632 DAVID BRYCE ON FIVE NEW SPECIES OF 



gather food from the increased area thereby brought within 

 the influence of the vortices set up by the cilia of the trochal 

 disks. This habit of extension for the purpose of increasing the 

 food supply reacts in two different ways upon the structure of 

 the body. One of these is very apparent in such species as 

 Rotifer vulgaris and its nearer relatives, where the foot and the 

 rump segments have become so elongated that they constitute 

 quite a large proportion of the whole length of the animal. 



In the central section of the Habrotrochae the head and neck 

 are lengthened to a marked extent, whilst the rump and foot 

 segments, and especially the latter, become or remain relatively 

 short and unimportant. The neck frequently becomes so 

 slender that the mastax is pushed backwards into the last seg- 

 ment of the neck when the animal is crawling, and into the 

 anterior segment of the trunk or central body when it is feeding. 

 The increasing distance from the mouth to the mastax necessi- 

 tates in turn a longer gullet, which is fully but not tensely 

 stretched out when the animal displays its corona. When the 

 latter is withdrawn, the inversion of the mouth reduces the 

 distance to the mastax, and the connecting gullet becomes slack. 

 In extreme cases the slackness is so great that the gullet, which 

 is usually a little stouter near the mastax, bends over, just above 

 the stouter part, towards the ventral side, and so forms a loop, 

 which is not straightened out until the corona is again everted. 

 It is very difficult to see the loop distinctly. The animal must 

 be observed in side view and at the moment when the neck is 

 fully extended, for the gullet is not truly " looped " if it straightens 

 out before the eversion of the corona. 



In some species a further modification is seen. Not only is 

 the mastax shifted rearwards, but also the brain, which is nor- 

 mally so placed that the narrow anterior end is close to the 

 dorsal antenna, while the broader posterior part more or less 

 overlaps the mastax. In II. insignis the anterior end of the 

 brain is placed about one-half the brain-length behind the 

 antenna, and this also is best seen in lateral view. I find 

 that this modification of the position of the brain does not 

 obtain in all the slender-necked Habrotrochae, and it seems 

 therefore to be a character useful for the differentiation of 

 such species. 



Habrotrocha insignis, again, is one of the few species of Bdelloid 



