SAPROLEGNIA IN RELATION TO THE SALMON DISEASE. 315 



filament is to be seen it is rounded, closed, and often no 

 laro-er than the rest ; or the filament may taper to its ex- 

 tremity. But the free ends of a greater or less number of 

 tlie filaments are slightly enlarged, so as to he club-shaped, 

 or they may be pyriform, or even almost spheroidal, and the 

 layer of protoplasm which they contain is very thick. The 

 cavities of some of these enlarged ends are shut off by a 

 transverse partition from the rest of the filament, thus giving 

 rise to a closed case. In others the protO[)lasm is broken up 

 into a number, greater or less, according to the size of the 

 enlargement, of equal sized spherical masses, each rather 

 less than 2-0V0 of an inch in diameter, which lie separate, 

 but closely packed in the interior of the case, like shot in a 

 cartridge (Fig. I, p. 316). In others the case is seen to be 

 open at the end, and a portion or the whole of the " shot " 

 have passed out. In yet others, again, a full unopened case 

 is seen to lie inside an empty one. 



The papyraceous mass is, in fact, what is known as the 

 mycelium of a fungus. It answers exactly to the similar, 

 wet-paper like crust which is formed by the common fungus, 

 Penicillium glaucum (usually known as " blue mould "), on 

 the surface of a pot of jam. The filaments are the stems of the 

 fungus, and are technically known as hyphcB. The enlarged 

 ends of the hyphse which are converted into the "cases," are 

 the sporangia, or fruits of the fungus ; and they are termed 

 zoosporangia, inasmuch as the spheroidal bodies or spores^ 

 under certain circumstances, are actively locomotive after 

 the fashion of many animalcules, and are, therefore, termed 

 zoospores. It is a peculiarity of this particular fungus that, 

 when a zoosporangium has emptied itself, the hypha ou 

 which it is supported begins to grow afresh, sends a pro- 

 longation through the centre of the empty sporangium, and 

 dilates into a nevv one within or beyond it. Hence the 

 appearance of a full sporangium, surrounded by one, or it 

 may be two or three empty ones, one inside the other (Fig. 

 II, p. 316). 



This structural feature is peculiar to the genus Saprolegnia 

 among fungi, and it enables mycologists to identify the 

 fungus, of which the papyraceous incrustation characteristic 

 of the salmon disease is a product, as a species of that 

 genus. 



Thanks especially to the labours of Pringsheim,^ Cornu,^ 



^ " Die Eutwickelungs-geschichte der Achlya prolifera." Nova Acta, 

 1851, and several later papers in the ' Jahrbiicher fiir Wissenschaftliclie 

 Botanik* for 1857, 1860, and 1874. 



^ ' Monographie Aiinales des Sciences Naturelles, Botanique,' 1872. 



