352 



FRESH-WATER BIOLOGY 



73 (7 2 ) Without a ventral epidermal pouch and ductus spermaticus. 



Mesostoma. 

 Only one species known in America. 



Mesostoma ehrenbergii (Focke) 1836. 



This species attains a length of 12 to 15 mm. in Europe. Greatest length recorded for Amer- 

 ican specimens is 6 mm. Very transparent. Color pale yellowish to brownish. Intestine 

 yellowish-brown. Thin, flat, and leaf-like in outline. Anterior end tapering, conical. Poste- 

 rior end tapering sharply and terminating in an acute caudal process. Conspicuous tracts of 

 rhabdites lead to the anterior end. Eyes black. Two shallow pits occur on the dorsal surface 

 of the anterior end, one on either side. Both summer and winter eggs are produced but 

 rarely at the same time. The summer eggs develop and the young embryos may be seen 

 within the body of the parent. From Illinois River; Lake St. Clair, Mich.; Ohio; and Elk- 

 horn River, Neb. 



A B 



FIG. 625. Mesostoma ehrenbergii. (A) Diagram from ventral side showing nervous, digestive, and 

 reproductive systems. Left side shows summer eggs, the right, winter eggs: be, bursa copulatrix; da, 

 anterior branch of intestine; da\, posterior branch of intestine; go, genital pore; k, ovary; pe, cirrus; 

 ph, pharynx; rs, receptaculum seminis; te, testes; , uterus; vd, vas deferens; vi, yolk gland; vs, 

 vesicula seminalis; wgc, female genital canal; co, subesophageal commissure of ventral nerves; din, 

 dorsal longitudinal nerve; dn, dorsal nerve of brain; g, brain; Inv, ventral longitudinal nerve; nr, pharyn- 

 geal nerve ring; MS, duct of uterus; yen, ventral nerve of brain; vid, duct of yolk gland; vn t and vn 2 , the 

 two pairs of anterior nerves of brain; x, chiasma of anterior nerves. X 6. (After von Graff, Vogt, 

 Fuhrmann, and Luther.) (B) From life, showing young worms in left uterus. X Q. (After Woodworth.) 



74 (31) With a genuine proboscis which lies within a sheath and communi- 

 cates with the exterior through an opening at the anterior 

 end. Pharynx rosette-shaped. 



Subsection CALYPTORHYNCHIA . . 75 



A small group easily recognized by the genuine proboscis. A bursa copulatrix is present. 

 The cirrus is divided into vesicula seminalis and vesicula granulorum. The rosette-shaped 

 pharynx lies on the ventral surface. 



