34 ILLINOIS BIOLOGICAL MONOGRAPHS [480 



substratum to ten times that length when the cercaria is attached. The 

 body is also about four or five times as long when extended as when 

 contracted. At average extension the body is slightly pointed anteriorly 

 and the acetabulum is but little more than half the distance from the 

 anterior to the posterior end. The preacetabular region is a little wider 

 than the postacetabular, and the postacetabular tapers slightly toward 

 its truncated posterior extremity. 



In mounted specimens the average length of the body is 0.4 mm., 

 and the widest diameter of the preacetabular region averages 0.13 mm., 

 while that of the postacetabular is 0.09 mm. The width of the tail at its 

 base varies from 0.03 mm. to 0.054 mm., depending on the state of 

 contraction. The oral sucker is lightly elongate, having an average 

 length of 0.05 mm. and a width of 0.045 mm. The acetabulum is slightly 

 larger and circular, averaging 0.054 mm. in diameter. 



The structure of the tail of Cercaria megalura (Figs. 30 and 32) 

 is interesting on account of its remarkable power of extension and the 

 modification of its posterior end for attachment. It is truncated and 

 there is an invagination at its tip. Into this inpushing open a clump of 

 from 15 to 20 unicellular club-shaped glands. These glands have an 

 average length of 0.017 "mm. and width of 0.009 mm., and contain nuclei 

 measuring 0.005 mm. in diameter. It is probable that these glands 

 secrete some substance which makes possible the adhesion of the end 

 of the tail. The tail is strongly attached at its base and except near the 

 tip appears to be filled with vesicles, which are stretched out when the 

 tail is extended and compressed when it is contracted. Figure 32 

 represents a cross section of the tail. The cuticula is very thin, the 

 muscle layers reduced, and the great bulk of the tissue is made up of 

 parenchymatous cells, the nuclei of which are surrounded by small 

 masses of protoplasm. These cells are connected by strands of proto- 

 plasm and the large intercellular spaces which are filled with clear fluid 

 give the appearance of vesicles. It is the looseness of the tissue of the 

 tail which makes possible its remarkable changes in shape. 



The cystogenous glands of Cercaria megalura are very highly de- 

 veloped and when the cystogenous material is still present in them they 

 from the bulk of the whole body. These glands (Fig. 33 cs) are large, 

 elongate, club-shaped cells, most of which open on the ventral surface, 

 a few only opening dorsally. They are full of tiny, rod-shaped, cysto- 

 genous granules. The cells have an average length of 0.036 mm. and a 

 width of 0.012 mm., and contain at about their centers large nuclei. 

 They are present all thru the body of the cercaria from the posterior 

 limit of the oral sucker almost to the posterior extremity. A comparison 

 of cross sections of two cercariae at the region of the esophagus shows 



