SPECIALIZATION IN HOSTS 293 



parasite, and by the decay of the larger animal, the 

 resistant forms of the smaller one may be set at 

 liberty. 



Even then, there are many perils to the parasite. 

 The resistant bodies may be set free, but in the wrong 

 environment. They can lie dormant for long periods, 

 but if they cannot get in contact with a new host of 

 the right sort, death results. Many parasites are 

 harmless to all races of animals except one. They 

 are specific to one host. Others are capable of 

 living in several hosts, more or less indifferent as to 

 which one they infect. The Myxidium of marine 

 fishes is a good example of such adaptability. In 

 contrast there is great restriction of hosts shown by 

 such interesting parasites as the Aggregate, found in 

 cuttlefish and octopus. In the bodies of these par- 

 ticular molluscs the parasites form large numbers 

 of gametes, which, after fusion, produce spores. 

 Some of these spores are voided with the excrement, 

 others remain in the body of their host. When a 

 squid or octopus is dead, it is a favourite meal of 

 many sea dwellers fish, other molluscs and crabs 

 alike. Should a special crab eat the infected 

 mollusc, it absorbs also spores of the Aggregata. 

 From each spore, three or more sporozoites issue, 

 commence to attack the crab, and form large cysts 

 that bulge into the body cavity. They form in- 

 ternally myriads of minute, club-shaped merozoites, 

 so arranged that in section they resemble large daisies. 

 Crabs are among the most common contents of the 

 stomachs of octopus and cuttlefish alike, and in the 

 process of digestion the cysts are broken open and 



