438 MARCCrS M. HARTOG. 



semination is effected by the movements of the water at no 

 cost to the organism. And we must bear in mind that a con • 

 siderable amount of dissemination is needed, lest the zoospores 

 should be uselessly attracted to the host on which their parent 

 thallus is living. 



In Achlya the limited first swarming forms a sort of 

 •globular colony of resting spores, easily broken away as a 

 whole by a slight impact, as of a dead animal floating down 

 stream. The swarmers when liberated form a host of invaders 

 effecting rapid and complete infection, which isolated ones 

 might fail to do. 



In Dictyuchus (gen.) the resting state of Achlya is 

 reached without the expenditure of energy required to swim 

 to the outlet of the sporange. A further economy is thus 

 effected. 



