210 PHYLUM ANNELIDA. 
Annelids with Bristles 
Segmented animals with sete developed in little skin-sacs, 
either on a uniform body wall or on special locomotor pro- 
trusions known as parapodia. The segments, indicated 
externally by rings, are often marked internally by parti- 
tions running across the body cavity, which is usually well 
developed. The nervous system generally consists of a double 
ventral chain of ganglia, connected with a pair of dorsal 
cerebral ganglia by a ring round the beginning of the gut. 
Two excretory tubes or nephridia are typically present in 
each segment, and they or their modifications may also 
function as reproductive ducts. The reproductive elements 
are formed on the lining membrane of the body cavity. The 
development ts either direct or with a metamorphosis. 
The two chief orders of this class may be contrasted :— 
Class CHATOPODA. 
OxicocHa&TA, e.g. Earthworm. 
Potycu ata, e.g. Nereis. 
With no parapodia, and with relatively 
few seta. 
Without any “jaw” apparatus in the 
pharynx. 
Head not highly developed. No tent- 
acles or cirri. Gills in a few forms. 
With complex hermaphrodite reproduc- 
tive organs, limited in number and 
definitely localised. 
Development direct. 
Living in fresh water or in the soil. 
With parapodiaand with very numerous 
seta. 
The pharynx is often armed with 
“jaws.” 
The head is much more developed, and 
bears tentacles and cirri. Gills are 
often present. 
Sexes usually separate, and reproduc- 
tive organs simple. 
A metamorphosis in development. 
Marine, with two or three exceptions. 
Type of OLticocHa#Ta. The Earthworm (Lumbricus) 
Habits.—Earthworms eat their way through the ground, 
and form definite burrows, which they often make more 
comfortable by a lining of leaves. The earth swallowed by 
the burrowers is reduced to powder in the gut, and, robbed 
of some of its decaying vegetable matter, is discharged on 
the surface as the familiar “‘worm-castings.” By the 
burrowing the earth is loosened, and ways are opened for 
plant-roots and rain-drops; the internal bruising reduces 
mineral matter ‘to more useful form; while, in covering 
the surface with earth brought up from beneath, the earth- 
