298 THE BIOLOGY OF THE SEA-SHORE 



Quiberon, Cherbourg, and other places. These algae are 

 spherical or ovoid, solid when young, later hollow and filled 

 with water. They may attain the size of a hen's egg, or 

 even that of a fist. These plants become attached when 

 young to the oysters, and gradually increase in size. When 

 they have attained a certain size and are left uncovered by 

 the retreat of the tide, they split from the pressure of the 

 water they contain, and this, on trickling out, is replaced in 

 part by air. The rising tide imprisons this air, and the alga 

 swells, rises, and carries up the oyster to which it is attached. 

 The whole is now drifted away by currents, and in this 

 manner, no less than 400,000 oysters have been (according 

 to Fabre-Domergue) removed from oyster-beds. The 

 same author states that this evil is to some extent mitigated 

 by the drawing of faggots of thorn bush over the beds, thus 

 breaking the *' balloons." 



Effects of Sewage, etc. — The system by which the sewage 

 of towns and efiluents of works, etc., is got rid of by being 

 led on to, or allowed to escape within, the tidal area has in 

 many instances led to serious results by infecting valuable 

 beds of mussels, cockles, and oysters with disease-causing 

 bacteria. 



Bacteriological and epidemiological evidence from many 

 sources has produced irrefutable evidence of the fact that 

 shell-fish, owing to their peculiar methods of feeding, are 

 capable, if infected by sewage bacteria, such as B. typhosus, 

 of transmitting such bacilli to human beings who may 

 consume them. This has led to much discussion and 

 experiment, and to the condemning of shell-fish from 

 infected sources by many Medical Officers for Health, and 

 considerable interruption of the usual trade has resulted. 

 Naturally, methods are being sought to eliminate this danger 

 and to place these frequently valuable fisheries on a safe 

 and sound basis. The Ministry of Agriculture and Fisheries 

 have for some years past carried out experiments at Conway, 

 the object being to cleanse mussels of dangerous bacteria 

 associated with sewage pollution, so as to render them " pure 

 in a bacteriological sense." As regards cockles, these also 



