ARENICOLA. 183 
The animal is plano-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 papille 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. 
External 
Features. 
The whole body is marked off by a great number of 
rings or annul, but these should be carefully distinguished 
from the far less numerous mefameric segments. In the 
greater part of the body there are five annuli to each 
segment. The number of segments, at least in (1) and (2), 
can be counted by enumeration of the appendages or the 
gills. 
The class of Polycheta 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 nofopodium and a ventral part or euro- 
podium. Each part bears a tuft of sete or bristles. In the 
active, swimming allies of Avenicola these feet are well 
developed, but in burrowing forms they tend to become 
reduced in size. Thus in Avenicola the notopodium is 
reduced to a small process with sete, and the neuropodium 
to a long pad with a single row of short hook-shaped sete. 
Arenicola has nineteen pairs of these appendages, and they 
are all similar except for the reduced size of the xeuro- 
podium in the first few segments. In the anterior region (1) 
the mouth is overhung by a small dorsal process, the pvos- 
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 zeuropodium, a minute excretory pore or xephridiopore. 
The middle portion (2) has thirteen segments, each 
having a pair of appendages, and the first three also have 
nephridiopores. All bear gills (or dranchie) which project 
