24 ILLINOIS BIOLOGICAL MONOGRAPHS [24 



There are two types of movement in the cercaria aside from the apparent 

 contraction and expansion of the body. One of these is concerned with the 

 forward movement of the animal and depends on the cooperative action of 

 the suctorial organs and the general bodily musculature. The other is caused 

 by the flagellate action of the tail. The movement of the cercaria along a 

 forward path reminds one of the rythmic action of a measuring worm. The 

 oral sucker is always used as one organ of attachment, and the ventral or 

 caudal suctorial disc supplies the other anchorage. With these two organs 

 of attachment, the larval worm applies the oral disc to the object of contact, 

 while it draws the posterior portion of the body forward by the contraction 

 of all the longitudinal muscles. This places the posterior attachment advan- 

 tageously near the oral disc, so that a relaxing of the longitudinal muscles 

 and a synchronous contraction of the transverse muscles throws the cephalic 

 portion of the worm far forward. In the forms with well developed muscula- 

 ture, such as monostome and echinostome species, the larva may appear 

 discoid on contraction, while the expanded worm will assume a length several 

 times that of the normal body. 



All groups of cercariae possess an oral suctorial organ. For the second 

 attachment organ there is a variety of accommodation. Undoubtedly the 

 most advantageously formed organ of this second type is the one found in the 

 Amphistomata, where there is a powerful suctorial disc at the posterior end of 

 the body. 



Among the distomes there are many types of posterior suctorial organ, 

 ranging from those with a prominent acetabulum not far from the caudal 

 extremity, as in Stomylotrema pictum (Crep.) (Looss 1899:629), to those with 

 a poorly developed acetabulum more cephalic in position. In the latter case 

 there are frequently found auxiliary locomotor organs, such as those in the 

 posterior pockets of the xiphidiocercariae. In some species there is only 

 a suggestion of a paired suctorial organ, as in the larvae, Cercaria crenata (Fig. 

 55), and C. diaphana (Fig. 76). In others there is the additional spinose 

 complement (C. glandulosa, C. dendritica, and C. micro pharynx). These spines 

 are of important function on rough surfaces where the disc can take hold 

 with difi&culty. Altho there is considerable difference in the rapidity of 

 movement of the various species studied, C. glandulosa was by far the most 

 rapid in movement of all the cercariae observed. The spinous outgrowth of 

 the acetabulum is of advantage in locomotion, catching hold where the un- 

 armed sucker can not operate {C . glandulosa, Y\g.6Q;dL,n.dC . gracillima, Fig. 142). 



Muscular development in the holostome larva is confined entirely to the 

 suctorial apparatus, since there is no distinct tail portion. This type of sucker 

 is derived from the distome type. With the translocation of the genital opening 

 to the posterior end of the body, the primitive genital pore has come to be used 

 as an accessory suctorial organ {Cercaria ptychocheilus, Fig. 47). The most 

 unique modification is found in the tetracotyle t>'pe. Here there have arisen 

 two lateral accessory suctorial grooves (Fig. 41), and lappet modifications of 



