THE DEVELOPMENT OF THE TREMATODA. 



181 



in which it remains quiescent, but underi^oes some further 

 advances in development, the coronal hooklets making their 

 appearance. 6. When a Paludina^ thus infested, is swal- 

 lowed by a water-bird and digested, the cj'sts are set free in 

 the alimentary canal of the bird ; sexual organs appear within 

 the included Dlstoma ; the body elongates and narrows an- 

 teriorly ; the sucker moves nearer the head, and the coronal 

 circlets reach their full development. The Dlstoma gradually 

 assumes the form of the parent, attaches itself by its hooklets 

 to the intestinal walls, and acquires complete sexual organs.^ 

 Thus the developmental stages of Dlstoma mllltare may be 

 summed up, as : 1. Ciliated larva. 2. Bedla. 3. Gercarla. 



4. Cercarla, tailless and encysted, or incomplete Dlstoma. 



5. Perfect Dlstoma. 



The stages of transition vary in different genera. Thus, 

 several generations of liedioB may intervene between the 



Fig. A^.— Bucephalus pohjmorphus oXihe fresh-water muscle.— J, ramified sporocj'st ; 

 B, portion of tlie same more inanniiied: a, outer coar, 6, inner; c, (?, sjerm- 

 masses in course of development ; f\ one of the jjerra-masses more highly mag- 

 nified ; Z*, Bucephalus : a, b, suckers ; c, clear cavity ; cl, caudal appeudagee. 



third and fourth stages ; or the mature animal may appear at 

 the close of this stage, having undergone no Cercarian meta- 

 morphosis. 



In Ducei^halus pohjmorpJms^ a parasite of the fresh- 

 water muscle (Fig. 45), two caudal appendages, which seem 

 to correspond wdth the tail of the ordinary Cercarlm^ become 



1 Van Bcnedeu, " Memoire sur les Vers Intcstinuux.' 



