300 AN INTRODUCTION TO THE STUDY OF FOSSILS 



two long), in the variable number of body segments, and in the 

 thinner and more flexible exoskeleton. 



Apus is similar to the crayfish in the growth of the hard parts, 

 in the presence of the upper and lower lips and in the five pairs 

 of head appendages, but is more primitive in having the pos- 

 terior pair of cephalic appendages biramous, and in the practical 

 absence of distinction between the thoracic and abdominal 



Fig. 132. — The primitive, fresh-water crustacean, Apus lucasanns Packard, from 

 Kansas. Section through the body, with intestine removed. Length of body, 

 exclusive of anal processes, is twenty miUimeters. an., anus ; an. pro., anal process ; 

 app., appendages; br., brain; cp., carapace; d.gl., digestive gland; ///., heart; 

 int., position of intestine; mo., mouth; n.ch., ventral nerve chain; ov., ovary; 

 St., position of stomach. (Redrawn from Packard.) 



appendages. All of these except a few at the apex are leaf-like 

 with a spinose projection (gnathobase) from the basal portion ; 

 these gnathobases, meeting the ones from the opposite side of 

 the body, function as a conveyor of food to the mouth. 



In the muscular system, its more primitive structure is shown 

 by the arrangement of the muscles in a tube, as in the Annulata ; 

 they do not fill up most of the abdomen as they do in the cray- 

 fish. 



The kind of food, its intake, the digestive system, absorption 

 of food and excretion are similar to the crayfish. There is, 

 however, but one division to the stomach, neither gastric mill 



