62 ALFRED GIBBS BOURNE. 



The pigment changes very slightly, at any rate for some 

 months, when the worm is preserved in spirit ; but the reddish 

 portions shown in the figures, which are due to absence of 

 pigment and great blood supply, naturally disappear when the 

 specimen is placed in spirit. When the worm is alive these 

 red portions are very noticeable, and indicate the greatly in- 

 creased blood supply in the prostomium and round the mouth, 

 and in the neighbourhood of all the genital apertures 

 (fig. 2). 



Number of Segments. — I have counted the number of 

 the segments in specimens varying from 10 inches to 32 

 inches in length, and have found it to vary between 250 and 

 290, being usually about 270. In fig. 1, 286 segments are 

 shown. 



Prostomium. — This is, as is often the case, of a blood-red 

 colour, and is broad and blunt at the extremity ; there is a 

 short transverse groove along its line of junction with the 

 peristomial (buccal) segment, and both it and the latter seg- 

 ment present in spirit-preserved specimens numerous longi- 

 tudinal grooves.^ 



Setee. — In shape the setae are, as compared with those of 

 many perichsete wprms, short, stout, and only slightly 

 curved. 



As to their position, they are of course arranged in rings, 



of contraction, and a contraction which produces a considerable alteration in 

 length produces a barely measurable increase of thickness. 



It is interesting to note that worms of some species possess a much greater 

 power of contraction than others. P. mirabilis, for instance, contracts and 

 extends itself to but a very slight degree, and has a movement like a Neumtoid, 

 while Moniligaster grandis can contract itself to about a quarter of its 

 fully extended length. 



1 We have at present few data to enable us to make much use of the pro- 

 stomium in classification. It will be necessary to compare its structure in a 

 large series of living worms. In preserved specimens it becomes contracted 

 to a large and very variable extent. Moreover, it never has when the worm 

 is alive the same shape for two seconds together. Many worms have a habit 

 of constantly protruding a large portion of the pharyngeal region, and there is 

 no line of demarcation between the prostomium on its ventral side and this 

 latter region. 



