ON MEGASCOLEX CCEBULEUS. 53 



and there is a small gap in the ring in the median dorsal and 

 ventral lines. The dorsal and ventral gaps are equal in the 

 greater part of the body to the interspaces between three or 

 four of the neighbouring setse. The seta gaps are throughout 

 smaller in the ventral than in the lateral and dorsal regions. 



In the most anterior segments the gaps, especially in the 

 lateral and dorsal regions, and above all the median dorsal 

 gap, are much larger than in the posterior segments. The 

 number of the setae is greater than in any other recorded 

 Perichaete. There are 120 to 140 in most segments, but con- 

 siderably fewer in the most anterior segments. I have found 

 as few as thirty-six in segment v.^ 



The most ventrally placed setae, especially in the neighbour- 

 hood of the male pores, are a little larger than usual, but I 

 should not speak of them as " modified " setae. There are no 

 setae in segment xviii between the male pores.^ 



I have never found the setae absent from the clitellum, as 



1 I am in the habit of counting the setse in segments v, ix, and xxv, the 

 latter segment serving as a type for the rest of the body, and I find that the 

 relation of these numbers to one another varies with other important characters 

 rather than the actual numbers themselves. 



The method I adopt in examining the arrangement of the setse is to cut 

 open the freshly killed worm on one side of the dorsal median line, to scrape 

 out the viscera, to flatten the body-wall of the most anterior twenty-five 

 segments between two glass slips, and to allow it to harden in spirit ; subse- 

 quently to treat this piece of skin with potash followed by glycerine, and 

 then to mount it. 



This process renders it possible to see the follicles, even where the actual 

 seta has fallen out. 



The shape of the setae varies so slightly in different species, that except in 

 special cases it is of little use for classificatory purposes. In a preparation 

 made as above, modified setse in the neighbourhood of the spermathecal aper- 

 tures or male genital pores should always be looked for. 



* The presence or absence of setse in segment xviii between the male 

 pores is a most important character among the Perichsetidse. In ail worms 

 which I think on other grounds it will be advisable to place in the genus 

 Pericliaeta s. sir. such setse are present. They are also present in certain 

 other worms which have other special peculiarities, but in a very large 

 number of species, e.g. P. armata, and the majority of Fletcher's Australian 

 species, they are absent, 



