138 MYCETOZOA. 



in collections between the genus Physarum and the genus Didymium, 

 but with a good microscope the distinction can be at once 

 determined. In Physarum the minute granules of lime appear as 

 deposits in the substance of the sporangium wall ; in Didymium 

 the lime is deposited in stellate crystals on the outside of the 

 wall. 



FIG. 5 : Craterium vulgare. 



The genus Craterium is very closely allied to Physarum. The 

 capillitium always contains " lime knots " and the spores are 

 similar to those of the latter genus, but in the species figured the 

 cup-like form and the cartilaginous substance of the outer 

 sporangium- wall, together with the pale calcareous lid, are very 

 distinctive characters. 



FIG. 6 : Dictydium cernuum. 



Unlike the species hitherto noticed we find in this genus that 

 there is no capillitium penetrating among the spores, but their 

 dispersal is assisted in a very striking manner by the construction 

 of the sporangium wall. This wall consists of nearly parallel ribs 

 running from the base to the apex of the sporangium, connected by 

 numerous delicate transverse bars, like the rungs of a ladder. 

 When first formed the sporangium is globose and the interstices 

 between the ribs are closed by an extremely delicate membrane 

 which breaks up and entirely disappears on drying, so that the 

 compact ball of spores, many thousand in number, lie in an open 

 cage ; they are very minute, about half the size of those usually 

 met with, and quite smooth. What is to prevent them from 

 consolidating into a hard lump 1 We observe that as drying begins 

 to take place the ribs bend inwards at the apex and bow outwards 

 at the sides away from the ball of spores within, and as we watch 

 the drying process we see the ball crack in all directions from the 

 gradual downward pressure of the inverted ribs. By this means 

 the spores are prevented from cohering, and break down into a 

 dusty mass which can be blown out through the basket work wall 

 of the sporangium, which looks, when empty, something like a 

 lobster pot with a deeply umbilicate top. 



