DISTOMUM HEPATICUM 13 



and swallow them as food, causing the death of the snail in 

 a shorter or longer time. During its migrations a redia 

 increases in size till it attains a length of i to 1*5 mm. 



The epithelium lining the body-cavity gives rise to germinal 

 cells, which pass into the cavity and undergo a development 

 similar to that of the germinal cells of a sporocyst. The first 

 generation of rediae usually gives birth to daughter rediae, and 

 these in turn may give rise to grand-daughter rediae, and so 

 on for several generations, the young rediae escaping from 

 the parent by a special aperture or birth-opening, situated at 

 one side of the body, a little behind the collar. Eventually 

 the rediae cease to produce daughter rediae, and give birth to 

 another form known as the cercaria.* 



A cercaria is developed from a germinal cell, in much the 

 same manner as a redia. When fully grown, it has the shape 

 and structure shown in fig. 2 (6), consisting of flattened heart- 

 shaped body with an exceedingly contractile tail, more than 

 double as long as the body. Anteriorly there is an oral 

 sucker, and some way behind there is a ventral sucker placed 

 in the middle of the lower surface of the body. In the centre 

 of the oral sucker there is a mouth, and posterior to the 

 sucker there is a muscular pharynx leading into a short narrow 

 oesophagus which bifurcates just in front of the ventral sucker 

 to form a pair of simple tubular intestinal limbs, one reaching 

 down on either side of the ventral sucker nearly to the hind 

 end of the body. The limbs of the intestine are at first solid, 

 being formed of single rows of thick discoid cells. The 

 cercaria is also provided with an excretory system, having a 

 terminal contractile vesicle, collecting ducts, and flame-cells ; 

 but the details of this and other organs are difficult to make 

 out, because of the presence of a number of large nucleated 

 cells crowded with coarse refractive granules which form two 

 irregular masses, one along each side of the body. These are 

 the cystogenous cells ; their function is to secrete the protec- 

 tive case or cyst with which the animal presently surrounds itself. 



As soon as a cercaria has attained a length of some "28 mm. 

 it escapes through the birth-opening of the parent redia, and 

 by the help of its suckers and contractile tail wriggles out 



* It is probable that temperature is the determining cause whether a 

 redia shall produce daughter redioe or cercaricE. In summer the redioe are 

 found full of daughter redise, in autumn of cercariae. 



