CHAETOPODA 151 



Aeenicolidae, Capitellidae, Chaetopteeidae, Teke- 



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-beariug region of 



13 segments, and a tail region of variable 



, . t . 1 .1 , i n Fig. de.—Areni- 



length, m which the segments are not well cola piscatorum. 



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-waU of each segment, into which the coelom is con- 

 tinued. The parapodium is usually divided into a dorsal and a 



