461] LARVAL TREMATODES—CORT 15 



of the outer cuticula. One of the structures (Fig. 8) is ovoid with 

 two cuticular walls, having a loosely fitted space between them, and 

 a very narrow central cavity communicating with the exterior by a 

 small pore at the tip of the projection. They apparently have no suck- 

 ing action, since no muscles are present and the central cavity con- 

 tracts when the projection is extended. They evidently have a function 

 in locomotion analogous to setae. Similar locomotor projections have 

 been described for Cercaria ephemera Nitzch and Cercaria imbricata 

 Looss, and for Leidy's (1877:200) (1904:143-144) Monostoma lucania 

 from North America they are merely mentioned as conical projections. 

 Ssinitzin (1905, Plate 4, figs. 75 and 76) figures these structures 

 in Cercaria ephemera Nitzch as clearly circumscribed projections with 

 a considerable cavity lined with spines. Figures 6a and b are copies 

 of his figures. Certainly the structures figured by Ssinitzin differ con- 

 siderably from those of Cercaria urbanensis. Marie Lebour (1907:442) 

 in the monostome cercaria from Paludestrina stagnalis which she con- 

 siders to be Cercaria ephemera describes the posterior locomotor pro- 

 jections as sucker like structures, which are circular in outline and 

 divided in two by a bar. Her figure shows them as structures quite 

 comparable to those of Cercaria urbanensis but very different from* 

 Ssinitzin 's figures. She assigns no especial reason for considering them 

 to be sucking structures. The posterior locomotor projections as described 

 by Looss (1896:194) for Cercaria imbricata offer still greater differences. 

 The cavity is comparatively large with but one cuticular wall and 

 divided at its center into two parts by a projection. Figure 7 is a 

 copy of Looss', Plate 14, Figure 151. 



Mature specimens of Cercaria urbanensis are heavily pigmented 

 especially at the anterior end, and have two lateral pigmented eyes 

 with lenses and a central anterior spot which is formed by a condensa- 

 tion of pigment. The eyes are situated dorsad at each side of the large 

 esophageal commissure and above the obtuse angles formed by the 

 large nerves which pass forward and backward. Each true eye is 

 formed by a mass of pigment in the form of a cup the bottom of which 

 is thicker than the sides. A lens fits into the opening of the cup, leav- 

 ing a space between its lower surface and the bottom of the cup. An 

 eye spot has a diameter 0.017 mm. and the depth of the pigment cup 

 is 0.024 mm. At the anterior end surrounding each eye are scattered 

 pigment granules extending in all directions and about as clearly defined 

 ventrally as dorsally. Anteriorly a condensation forms the so-called 

 anterior eye spot, and scattered granules reach well beyond the limit 

 of the oral sucker. Posteriorly the pigment granules become more 



