560 



FRESH-WATER BIOLOGY 



malleate type (Fig. 860, ^), on account of the great development of 

 the mallei. (2) In other species all parts of the trophi are long and 

 slender; the unci end in a single sharp point, which may be thrust 

 out of the mouth to seize upon living prey. The two rami Hke- 

 wise form a pair of strong, blade-Kke jaws. Such trophi are said 

 to belong to the forcipate type (Fig. 860, B) ; they are found in active 

 rotifers of predatory habits. There exist many modifications of 

 these two types, and many jaws intermediate between the two. 

 Both types of jaws are found in the Notommatidae. 



The mastax usually bears near its posterior end a pair of small 

 glands that are known as salivary glands (Fig. 856, sg). From the 

 mastax the food passes into the slender esophagus (Figs. 856 and 

 857, 0), which leaves the mastax on its dorsal side. Through the 

 esophagus the food reaches the large stomach (st), where digestion 

 takes place. Attached to the anterior end of the stomach are the 

 two large gastric glands (gg). From the stomach the undigested 

 remnants of the food pass back into the straight slender intestine, 

 and thence to the outside at the cloacal opening (c). This lies on 

 the dorsal side of the body, above the foot. 



The body cavity is enclosed by but a single layer of cells, which 

 form the body wall, so that each cell is bathed on its outer surface 

 by the outer water and on its inner surface by the fluid of the body 

 cavity. By this arrangement the processes of respiration are 

 made very simple. Oxygen doubtless passes from the surround- 

 ing water through the single layer of cells into the body fluid, while 

 the waste carbon dioxide produced within is given off in the same 

 way to the outside. 



The nitrogenous waste products are not so easily eliminated as 

 is the carbon dioxide; for removing these the rotifers have a set of 

 excretory organs. These consist of fine tubules running through 

 the body cavity at the sides of the alimentary canal (see Fig. 857, 

 ex, and Fig. 861). On each side there are usually two tubes, one 

 with thick walls (a), the other with very thin ones (b). These two 

 are usually connected (c) in the anterior part of the rotifer. They 

 commonly bear at intervals along their course certain minute 

 club-shaped organs (Figs. 85 7 , 5 ; 86 1 ,/c) . These are closed at their 

 free ends, and contain within them either a vibrating membrane 





