which 

 is parasitic on the gills of the fresh-water crayfish, is a somewhat 

 aberrant member of the group. The Megadrili include the larger 

 Oligochcetes, mostly living in earth, and commonly designated as 

 " earthworms." The largest form is a Tasmanian species {Megascolides 

 gippslatidicus], measuring about 6 ft. in length, and said to make 

 a gurgling noise as it retreats underground. 



II. Polychseta. As contrasted with the more or less subterranean 

 earth- and mud- 

 worms, the marine 

 Polychreta have a 

 richer development 

 of external structures 

 and a more complex 

 life history. The ex- 

 ternal appearance is 

 greatly modified by 

 the relative degree of 

 development of the 3 _ 

 parapodia, which are 

 lateral outgrowths 

 typically functioning 

 as walking " legs," 

 or as swimming 

 organs. A para- 

 podium when fully 

 developed, is divi- 

 sible into a ventral 

 neuropodium and a 

 dorsal notopodium. 

 Each of these is 

 bilobed, bears a 

 tactile process or 

 cirrus, and is fringed 

 with firm bristles or 

 setae. Within the 

 substance of each 



lobe is embedded a stout needle-shaped "aciculum," which functions as 

 an internal skeleton, both by giving support and by serving as an attach- 

 ment for muscles. With the notopodium, further, true gills containing 

 prolongations of the body cavity are often associated. Such typical 

 parapodia occur especially in the active free-living forms like Nereis 

 and its allies, but in the order in general the parapodia show much 

 variation, and may be almost suppressed, as in Arenicola. Parapodia are 

 absent from the " prostomium, '' and are rarely fully developed on the 

 first true segment or peristomium. In both cases, however, tactile cirri 

 and tentacles are often present. The prostomium varies greatly in 

 development and structure, and is of great systematic importance ; it is 

 frequently furnished with eyes and other sense organs, but these may 



c* 



FIG. 105. Parapodium of " Heteronereis" of 

 Nereis pelagica. After Ehlers. 



i, 2, 3, 4, the leaf-like outgrowths; c 1 ., notopodial 

 cirrus; c-. , neuropodial cirrus; a 1 ., a-., acicula or 

 supporting bristles of notopodium and neuropodium ; 

 s., setae. 



