344 Illinois State Laboratory of Natural History. 



power of protruding the tropin through the buccal orifice, 

 and they may often be found nibbling at algae and other 

 aquatic plants. Although the tropin are used by many 

 rotifers to crush their food, there are some large forms, like 

 Asplanchna, that swallow their food entire, as is shown by the 

 uninjured rotifers, protozoans, and algre found in their 

 stomachs. 



The chitinous jaws or tropin (Fig. 3) consist typically of 

 two hammer-like lateral parts, the mallei, bearing one to 

 seven comb-like apical teeth, and working upon the two 

 halves of a divided central part, the incus. The mallei are 

 usually separable into an apical part bearing the teeth, the 

 uncus, and a basal part, the manubrium. The two divisions 

 of the incus are the rami and its basal projection is the ful- 

 crum. These inner and outer pairs vary greatly in relative 

 development, the mallei in some groups disappearing entirely. 

 Hudson and Gosse recognize seven types of tropin, as shown 

 in the figure below. Dr. Hudson's summary of the distin- 

 guishing features of these types is given on the next page. 



un+wa 



Fig. 3, Diagram of trophi: A, malleate; B. sub-malleate: C.virgate; D, 

 forcipate; E. raalleo-ramate ; F, incudate; G, uncinate; ET, ramate. 

 f, fulcrum; i, incus; wia, manubrium: r, ramus; un, uncus. (After 

 Hudson, from the Cambridge Natural History.) 



