29 



ponds dried up, while by next season they again become abundant. 

 Whatever means of distribution there may be, it is very probable 

 that this renewed life proceeds in a great measure from ova. That 

 the ova retain vitality for a long time is certain. The late Dr. 

 Baird, of the British Museum, showed Dr. G. S. Brady and myself 

 a jar containing numerous forms of animal life winch had made their 

 appearance from mud taken from a dried-up canal in India during 

 the hot season, and kept in the dry state for a considerable time 

 after reaching this country before being subjected to water. Shortly 

 after water was supplied, many living forms made their 

 appearance. Where the pools are small and subject to be dried up 

 during summer they seldom contain many species, although in 

 such cases one species may prevail greatly. Limestone districts 

 are favourable to Ostracoda, but all rock or clay surfaces are better 

 than peat. Where there is nothing but pure peat, or peaty ponds 

 fringed with Sphagnum, few or no Ostracoda may be expected. 

 They are seldom searched for successfully where the lakes or pools 

 have risen much by heavy rain-falls, nor in mill-dams, where the 

 water is drained off rapidly, leaving broad, bare margins. It is 

 otherwise where the water in the pools is decreasing gradually by 

 evaporation. Then these animals appear to have time to follow 

 the water, and may be taken abundantly when thus brought closer 

 together in the small shallow pools left here and there. Moorland 

 roadside ditches are more promising than those at some distance 

 from the road. This may arise from a supply of material from the 

 drainage of the road, which may be requisite to build up the shells 

 of these minute crustaceans. Ostracoda are seldom absent in ditches 

 or marshes which contain a little ochreous deposit with a metallic 

 bluish scum on the surface of the water ; they are more common in 

 broad shallow ditches than in those more narrow and deep, and 

 are rarely met with in springs or in ponds abounding with fish. 

 Neither do they thrive where amphipods prevail. These little 

 bivalve crustaceans are not always fastidious in their choice of 

 habitat, sometimes disporting in pure fresh water, at other times 

 revelling in water of very questionable character, while others 

 affect brackish water, although they live in very different degrees 

 of the saline element. 



The Govan Colliery Dam, which is close to the terminus of a 

 railway, is subjected to the dust from loading and unloading of the 

 waggons, and to the deleterious fumes of a range of brick kilns 



