266 PHYLUM ANNELIDA 



to variation in the structure of the excretory system. In all, with the 

 exception of Molosoma, certain of the nephridia are modified to serve 

 as genital ducts, while in the Megascolicid^e the nephridia tend to be 

 reduced to a mass of minute tubules ramifying over the inner surface 

 of the body wall. In general the Ohgocha^ta, however, show more 

 uniformity of structure than their marine alhes. 



They may be divided into two main groups — (i) the Microdrih, 

 and (2) the MegadriU. The first group includes the small aquatic 

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

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

 power of asexual budding. Some Microdrili live between tide-marks. 

 The leech-like Branchiobdella, which is parasitic on the gills of the 

 fresh-water crayfish, is a somewhat aberrant member of the group. 

 The MegadriU include the larger Oligochaeta, mostly living in earth, 

 and commonly designated as " earthworms." The largest form is a 

 Tasmanian species {Megascolides gippslandicus), measuring about 6 ft. 

 in length. 



II. Polychaeta. — As contrasted with the more or less subterranean 

 earth- and mud-worms, the marine Polychaeta have a richer develop- 

 ment of external structures and a more complex life-history. The 

 external appearance is greatly modified by the relative degree of 

 development of the parapodia, which are lateral outgrowths typically 

 functioning as walking " legs," or as swimming organs. A para- 

 podium, when fully developed, is divisible 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 seta^. 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 attachment for muscles. With the notopodium, further, 

 true gills containing prolongations of the body cavity are often associ- 

 ated. 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 also occur on other regions of the body. Apart from the 

 parapodia, the shape and appearance of the body are most affected by 

 the condition of the septa. In the active free-living forms (Errantia) 

 these are usually present throughout the body, and give a characteristic 

 worm-like appearance. In burrowing and tubicolous forms (Sedentaria) 

 the septa tend to be suppressed. Their absence facilitates burrowing, 

 by permitting free movement of the ccelomic fluid, and is often associ- 

 ated with a division of the body into regions, and a loss of the typical 

 uniform shape (cf. Arenicola). 



With regard to internal organs, the gut is frequently branched and 

 of large calibre. In some cases (Capitellidas) it possesses an accessory 

 communicating tube (Nebendarm), which is of interest, because it has 

 been compared to the notochord of Vertebrates. There is typically a 



