CHAETOPODA I5I 
ARENICOLIDAE, CAPITELLIDAE, CHAETOPTERIDAE, TERE- 
BELLIDAE, SERPULIDAE. 
The Polychaeta include a vast variety of worms, which 
either swim about freely in the sea or inhabit tubes, from the 
open mouth of which they often protrude the anterior end of 
their bodies. They are very generally brightly coloured, and 
many of them, especially the fixed forms, with their feathery 
tentacles and branchiae, are objects of great beauty. With 
three exceptions, they are exclusively marine; a few are pelagic, 
and, as is usual with such a habit of life, their body is trans- 
parent. One or two only are parasitic, one 
living in the coelom of the Gephyrean Bonellia, 
another in the branchial cavity of a barnacle, 
Lepas. 
Arenicola piscatorum, the common lugworm, 
is a member of the sub-division Sedentaria, 
which tunnels out tubular passages in the 
sand, boring down into it with its head, and 
then turning the anterior end of the body 
up again, thus assuming the shape of a U. It 
can be dug up in considerable quantities in 
sandy places round our coasts when the tide 
is low; its presence being indicated by numer- 
ous little heaps of cylindrical sand castings, the 
undigestible remnants of its food. 
The worm may attain the length of ten or 
more inches, and is of a blackish-brown colour 
with a tinge of green. 
The body of the animal is divisible into 
three regions (Fig. 96): an anterior or neck of 
6 segments, a middle or gill-bearing region of 
13 segments, and a tail region of variable 
- c Fic. 96. — Areni- 
length, in which the segments are not well gota piscatorwm. 
marked. 
The chief characteristic which separates Polychaetous 
from Oligochaetous worms is the presence of parapodia. 
These, when typically developed, are lateral outgrowths of 
the body-wall of each segment, into which the coelom is con- 
tinued. The parapodium is usually divided into a dorsal and a 
