A REN ICO LA. 183 



The animal is piano-symmetric, with a mouth at the 



anterior and an anus at the posterior end. The pharynx 



is protrusible and is used by the animal for 



"rolling" sand and food into the alimentary 



canal. In the living animal its action may 



often be seen. It is covered with rough papillae on its 



front part and with hooks further back. 



The body is differentiated by its structure into three 

 portions : — 



( 1 ) The anterior region with appendages but no gills. 



(2) The middle part with gills and appendages. 



(3) The posterior region or tail with neither gills nor appendages. 



The whole body is marked off by a great number of 

 rings or annuli.hvX these should be carefully distinguished 

 from the far less numerous 7neta7neric segme?tts. In the 

 greater part of the body there are five annuli to each 

 segment. The number of segments, at least in (i) and (2), 

 can be counted by enumeration of the appendages or the 

 gills. 



The class of Polychceta has each segment typically pro- 

 vided with a pair of lateral appendages, called feet or 

 parapodia^ and each parapodium usually has two parts — a 

 dorsal portion or notopodium and a ventral part or neuro- 

 podium. Each part bears a tuft of setce or bristles. In the 

 active, swimming allies of Arenicola these feet are well 

 developed, but in burrowing forms they tend to become 

 reduced in size. Thus in Arenicola the notopodium is 

 reduced to a small process with setae, and the neuropodium 

 to a long pad with a single row of short hook-shaped setae. 

 Arenicola has nineteen pairs of these appendages, and they 

 are all similar except for the reduced size of the neuro- 

 podium in the first few segments. In the anterior region (i) 

 the mouth is overhung by a small dorsal process, ^^ pros- 

 tomium, and immediately behind this is the peristomium 

 which differs from the true segments in having no 

 appendages. Then follow six true segments, each having 

 appendages, and the last three of which have, just above 

 the ?ieuropodium, a minute excretory pore or nephridiopore. 



The middle portion (2) has thirteen segments, each 

 having a pair of appendages, and the first three also have 

 nephridiopores. All bear gills (or branchice) which project 



