SEA-FISHERIES LABORATORY. 215 



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 cirri. 



On the last or anal segment of the body, the " tail," 

 the anus opens posteriorly, and though without parapodia 

 or setse 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 Arenicola are 

 reduced compared with those of Nereis, 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 liable 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 setse, but the limits of the peristomium and the 

 composition of the region between the prostomium and the 

 first chsetigerous annulus will be discussed below (p. 217). 



Segmentation, A n n u 1 a t i o n, Parapodia. 

 Leaving out of account the prostomium, the peris- 

 tomium, and the following achaetous segment, Arenicola 

 may be, for descriptive purposes, divided into three por- 



