CLASSIFICATION OF CH^TOPODA. 209 



General Survey of Chcetopoda. 



I. Oligoclucta. 



Of Liiinbricus there are many species, e.g., the common earthworm 

 L. terrestris, the dimghill worm L. fcettdtis, and L. coiiiniiinis or 

 trapezoides, whose ova usually form twins. We may conveniently 

 include under the title " earthworms " a great array of animals more or 

 less like Iiuiibricus, and usually described as terricolous 01igoch;eta. 

 These are arranged by Beddarcl in four main groups — LuMBRICINI, 

 Geoscolecini, Acanthodrilini, and Eudrilini, with a divergent 

 group MoN'ILIGASTRES. It is enough for us to notice here that the 

 modern classification is chiefly based on the modifications of the 

 excretory system. The largest " earthworm" is a Tasmanian species — 

 Megascolides gippslandiciis — measuring about six feet in length, said to 

 make a gurgling noise as it retreats underground. 



To these must then be added a number of families, Tubificidce, 

 Enchytneida:, &c., which live in mud and water, and are often called 

 limicolous Oligocha^ta. Of these a very common representative is the 

 little river worm Tiibifex rivii/onim, often found in the mud of brooks, 

 and well suited in its transparency and small size for microscopic 

 examination. Also notable is the fresh water Nais, with remarkable 

 powers of asexual budding. Another interesting ally of Tiibifex is 

 Bramhiura, which has paired contractile gills on each of the posterior 

 segments of its body, thus resembling a Polycha;te. 



The leech-like BranchiobdeUa, which is parasitic on the crayfish, is 

 apparently an abnormal Oligochaste. 



II. Polyclucta. 



I/i\ing in surroundings usually very different from those of the more 

 or less subterranean earth- and mud-worms, the marine Polychteta have 

 a richer development of external structures, and a more complex life 

 history. From the sides of the body rings distinct outgrowths form 

 the first genuine legs. These, known as parapodia, bear bundles of 

 firm bristles, and are typically divisible into a ventral neuropodium and 

 a dorsal notopodium. Each of these is usually furnished with a probably 

 tactile process, the two being known respectively as the notopodial 

 cirrus and the neuropodial cirrus. With the notopodium the first true 

 gills, which contain prolongations of the body cavity, are often associated, 

 but the respiratory plates which occur in the sea mouse, &c. , are probably 

 metamorphosed dorsal cirri. Parapodia are absent from the anterior 

 region, but this is frequently well furnished with tactile cirri, as well as 

 with eyes, " ear sacs," and other sensitive structures. The eyes show an 

 interesting series of gradations from simple pigment spots to very com- 

 plicated structures (e.g., Alciope), exhibiting cornea, crystalline lens, 

 retina, &c. In many cases in these marine worms the blood is red as it 

 is in most Oligochi^tes, but it may be colourless [Aphrodite), green 

 (Sahella), or yellow. The pigment is usually dissolved in the plasma, 

 and its variations in character and amount among nearly allied forms are 

 of great interest to the comparative physiologist. The gut is frequently 

 branched and of large calibre. In some cases (Capitellidce) it possesses 

 an accessory communicating tube (Nebendarm) which is of interest, 



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