168 COMPARATIVE ANATOMY 



boat-shaped diatoms of the same name, have long been known 

 to naturalists as pseudonavicellae. 



The eight minute nucleated bodies contained in each spore 

 were formerly called the falciform (i.e. sickle-shaped) young 

 but are now known as the sporozoites. Protected by the 

 tough and impervious sporocyst they are incapable of further 

 development until transferred to the body of another earth- 

 worm, but are in a condition to withstand such adverse influ- 

 ences as they may encounter during the transference. Their 

 actual history at this stage has not yet been traced. It seems 

 certain that the spores are by some means scattered broad- 

 cast in the earth and are swallowed by another earthworm, 

 the action of whose digestive juices causes the sporocyst to split 

 open and set free the contained sporozoites. The sporozoites 

 are not only active but have the power of boring their way 

 through the walls of the gut and other tissues of their new host 

 till they reach the reproductive organs, where they penetrate 

 into the sperm-mother cells, become quiescent, and enter 

 upon the trophic phase of existence. But the manner in which 

 the spores are scattered in the earth is not accurately known. 

 There is no evidence that they are passed to the exterior 

 through the sperm-ducts. It has been suggested, and the 

 analogy of other Sporozoa whose life histories are better known 

 affords good grounds for the suggestion, that they are dissemin- 

 ated by the agency of worm-eating birds. The protective 

 sporocyst would enable the spores to pass uninjured through 

 the alimentary tract of a bird and pass out with the faeces, 

 whence they would be washed down into the soil and readily 

 swallowed by other earthworms. However this may be, the 

 transference of the spores from one host to another remains a 

 problem which still awaits experimental solution. 



A consideration of the foregoing description will show that 

 the life history of the common species of Monocystis of the 

 earthworm is really a simple one. Much of the apparent 

 complexity is due to the fact that different names are used 

 to indicate the different stages of growth of one and the same 

 individual. Thus sporozoite, trophozoite, gametocyte, are 

 names used to designate successive stages of growth of a 

 single individual and though they may be confusing at first 

 because of their novelty, when once they are understood they 

 should no more embarrass the reader than such familiar names 



