SEGMENTATION OP CERTAIN POLYCH^.TES. 541 



"wedges "" ). I was confirmed in this opinion by the examina- 

 tion of a specimen of Pentastomum which I happened to have 

 by me (from Professor Haddon's collection), and which is 

 apparently a female of the same species. In this one specimen 

 there are three spirals to be traced : 



Baldwin Spencer's casual mention of variation in symmetry, 

 and the fact of my finding three spirals at once in the only 

 specimen of Pentastomum available, lead me to infer that 

 spirals and wedges are probably as common, at any rate in this 

 one species of Pentastomum, as in the special Polychaetes we 

 have been considering. And it seems to me interesting for 

 the whole theory of segmentation to find such asymmetrical 

 variations occurring almost normally in two very different 

 groups of segmented animals. Cori has already drawn atten- 

 tion to the intercalation of iialf-segments in the Cestodes 

 (and it would seem that it is especially in Bothriocephalus 

 latus that such variations occur). So that we have here a 

 third group of segmented animals in which variations in 

 symmetry may be common. 



With regard to Cori's suggestions as to the origin of these 

 variations, we have as yet too few data to say anything either 

 for or against them. His statement that the irregularities 

 occur more frequently in the middle of the body than at either 

 end is not confirmed by the specimens I have examined, as 

 reference to the above tables will show. As far as Ehlers' 

 suggestion is concerned, that such irregularities are the result 

 of regeneration of lost parts, 1 will mention that 1 have fre- 

 quently seen Polychaetes regenerating heads or tails (and 

 amongst others four or five Amphinomes, one Halla, and 

 one Lumbriconereis inipatiens), but in all cases except 



