210 ILLINOIS BIOLOGICAL MONOGRAPHS [210 



This material was obtained from Ambly stoma tigrinum (Green) 

 caught in ponds in Nebraska, both at Crete and in Cherry County, also 

 in a pond at Belleville, Kansas. The specimens were taken during the 

 summers of 1907 and 1908. The data presented here are for the most 

 part extracted from the former article (La Rue, 1909) but the data 

 regarding the character of the fifth sucker are new. 



These cestodes are attenuate, thin and flat. In life they are white, 

 at times somewhat translucent. The chain presents no evident strobi- 

 lation, for the proglottids are closely joined by their full breadth and 

 the intersegmental furrows are shallow. There are no longitudinal or 

 transverse furrows in preserved specimens. The strobila measures 80- 

 110 mm. in length by a maximum breadth of 0.80-0.90 mm. The scolex 

 (Fig. 26) is globose, flattened somewhat dorsoventrally, and possessing 

 a smooth conical apex in which there is no depression nor fifth sucker. 

 There is no rostellum and no spines. The surface of the head is usually 

 smooth and but rarely marked by furrows. The head measures 0.366- 

 0.40 mm. broad. Four oval suckers are borne at the broadest part of 

 the head. They measure 0.165-0.184 mm. in the maximum dimension. 

 The suckers have deep cavities and their musculature is well developed. 



As in many other species of this group which possess no functional 

 fifth sucker there is in the tissue below the surface of the apex of the 

 head a structure which the writer formerly called an endorgan. This 

 structure appears as a small mass of tissue surrounded by a definite 

 membrane. The mass of tissue contains a few nuclei and a few scat- 

 tered fibers which may be muscle fibers. In the adult this structure is 

 about 0.063 mm. long by 0.034 mm. broad. It has no opening to the 

 exterior. Two histological drawings of the adult organ are to be found 

 in a previous paper (La Rue 1909, Figs. 13 and 17). In the plerocercus 

 this structure is much larger than in the adult. This fact was pointed 

 out in the author's paper of 1909 and figures were given to show the 

 difference in size. These figures have been reproduced (Figs. 27, 28). 

 If sections through the head of plerocercoids of this species be examined 

 one notes that the endorgan has many points in common with the func- 

 tional fifth sucker. There is a sucker cavity communicating with the 

 exterior, a basement membrane, muscle fibers (sub-cuticular and radial). 

 The musculature about the organ is also like that about other suckers. 

 These histological features are shown in drawings which are reproduced 

 (Figs. 43-46). This structure is, however, undergoing a marked modi- 

 fication. The sucker cavity is completely or partially filled with a gran- 

 ular mass of apparently the same texture as that which makes up the 

 greater bulk of the sucker itself. The radial muscles are no longer 

 arranged in such beautiful order as in other suckers but they seem to 



