310 . 122 



emancipate themselves from a substratum and pass over to being strongly freeswim- 

 ming animals, the more vertically is the wheel-organ placed. In the families of 

 the Notommatidce, in the developmental series Hydatina, Rhinops, Notops brachionus, 

 and the Brachionidce, we find excellent examples of this phenomenon. A wheel-organ 

 of these hitherto mentioned types is really a swimming organ, but has no importance 

 as an organ by means of which the nourishment may be seized. This is only done 

 by means of the mouth parts, which are near the mouth opening, and are often 

 capable of being protruded as catching organs. Very many of the wheel-organs of 

 the freeswimming Rotifers are really modifications of this very tj'pe. 



The more the animals pass over to being freeswimming individuals, quite 

 emancipating themselves from a substratum, the more often is the disc, in accor- 

 dance herewith, placed terminally. The homogeneous cilia-covering of the disc disap- 

 pears, giving place to elevations of different kinds, hills, plates, etc., covered with 

 much stronger cilia; simultaneously herewith the bordering band of cilia of the 

 disc, commonly reduced in the middle line dorsally and ventrally, is more strongly 

 developed, and great parts of the disc itself may be quite destitute of cilia. In 

 different ways the strong cilia of the disc planted upon small elevations, hairpads 

 are of significance as stopping apparatus for the catching of prey; but still the 

 mouth parts play a prominent part in this waJ^ Wheel-organs of this type we meet 

 with in the Euchlanidcc, Salpinadce, Cohiridœ, Cathijpnadœ, Dinocharidœ, Ploesoinatidce, 

 Gastropodidce, Rattiilidœ, Hydatinidœ and Brachionidcv. Almost all these Rotifers belong 

 to smaller ponds rich in plants and whose surface is divided by means of floating 

 leaves in innumerable small pelagical regions in which most of these animals live 

 their lives, half swimming, half creeping, often fastened to the leaves. Many of them 

 are typical vegetarians, gnawing the diatoms coatings etc. upon the leaves, most of 

 them get their food when, half swimming half creeping, they move over the sub- 

 stratum using their wheel-organ mainly locomotorically; the most elaborate forms 

 are really plancton organisms, inhabiting the pelagic regions in ponds, pond lakes 

 and exceptionally also real lakes. As such vnay mainlj' be mentioned most of the 

 Brachionidce, and a few Gastropodidæ and Ploesomatidæ. In the Biachionidœ the wheel- 

 organ, in accordance with its structure, is used as well locomotorically as for the 

 catching of nourishment; in the two last named families mainl}' locomotorically, the 

 mouth parts being here used as organs which catch the prey when the animal crosses 

 its way through the water layers. In one of the most typical groups of plancton 

 Rotifers the Asplanchnadce the ciliary disc is quite destitute of cilia, a wreath of 

 cilia encircling the nude disc, only provided with some tufts of cilia with sensorial 

 functions; but also this wheel-organ is only locomotive, the prey being caught with 

 the mouth parts during swimming. 



In case the wheel-organ in freeswimming or sessile Rotifers is to act simulta- 

 neously as an organ fitted both for locomotion and for provision of food, two ciliary 

 wreaths, separated from each other by a cilia covered furrow, are developed. The 

 material caught by the wheel-organ is carried down into the furrow between the 



