186 



EMBRYOLOGY 



The CercarifB produced in gei-m tubes present a variety 

 of forms. This applies chiefly to the caudal appendage, 

 as can be recognized in the peculiarly formed Cercaria 

 represented in Figs. 90 and 91. One of these, Cercaria 

 setifera Villot,^ a marine foi'm, which arises from a sporocyst 

 inhabiting Scrohicularia tenuis, possesses an extraordinarily 

 large tail, beset with bristles. The other (Fig. 91) has 

 two tails, which are directed forwards, howerer, in swim- 

 ming. This is the Cercaria of Gasterostomtim Jimhriatum, 

 and is known under the name of Bucephalus polymorplius. 



Fig. 91.— Cercaria of Ga«terosfO(num ^mbriafiuii (after Ziegi.er). 



Under certain conditions the tail is entirely wanting in 

 the Cercaria stage. This is the case when the Cercarise 

 are not compelled to undei-take a migration, but remain in 

 their host until, along with it, they are consumed by another 

 animal, the final host. Since they do not pass through a 

 free stage, they do not require any special organs of loco- 



> The Cercaria setifera of Viluit is called Cfrairla Villoti by Monti- 

 CELLi, for the term setifera occurs in another species (Monticklli, 

 " Sulla Cercaria setifera Miiller," BoUetino di Naturalt>ti in NapoH, 

 vol. ii., 1888). 



