36 Arenicolidae 



" Seitenorgane " of other Polychaeta, but it has not been found 

 possible to determine, on the material available, if the pits are 

 really sensory structures. 



The number of segments characteristic of each of the caudate 

 species is, on the whole, constant, only very few variations having 

 been observed. A. cristata and glacialis have 17 segments, A. marina, 

 pusilla, loveni and assimilis var. affinis have 19 segments, while the 

 typical form of A. assimilis has 20 segments. 



In the ecaudate species there are considerably more segments, 

 the full number of which about 42 in A. branchialis and 64 in 

 A. ecaudata is already present at the end of the post-larval stage. 

 The number of segments in these species does not increase with 

 age, on the contrary, segments are usually lost posteriorly and 

 are not regenerated, so that specimens are generally found to 

 have fewer segments than stated above ; average specimens of 

 A. foanckialis have about 30, and of A. ecaudata about 45 to 50 

 segments. 



Each chaetiferous segment, except the first two or three, is sub- 

 divided into five rings, 1 and a consideration of the internal anatomy 

 shows that the posterior limit of the segment is at the posterior edge 

 of the ring which follows the chaetiferous one. Thus each of the 

 segments, posterior to the third, consists of the chaetiferous annulus 

 together with the three annuli in front of and that behind it. 

 The third segment consists of three or four rings, the penultimate 

 being chaetiferous ; the second segment is generally subdivided into 

 three rings, of which the middle one bears the parapodia. The first 

 segment 2 is usually composed of two rings, the anterior of which 

 is chaetiferous; in rare cases another small ring is present and 

 lies in front of the chaetiferous annulus. The limits of these 

 segments are ascertained chiefly by an examination of the internal 

 septa, which are a constant feature of this region in all species 

 of Arenicola. 



Between the anterior margin of the first chaetiferous segment 

 and the prostomium there is a region which, in most adult specimens 

 of Arenicola, is subdivided by encircling grooves into three or four 

 (or more) rings. There are good reasons for stating that this region 

 is composed of the peristomium and a body-segment which is with- 



1 Occasionally the last one or two segments in A. branchialis, and the last 

 one to four segments in A. ecaudata, are subdivided into fewer than five 

 annuli. 



2 That is, the first chaetiferous segment. The word " segment " throughout 

 the following pages means " chaetiferous segment." 



