4i6 



FRESH-WATER BIOLOGY 



198 (197) Tail long with terminal organ for attachment. 



Megalurous cercariae. 

 Single American species known. 



Cercaria megalura Cort 19 14. 



Develops in rediae. Cystogenous glands abundant. Does not swim in open water or use 

 tail as swimming organ, but as a stalk, becoming attached by the adhesive organ, a group of 

 unicellular glands at the tip. The tail has the power of elongating very greatly. In this 

 position the worm waves or wriggles about in a serpentine fashion. When taken up in a pi- 

 pette it encysts quickly and this seems to be normal on contact with fresh water. 



From Pleurocera elevatum, Sangamon River, 111., and Goniobasis virginica, Princeton, N. J. 

 Adult unknown. Reproductive organs indicated by two masses of nuclei joined by line. 



199 (195) One or more spines present at anterior end 200 



200 (224) xA.nterior end provided with single median boring spine. 



Stylet cercariae . . 201 



These forms called Acanthocephala by Diesing and Xiphidiocercariae by Liihe are numer- 

 ous and perhaps not closely related; even if the stylet cercariae do belong to different adults, 

 their assemblage in a single group is convenient. 



Small, slender-tailed cercariae with rounded anterior margin, bearing a dagger-shaped 

 boring spine or stylet, usually in the upper lip of the oral sucker. The form of this organ 

 is very definite in each species and varies between different species distinctly enough to form 

 in many cases a valuable mark for diagnosis. Eye spots are usually wanting. Development 

 in sporocysts is most frequent and encystment in a second intermediate host usual in species 

 of which the development is known. 



201 (217) Tail slender, not provided with special organs (bristles, fin-fold) 



or regions 202 



202 (203) Stylet glands few in number, not more than four on each side. 

 Tail attached to median posterior extremity, not arising 

 from distinct caudal pocket. . . Cercariae microcotylae. 



Very small. Body less than 0.2 mm. long. Stylet glands 3 to 4 only, 

 near acetabulum. Excretory bladder small, forking more or less acutely 

 at anterior end. These forms are all minute and further study may dis- 

 close the presence of a caudal pocket with minute spines in some or all 

 species. Two species; not analyzed in key. 



Median stem of excretory bladder elongate, club-shaped. 



Cercaria leptacantha Cort 1914. 



Body oval; circular in cross section, 0.12 mm. long by 0.063 mni- wide. 

 Tail slender, shorter than body. Stylet small. Not fully developed. Sur- 

 face in living specimen with highly refractive prominent globules of differ- 

 ent size. 



Produced in oval thin-walled sporocysts from Campeloma decioum 

 Hartford, Conn. 



Fig. 718. Cercaria leptacantha, immature, ventral view, a, stylet, ventral view. 

 X 320. (After Cort.) 



Median stem of excretory bladder short. 



Cercaria caryi Cort 19 14. 



Very small; stylet glands present, few in number. Acetabulum small; 

 develops in sporocysts. From Goniobasis virginica, Princeton, N. J. 



Fig. 719. Cercaria caryi, ventral view. From Gary's material. X 140. (After 

 Cort.) 



203 (202) Stylet glands more numerous, six or more on each side. Tail aris- 

 ing from posterior caudal pocket, ventral to excretory 

 bladder Polyadenous cercariae . . 204 



