135 
in shape to a body segment, though frequently larger and 
always without parapodia, bears on each side at its 
anterior edge four long slender filiform sensory ciz71. 
On the last or anal segment of the body, the “ tail,” 
the anus opens posteriorly, and though without parapodia 
or sete this segment bears a pair of long anal cirri, which 
probably correspond to the ventral cirri of the trunk 
segments. 
The prostomium and parapodia of <Arenrcola are 
reduced compared with those of Mere’s, and the peris- 
tomium and the segmentation of the body are much less 
distinct in the former than in the latter. 
The prostomium of Arenicola may be seen on exam- 
ining the anterior end of the animal in the mid-dorsal 
line. It is a trilobate structure which, in fully-grown 
Specimens, is about 1 mm. in length by nearly 2 mm. in 
breadth. It is hable to be retracted within the crescentic 
nuchal groove which lies immediately behind it, and in 
that case is difficult to see, but if well-expanded the pros- 
tomium assumes the form shown in fig. 5. — It does not 
bear any sensory processes, and eyes, though present, are 
minute and sub-epidermal, and are not visible in the 
adult except in sections. 
There is good reason for believing that the peri- 
stomium is represented by the first two annuli, and that 
the third and fourth annuli represent a segment which has 
lost its sete, but the limits of the peristomium and the 
composition of the region between the prostomium and the 
first cheetigerous annulus will be discussed below. 
meenientatran, Annulation, Parapodia. 
Leaving out of account the prostomium, the peris- 
tomium, and the following achztous segment, Arenicola 
may be, for descriptive purposes, divided into three por- 
