220 THE INVERTEBRATA 



seeks out a tadpole, dying within twenty-four hours if one is not found. 

 If a tadpole is reached, the parasite fastens itself on to the gills, where 

 its ciliary covering is cast and it then creeps into the bladder to wait for 

 three years before becoming sexually mature. The larvae may, how- 

 ever, attach themselves 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 metamorphoses, and thus it never reaches the bladder. 

 In Dtplozoon, 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. 



Another form which displays a variation of the usual type of 

 history is Gyrodactylus which occurs on the gills of freshwater fish. 

 In Gyrodactylus the ovary and the vitellarium are not separated, as 

 is the general rule in the Trematoda, but constitute one organ. A 

 single egg ripens at a time and, after fertilization, develops into an 

 embryo in the uterus. Before the first embryo leaves the mother a 

 second younger one appears inside it so that we thus have a con- 

 dition of three generations one inside the other, and the conditions 

 are such that the youngest embryo must develop without fertilization. 

 This feature of the development of one larva with another without 

 the agency of fertilization is common in the life histories of the 

 Malacocotylea but Gyrodactylus is the one member of the Hetero- 

 cotylea in which it occurs. 



Order MALACOCOTYLEA 



The life history of Fasciola (Fig. 159) 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, with rare exceptions, 

 parasitic in some vertebrate host, the sporocyst and redia stages are 

 always parasitic in a mollusc. Two hosts are always, and three 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 forming by budding 

 a second generation of sporocysts within which the cercariae arise, 

 (iii) the cercaria requiring 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 Gasterostomum fitnbriatum, where the sporocyst develops in 



