128 A. r. THOMAS. 



approaching complete development. On one occasion I counted 

 as many as six. 



2. Free Cercaria. — As soon as the cercaria has reached 

 the limit of development within the redia, it escapes from the 

 parent by the birth-opening (fig. 13^ v^) and then by the aid of 

 suckers and tail^ crawls or wriggles its way out of the host. 

 The free cercaria is very active, and its tissues so contractile 

 that the form and dimensions of the body are constantly 

 changing. When in a relatively quiescent condition, the body 

 has a depressed oval form (Plate III, fig. 19), its average size 

 is "28 mm. long and '23 ram. broad, though the largest may 

 measure over "3 mm. in length. The tail is more than double 

 the length of the body, and is exceedingly contractile. The 

 oral sucker is subterminal, the opening of the mouth being 

 directed downwards and forwards, and has a diameter of 06 

 ram. ; the pharynx is 'OS^ mm. in diameter. The ventral sucker 

 is situated slightly behind the centre of the ventral surface, and 

 is equal in size to the oral, or is sometimes a little larger. As 

 is the case with all the cercarise produced in rediae (with the 

 partial exception of Distom a Paludinse impurse armatum) 

 the cercaria has no head spine. In the most mature specimens, 

 and especially in such as have left the redia in the natural 

 course, and have not been disturbed by the dissection of their 

 host, the surface of the body is beset anteriorly with exceedingly 

 minute spines. But the most striking character is due to the 

 presence of the cystogenous cells, large nucleated cells so 

 crowded with coarse, highly refractive granules as to be 

 rendered quite opaque. They are arranged in two lobed masses 

 extending along each side of the body (Plate III, figs. 19 and 21), 

 from the level of the pharynx to the posterior end of the body. 

 Just in front of the ventral sucker is another group of these 

 cells, which is often large enough to connect the two lateral 

 masses, and behind the ventral sucker others are scattered. 

 Cells of the same kind, and showing a similar arrangement, are 

 found in Cercaria tuberculata (inhabiting Bythinia 

 tentaculata), a species which shows at first sight a remarkable 

 resemblance to the cercaria of Fasciola hepatic a. I have. 



