CLASSIFICATION OF CH^TOPODA. 185 



General Survey of Chatopoda. 



I. Oligochceta. 



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

 terrestris, the dunghill worm Z. fcetidus, and Z. communis or trapezoides, 

 whose ova usually form twins. We may conveniently include under the 

 title "earthworms" a great array of animals more or less like Z«/«/;?-«V«j, 

 and usually described as terricolous Oligochoeta. The senior student 

 should make himself acquainted with the four groups — LUMBRICINI, 

 Geoscolecini, Acanthodrilini, and Eudrilini, and with the 

 divergent branch Monit.igastres, but it is enough for us to notice here 

 that the modern classification is mainly based on the modifications of 

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

 — Megascolides gippslandicus — 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, Tubificidm, 

 Eiuhytrmidce, etc. , which live in mud and water, and are often called 

 limicolous Oligochseta. Of these a very common representative is the 

 little river worm Tubifex rivulorum, 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. 



The advanced student should take note of the leech-like Branchiobdella-, 

 which is parasitic on the crayfish, and apparently an abnormal Oligochaete. 



The two sets of which Lumbricus and Tubifex are types, are united as 

 Oligochseta, i.e., with few setse, in contrast to the marine Chjetopods 

 where the bristles are numerous, the Polychasta. 



II. Polychceta. 



Living in surroundings usually very different from those of the more 

 or less subterranean earth- and mud-worms, the marine Polychasta 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 

 bristles, and are typically divisible into a firmer ventral neuropodmm, 

 often used for creeping, and a more leaf-like dorsal notopodmm often 

 respiratory. Special outgrowths of skin, on which blood vessels are spread 

 out, form the first genuine gills, and soft processes or cirri are present on 

 some or all of the rings. The head is equipped with antennae and other 

 tactile organs, and not unfrequently with eyes, ear-sacs, and other 

 sensitive structures. The sexes are usually separate, and the develop- 

 ment includes a metamorphosis, the larval trochosphere bemg quite 

 different from the adult worm. 



(a) Some of these marine Polychsetes lead ^ free and more or less 

 active life, crawling between tidemarks or on the sea-bottom, burrowing 

 in the sand, or swimming in the open water. These Errantia have well- 

 developed appendages, and a large pre-oral segment, and are genera y 

 furnished with eyes and well-developed antennae. Gills are usually 

 associated with the dorsal parts of the parapodia. Most of them feed on 



