504 University of California Publications in Zoology [Vol. 18 



they were exploring with the anterior end to find a point where they 

 could push in. That the pushing out of the anterior tip exerts con- 

 siderable force was shown by the observation that when the cercaria 

 was in contact with loose pieces of debris it would push them away 

 by this movement. I also saw cercariae forcing their way through the 

 loose tissue of the snail or disintegrating sporocysts. The extension 

 of the body combined with the pushing out of the anterior tip forced 

 the tip of the worm into the tissue. The backward pointing, cuticular 

 spines of the body held what was gained and the new extensions of 

 the body produced further progress. Several times in connection with 

 the progress of cercariae through tissue, bubbles of secretions from the 

 cephalic glands were seen to be forced out when the anterior tip was 

 extended to its utmost. The above observations and a consideration 

 of the relations of the adaptive larval characters of the cercaria of 

 S. japonicum gives us an idea of how it penetrates the human host. The 

 cercariae freed from the snail in the rice fields would be stirred into 

 activity by the passage of the host. Coming in contact with the sur- 

 face of the skin the cercaria would catch hold with the ventral sucker 

 and by the extension of its body and the butting with the spines- of the 

 tip would produce a slight opening. Aided by the cytolytic secretions 

 of its glands, the backward pointing spines and its movement, the 

 cercaria would rapidly take advantage of the opening to penetrate 

 through the skin of the host. In fact, the cercaria of S. japonicum 

 is primarily a machine for skin penetration and its structure is com- 

 pletely dominated by those adaptive larval characters which make 

 possible its penetration into the human host. 



