(304 Tj. R. Gary, 



3. The cere aria. 



The development of the cercaria will be discussed in a later 

 section of this paper. The present description will, therefore, be 

 confined to the full}' developed cercaria at the time of its libera- 

 tion from the sporocj^st or redia. 



The mature cercaria (Fig. 6) is about 2 mm, long — including the 

 tail — and 0,15 mm in its greatest breadth. It is elongate oval in 

 shape and flattened in the dorso ventral direction. The tail is 

 quite slender and is about three fourths the length of the body. 



The cercariae are very active when first liberated from the 

 redia or sporocyst, and are constantly altering their shape. Swimming 

 is accomplished by means of a vigerous lashing of the tail, suppli- 

 mented by a rhythmical bending of the body. That this last 

 mentioned process is sufficient to bring about progress through the 

 water is established on the examination of almost any lot of living 

 cercariae. It will be found that some of the actively swimming 

 larvae will have lost their tails, but that in spite of this fact they 

 are able to make quite rapid progress through the water. The 

 anterior end of the body is always directed forward in swimming, 

 whether or not the tail is present. 



The living cercariae are colorless when viewed by transmitted 

 light and are very transparent except over the areas occupied by 

 some of the denser organs. 



At the anterior end of the body there is a small sucking disc. 

 The posterior sucking disc is situated on the ventral side of the 

 body a little posterior to the centre. 



The body is covered by a thin cuticular layer. 



Digestive sy stem. 



The mouth is situated at the anterior end of the body at the 

 centre of the anterior sucking disc. It opens backward into right 

 and left pharyngeal pockets. From these a narrow oesophagus runs 

 backward for about one third the length of the body. The oeso- 

 phagus ends in a pharynxlike swelling which in turn opens into the 

 paired intestinal caeca. These extend backward nearly to the 

 posterior end of the body. 



