LIFE-CYCLE OF " CYSTOBIA " lEREGUJiAEIS (uiNCH.). 67 



(h) Biological Considerations. 



The reason for neogamy. — The reason for tbe gradually 

 increasing precocity and intimacy of association, as we can 

 trace it in different spp. of Diplocystis and Diplodina, 

 is undoubtedly to be found in the absence of any power of 

 movement in these coelomic parasites. The whole process, in 

 short, is the result of an endeavour to insure a suitable and 

 durable association, since the animals are not able to move 

 about themselves in search of a partner when ready to com- 

 mence sporulation. The fact that neogamy is only known 

 to occur in these non-motile forms bears out this view. 



Let us consider D. major, for example. It is purely a 

 matter of chance whether two individuals of the right age are 

 brought near enough— as they are passively washed about in 

 the body-cavity — to attract each other reciprocally and asso- 

 ciate. Besides this, there is the danger of unsuitable and 

 indiscriminate grouping, and also of a separation of two 

 pai'tners (if not ripe for encystment) when violently dis- 

 turbed ; iu such cases a second association might be impos- 

 sible. Probably, therefore, a considerable percentage of 

 individuals thus isolated from one reason or another is 

 doomed to phagocytic destruction. A method so haphazard 

 must prove very expensive, and in the other two species 

 association is modified as above described. Thus unequal 

 and multiple associations are, as a rule (though not entirely) 

 prevented, but the membrane probably interferes, to a 

 certain extent, with the absorption of nourishment by the 

 trophozoites. 



At all events, this plan is not followed in Diplodina and 

 Cystobia. In these parasites the difficulties in the way of 

 association between adult trophozoites would be even greater, 

 because of the more confined situation. We find, corre- 

 spondingly, in these genera no stage equivalent to that seen in 

 D. major. Those races of individuals which did not adopt 

 early and permanent association appear to have all died off. 



Effect on the individuals. — Notwithstanding the appa- 



