251 LIFE HISTORY OF TREMATODES— FAUST 25 



the acetabulum. All of these come to be enclosed in a common pocket which 

 acts as a large sucking cup (Fig. 40). There is practically no locomotion in 

 these species, since movement is confined almost exclusively to the sucking 

 reflex. 



In the monostome no acetabulum is present, yet the cercaria performs 

 the processes of locomotion par excellence. The pair of posterior locomotor 

 organs replaces the acetabulum in the measuring worm movement. In Cer- 

 caria pellucida and C. konadensis, as well as in C. urbanensis Cort, these organs 

 consist of posterior inpocketings of the integument. In C. imbricata, Looss 

 (1896, Fig. 151) there is an internal pocket. In C ephemera Nitzsch (Ssinitzin, 

 1905, Fig. 75, 76) there are hook-shaped spines. Cort (1915:15) finds that 

 they " apparently have no suctorial function, since no muscles are present and 

 the central cavity contracts while the projection is extended." A careful 

 study of living and preserved specimens of C. pellucida, C. konadensis, and 

 C. urbanensis shows that these three American species have no spinose or 

 other integumentary modifications. However, their function is found to be 

 distinctly suctorial, and not "analogous to setae," as Cort believes. Typical 

 drawings for the locomotor organs of any of these three species are shown 

 (Figs. 16, 17). As will be seen in figure 16, there are four muscles which are 

 attached to the pockets. By a contraction of the pair xx the pocket disc 

 is applied to the surface of the contact body; by a relaxation of xx and a con- 

 traction of yy the pocket is released and pulled forward by the general bodily 

 contraction. This has been observed repeatedly in so convincing a manner 

 that it leaves no doubt as to the structure or function of the organ. In addition, 

 in C. konadensis (Fig. 21) a group of glands just anterior to the locomotor 

 pockets pour out a mucous secretion at the time when the disc is applied 

 to the contact organ. The locomotor pockets perform a similar function 

 and in a similar manner to that of the secondary suctorial disc or acetabulum 

 of amphistome or distome, altho these organs are in no sense homologous. 

 The significance of the spines in connection with the caudal locomotor 

 pockets of distomes has been regarded by Leuckart (1886:128) as deserving 

 special consideration. In Cercaria armata he considers them as serviceable 

 in keeping the tail attached to the body after the constriction between the 

 two parts has become deep. Looking into the phylogenetic significance of 

 the spines of the same cercaria species, Ssinitzin (1911:68) regards them as 

 indicating a bifid ancestral appendage of a caudal nature. In view of the 

 fact that these pockets actually function similarly to the locomotor pockets 

 of the monostomes, and are more than likely the ancestors of the monostome 

 type of pocket (Fig. 12), it seems hardly worth while to find a more obscure 

 meaning in the structures. 



The tail is the portion of the cercaria showing preeminently the adaptation 

 of the organism to free-swimming life. In such forms as C. setifera (Monticelli, 

 1914), C. pennata and C. plumosa (Ssinitzin, 1911, Figs. 76-79), the prolonged 

 free-swimming existence has given rise to setae, spines and scutes. 



