582 APPENDICES. 



These cells again grow into hyphse, which penetrate the skin, each 

 forming a conidium, which is cast off with considerable force. The 

 parasite is fatal to flies, especially in the autumn. They are often 

 observed attached to the walls or window-panes, surroiinded by a 

 powdery substance, consisting of the extruded conidia. 



Empusa radicans. — The spores form long hyphse, which pierce 

 the transparent skin of the caterpillar of the cabbage white butter- 

 fly. The terminal cells ramify, and fill the body of the caterpillar 

 with a network of mycelial filaments. The caterpillars attacked 

 become restless, then motionless, and death ensues. 



Tarichium megaspermum. — The spores are black in colour, 

 and provided with a thickened episporium. They occur at the 

 sides and ends of myceHal threads, attacking caterpillars {A gratis 

 segetum). 



Phycomtcetes. 



Saprolegnia.^ — Colourless threads, forming dense radiating tufts, 

 occur on living and dead animal and vegetable matter in fresh 

 water. The filaments penetrate into the substratum, and branch 

 more or less in the surrounding water. The cylindrical ends of the 

 threads are shut off by a septum — forming zoosporangia, or mother- 

 cells, in the interior of which a number of spherical zoospores 

 develop. These are set free through an apical opening in the 

 thread, and after a time coming to rest, give rise to new plants. 

 In the sexual mode of reproduction a spherical bud, the oogoniwm, 

 develops at the end of a mycelial thread ; from the thread small 

 processes or antheridia sprout out laterally towards the oogonium 

 and blend with its protoplasm. The latter breaks up into a number 

 of oospores, which clothe themselves with a membrane while still 

 within the mother-cell, and, eventtially being set free, grow into 

 fresh mycelial filaments. The fungus attacks fish and tritons, 

 and produces a diseased condition of the skin, which may be 

 ultimately fatal. In salmon it produces the common "disease of 

 salmon." 



Peronospora infestans The conidia -bearers of this fungus 



have as many as five branches, each bearing an egg-shaped 

 conidium. The contents of the conidia falling off and reaching a 

 drop of moisture, break up into a number of swarming zoogonidia, 

 which in turn develop upon plants. Fixing themselves to the 

 cuticle of the host, they throw a germinating filament into an 

 epidermal cell ; after piercing first its outer wall, and then its inner 



