194 THE INVERTEBRATA 



way to some particular spot on the host after being free-swimming 

 for a time. As soon as they attach themselves the ciliary covering is 

 cast off and the generative organs develop. The larva of Polystomum 

 seeks out a tadpole and it dies within twenty-four hours if one is not 

 found. It fastens itself on to the gills, where its ciliary covering is 

 cast and then it creeps into the bladder to wait for three years before 

 becoming sexually mature. The larvae may, however, attach them- 

 selves to the external gills, where a copious supply of nourishment 

 induces such rapid growth that the animal becomes sexually mature 

 in five weeks and produces eggs. But it dies when the tadpole meta- 

 morphoses, and thus it never reaches the bladder. In Dlplozoon, 

 which lives attached to the gills of the minnow, the larvae attach 

 themselves to the gills of the host, but they do not develop generative 

 organs until they meet another larva. If such a meeting occurs the 

 larvae fuse across the middle. After fusion the generative organs 

 develop and the animals grow in such a manner that the vas deferens 

 of one form is permanently connected to the genital atrium of the 

 other. They thus remain throughout their lives in permanent 

 copulation. 



Order MALACOCOT YLEA 



The life history oiFasciola (Fig. 153) may be taken as the type of life 

 history commonly found in the group. For details of this life history 

 the reader is referred to elementary textbooks. 



In the Malacocotylea the adult is always parasitic in some verte- 

 brate host, the sporocyst and redia stages are always parasitic in a 

 mollusc. Three hosts may be necessary for complete development. 

 Divergence from the type of life history recorded for Fasciola may 

 come about by (i) a generation, the redia stage, being omitted, (ii) the 

 sporocyst may form by budding a second generation of sporocysts 

 within which the cercariae arise, (iii) the cercaria may require to 

 encyst in a host and to await this host being eaten by the final host 

 before reaching sexual maturity as in the case of G aster ostomutn 

 fimhriatum^ where the sporocyst develops in the liver of Anodon, 

 the cercaria encysts in the roof of the mouth of the roach and only 

 reaches sexual maturity when the roach is swallowed by a perch. 



In Distomum macr ostomutn^ which is parasitic in the gut of thrushes, 

 there is no free-living stage in the life history. The eggs, passed out 

 withthe faeces of the bird, are eaten by a snail, inside which the sporo- 

 cyst develops. The sporocyst'finds its way into one of the tentacles. It 

 develops pigment, being bright coloured in bands of green and red, 

 while its presence stops the snail from withdrawing this tentacle. 

 Presumably this bright object attracts the bird which devours the 

 snail and infects itself by setting free the cercariae from the sporocyst. 



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