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XIII.~A GEEGAEINE PARASITE OF THE EARTHWORM. 

 By M. T. Denne, O.B.E., A.Inst.P., E.R.M.S. 



(Bead June 15, 1921.) 



I WAS led to clioose Monocystis agilis as the subject of tins paper 

 because its life-history has not yet been fully worked out, while 

 the further work necessary to complete our knowledge of this form 

 lies not so much in the application of the usual methods of in- 

 vestigation into structure and function, as in the direction of 

 experiments to determine the method of conveyance to new hosts, 

 coupled with the systematic breeding of the worms, and it is hoped 

 that this note will draw attention to the fact and result in the 

 work being taken up, 



Monocystis agilis, the spore of which has not been observed 

 outside the sporocyst, first appears as a minute amoeboid tropho- 

 zoite in the seminal vesicles of the worm, embedded in a cell of 

 the sperm elements. In growing it leaves the cell and moves 

 freely in the contents of the vesicle, taking in food material by 

 absorption through its epidermis. 



In this stage it consists of an elongated sac with a zone of 

 ectoplasm divided into three layers, the inner being contractile. 

 Its endoplasm is granular, with food material stored in reserve for 

 the reproductive process. 



The association of two trophozoites may be abortive, but 

 generally results in spore formation, commencing with encystment 

 of the two individuals. No sexual union occurs at this stage, but 

 the nucleus of each commences to divide, the resultant daughter 

 nuclei being peripherally disposed, until a large number are present, 

 all arranged immediately beneath the surface of the cyst. 



As has been observed by Brasil, there is now a small difference 

 observable between the gametes of the two associates, one group 

 being smaller than the other, and containing more darkly staining 

 material. 



True sexual unions now take place, and the resultant zygotes 

 gradually assume the form of the mature sporocyst. During this 

 growth the nucleus divides to produce eight elongated spores with 

 extremely fine pointed ends in wdiich the darkly staining elements 

 are collected at one end, leaving the remainder clear, but slightly 

 granular with cytoplasm, very faintly stained. The eight sporo- 

 zoites are arranged in two groups of four in the cyst, which has 

 meanwhile developed a strong resistant envelope and taken its 



