SOME SCOTTISH ROTIFERS 161 



rostrum ; Philodina has two eyes, placed in the neck ; 

 Callidina has no eyes. Though convenient in use, there can 

 be no doubt that this classification is artificial, and associates 

 species having only distant affinities, while separating others 

 which are closely related. Many observers, from Dujardin 

 onwards, have pointed out the unreliability of the eye spot 

 as a generic distinction, but no more satisfactory arrange- 

 ment has yet been proposed. Milne proposed to amend the 

 definition of Philodina so as to include all the four-toed 

 Philodinadas, and I think this is a step in the right direction. 

 The two genera, Rotifer and Philodina, are on the whole 

 natural groups. There are but a few species having the 

 eyes placed as in those genera which would have to be 

 excluded from them in a natural classification. The genus 

 Callidina, on the other hand, comprises a most heterogeneous 

 body of species, many of which have very little affinity one 

 with another. Though there is a pressing need for a 

 rearrangement of the genera, and the time is almost ripe for 

 the attempt, it is not my intention to do more than indicate 

 some groups of species which seem to me to be so closely 

 related that in a natural classification they would be 

 associated. If the four-toed species at present included in 

 the genus Callidina were joined to Philodina, and those 

 species which resemble the genus Rotifer in the long foot 

 and conspicuous toes were placed in that genus, the genus 

 Callidina would be much more compact, but would still 

 include several groups of species having only distant 

 affinities. The best marked group is that in which the 

 food is moulded into pellets. All the species having this 

 habit resemble one another in many other points. Another 

 group showing characters of possibly generic value is that in 

 which the toes are modified to form a powerful adhesive 

 disc, the so-called symbiotic rotifers. Other groups will 

 doubtless suggest themselves as our knowledge of minute 

 details of structure increases. Among the four - toed 

 Bdelloids it is probable that a genus will have to be 

 formed to include Philodina niacrostyla, P. 'acnleata, and 

 Callidina spinosa, three closely related species which differ 

 from the typical group of Philodina in the long spurs, three- 

 barred ramus, long tactile setse on the rostrum, absence of 

 51 D 



