150 



INVERTEBRATE MORPUOLOOT. 



yet been observed, but there is no reason for doubting that 

 it is the result of the parthenogenetic development of one of 

 the germ-cells which occur in the body-cavity of the embryo. 

 So long, however, as it remains in the tissues of the Mollusk 

 it undergoes no further development; it can only reach ma- 

 turity in a second host, in this case some water-bird which 

 swallows the Mollusk and its encapsuled parasite, when the 

 latter, its capsule being dissolved by the digestive juices of 

 the bird, is set free, fastens itself to the wall of some of the 

 cavities of its host and becomes sexually mature. 



In this species of Trematode but two hosts are required 

 in the life-history ; in ttie majority 

 of the Distomese a third occurs, an 

 additional stage of development in- 

 tervening between the Kedia or Spo- 

 rocyst and the encapsuled immature 

 worm. The germ-cells of the Eedia 

 or Sporocyst while in the interior of 

 the Mollusk develop into a form 

 resembling an immature Distome, 

 but provided with a mobile muscular 

 tail whose axis is formed by a fibrous 

 rod resembling somewhat in appear- 

 ance the Vertebrate notochord. Such 

 an organism is known as a Cercaria 

 (Fig. 80), and when fully developed 

 the Cercaria brood leaves the body 

 of the parent Eedia or Sporocyst, 

 makes its exit from the tissues of 

 armaia the MoUusk and leads for a time 

 a free-swimming existence. Eventu- 

 ally the Cercaria makes its way into 

 the body of a second host, usually 

 like the first a Mollusk, and there 

 becomes encapsuled in the tissues, 

 losing at the same time its tail, and 

 it reaches its maturity only after the 

 Mollusk has been swallowed by the definitive host, as was 

 the case in Monostomum. 



Fig. 80. — Cercaria 

 (after Schwarze). 

 as = anterior sucker. 

 cv = contractile vesicle 



d = intestine. 



n = nephridial tube. 

 ph = pharynx. 

 sp = spine. 



m = ventral sucker. 



