; 



Reproduced from " The Wonder of Life" by Professor J. Arthur Thomson (A ndrew Melrose, Ltd.). 



ONE VALVE OF THE FRESHWATER MCSSKI. 



This bivalve lives a sluggish life in rivers and ponds. Its two shell-valves, one to the right and the other to the left of its body, are 

 very solidly built of carbonate of lime and an organic substance called conchin. To the outside there is a layer without any lime, then 

 a thick layer with lime arranged in prisms, and then innermost the mother-of-pearl layer with iridescent colouring. The shell is always 

 being added to at the margin, as the animal grows bigger. It shows concentric lines of growth. Several fixed pearls are seer, near the 

 margin (see above illustration), but really fine pearls lie embedded in skin-pockets of the mollusc. Truly fine pearls are believed to be 

 formed (a) in some cases around the larva of a parasitic worm, and (6) in other cases around a minute blob of conchin. The larva of 

 the freshwater mussel have to spend a short time as temporary parasites on a freshwater fish. 





Reproduced by permission of Charle. 



b-Co., Ilayvards /lt,itlt. 



A HIGHLY EVOLVED ORCHID 



(Odontoglossum cnspiini) 



Many of the Orchids deserve to be called the aristocrats of the floral world. They are beautiful in form and colouring and fine in 

 fragrance, and they have a strong individuality. Their arrangements in connection with the visits of insects are often highly special- 

 ised; and Darwin, who made a careful study of their pollination, notes that "a study of their many beautiful contrivances will exalt 

 the whole vegetable kingdom in most persons' estimation." It is a case of flower and insect being fitted as glove and hand. While the 

 Orchids of temperate countries always grow in the ground, in meadows and woods, many of those which are found in warm and moist 

 countries live as "epiphytes," perched on the stems and branches of trees. Their aerial roots have a spongy outer envelope which 

 readily absorbs water and also air laden with water-vapour. In Odontoglossum there is a partner filamentous fungus which lives in 

 the outer part of the root, and a striking fact is that some of the cells of the Orchid-root digest the fungus and seem to keep it from 

 spreading too exuberantly. It will be understood that epiphytic Orchids are not parasites on the trees; they are simply perched plants. 



