184 



FRESHWATER FAUNA 



tions, were not sometimes made use of in the same way. 

 Our freshwater genera had attained a wide distribution as 

 early as the close of the Mesozoic sera, or even earlier ; 

 and hence, so far as this was brought about by the inter- 

 vention of animal agency, it is to the extinct and not the 

 existing forms of life that we must turn for an explana- 

 tion. Amongst these, reptiles were so preponderant 

 during the period when distribution was in progress that 

 it seems not unnatural to attribute to them some share in 

 the process. Whirlwinds sometimes act as a means of 

 transport, whirling bodies of water, full 

 of living creatures, into the air, and 

 showering them down in some distant 

 place. Thus on February 9, 1859, a 

 shower of minnows and sticklebacks fell 

 in the valley of Aberdare ; frogs have 

 been rained down in the same way, 

 and so has the freshwater mussel. * 



Let us now descend to a lower stage 

 of the animal kingdom, and consider 

 first the freshwater sponges. Those of 

 our islands, like the great majority of 

 the family, are distinguished, whether 

 they give rise to free larvae or not, by 

 a peculiar mode of propagation. To- 

 wards the end of autumn a number of the living cells, 

 of which the sponge is composed, wander to certain 

 points within it and accumulate in little heaps, as a 

 preliminary to going into winter quarters (Fig. 57). 

 Other cells then travel towards these heaps and invest 

 them with a horny coat (Fig. 57, A) ; then still others 

 come, each bringing with it a siliceous spicule which it 

 has secreted within itself (Fig. 57, B) ; these spicules 



-'' For a full account of these events see H. W. Kew, " The Dispersal 

 of Shells," International Science Series, 1893. 



FIG. 56. Ijimncca 

 stagnalis, Linn., 

 a Freshwater 

 Snail. Natural 

 size. From 

 Bronn's "Thier- 

 reichs." 



