16 



INTRODUCTION. 



caecum, (where such a structure exists,) contains nothing but 

 young Cercarice. These young are developed, not from ova, but 

 from gemmae, which differ essentially from ova. They are 

 solid, round, and somewhat flattened discs, which, growing and 

 developing, become little caudate worms, resembling in form and 

 organization certain Trematoda (Distomum, Monostomum, Diplo- 

 discus, Gastrostomum). 1 



Fig, 9. 



Figs. 4 10. The various stages of development of Cercaria ephemera, from the 

 agamozobid, fig. 1. Fig. 4. Sporula. Fig. 5. Sporula thinned at the hinder end. 

 Fig. 6. Sporula with this taper posterior extremity elongated into a tail. Fig. 7. The 

 sporula in this stage has assumed the form of a Cercaria. The tail is already defined. 

 Two black pigment-spots appear on the fore part of the dorsal surface. Fig. 8. A still 

 further developed Cercaria. a. The oral aperture, c, d. The urinary organ, e. The 

 Tail. f. Two pigment-spots. Fig. 9. A fully formed C. ephemera (one millimetre long). 

 a. Oral cavity, b. Alimentary canal, c, d. Urinary organ filled with granular urine. 

 e. Tail. /. Three hlack spots on the anterior part of the dorsal surface. The median 

 pigment only begins to be developed in the last stage of development. The whole figure 

 of the body of Cercaria ephemera corresponds with that of Monostomum. Fig. 10. 

 Four cercarise, after Filippi, from Planorbis nitidus, whose posterior sucking apparatus 

 (composed of two suckers, one enclosed within the other) is seen in different stages of 

 contraction and expansion. When the tail is cast off these Cercarice are altogether 

 similar to Diplodiscus. 



1 The Cercarice and the sacs have been so often referred to of late that I mav leave 



