ANNELIDS. 
433 
the segments of the body, and that the latter are defined externally by transverse 
grooves, and internally by septa or partitions, which divide the body-cavity into a 
series of compartments. 
Many-Bristled Group, —Order Polycileta. 
The Annelids of this order live exclusively in the sea, and may be referred to 
two sections or suborders, the one being the wandering or roving species, which 
have no fixed abode (Errantia), while the other sedentary forms live habitually in 
tubes which they construct for the purpose. In the former of these two suborders 
in accordance with the free 
roving life, the front end of 
the body is furnished with 
a conspicuous lobe, or pro- 
stomium, overhanging the 
mouth, which bears eyes and 
feelers. Those forms that 
are carnivorous seize their 
prey with sharp - hooked 
teeth, visible at the end of 
the proboscis. The struc¬ 
ture of one of the parapodia 
is shown in the accom¬ 
panying figure. It consists 
of two principal branches, 
an upper {A) and a lower 
( B ), each being supported 
by a long stout bristle (e, i). 
The branches are further 
divided into several well- 
defined parts. For instance, 
in the upper there is a con¬ 
spicuous feeler or sensory 
cirrus {a), and a bilobed 
leaf-like plate ( 6 , c), from 
the lower lobe of which 
(c) projects a cluster of arrow-shaped bristles. Analogous parts may be recognised 
in the lower branch, f being the feelers, at the base of which is the leaf-like plate 
(k ); while the larger leaf-like plate (g) supports a second and larger tuft of 
similarly-shaped bristles. A well-known example of this group is the sea-mouse 
of the British shores {Aphrodite aculeata), a broad-bodied, somewhat slug-shaped 
creature, commonly 3 or 4 inches in length. Like many of the species of marine 
worms the sea-mouse is ornamented with iridescent hues, revealed by cleansing 
the skin of the mud and sand with which it is usually coated. The back is 
furnished with a double row of large overlapping scales, but in the British 
species these scales are concealed by a close felt of hairs, although in another kind 
VOL. vi .—28 
PARAPODIUM AND BRISTLES OP A POLYCH.ETOUS ANNELID, IlcterOliereis 
cerstedi (much enlarged). 
