THE WHEEL ANIMALCULES (ROTATORIA) 559 



The trophi consist of a number of pieces, so arranged that two main 

 parts may be distinguished. There is a middle portion, somewhat 

 fork-shaped, which is known as the incus (Fig. 860, in), and two 

 lateral parts known as the mallei {ma). 



In the middle portion or incus may be distinguished a single 

 basal piece, comparable to the handle of the two-tined fork; this 

 basal piece is known as the fulcrum {fu, Fig. 860). The two blade- 

 like pieces resting on it, i.e, the tines of the fork, are the rami ira). 

 A B 



ma.: 

 -rnu. 



~mu. 



Fig. 860. Trophi or jaws of rotifers. ^, Malleate type. (From Wesenberg-Lund, after Hudson and 

 Gosse.) B, Forcipate type, from Z)ig/e«a/orc//>a/a Ehr. (After Gosse.) /m, fulcrum; in, incus; m<i, mallei; 

 mil, manubrium; ra, ramus; un, uncus. 



The rami are joined to the fulcrum in such a way that they may 

 move back and forth, like the blades of a pair of shears. They 

 often bear teeth. 



In the lateral parts or mallei one may likewise distinguish two 

 parts. The basal piece, serving as a sort of handle, is known as the 

 manubrium (Fig. 860, mu). Joined to the top of this, but placed 

 nearly at right angles to it, is the piece known as the uncus {iin)\ 

 the two unci usually lie across the tops of the rami, their points 

 meeting in the middle. Each uncus may bear one or more points, 

 or a number of sharp ridges serving as teeth. The food passes 

 between the teeth of the unci and rami and is cut and ground by 

 them. The jaws are worked by muscles which are attached to 

 the manubria and to other parts of the apparatus; these muscles 

 make up the main part of the mastax. 



In different rotifers the trophi vary much in the form and rela- 

 tive development of the t>T)ical parts; this is true even within the 

 Notommatidae. There are two main lines of divergent develop- 

 ment: (i) In many rotifers the parts of the trophi become thick 

 and stout; the unci are broad plates bearing a number of ridges. 

 Such jaws are used mainly for grinding, and are said to belong to the 



