64 



YVES B. PLESSIS 



VUIIJIIIIIII/I 11111,11/11 IIIUIIll/IJI/JIIIIIIIDIIlll, 



Figure 8. — The "tidal" tank (A) into which is 

 immersed the bottomless tank {B) which rests 

 on the sand (-B). A plate of glass {F) covers 

 the upper part. The air enters and leaves at 

 the cover {¥) without having to pass the 

 small breeding animals. Since the saturated 

 atmosphere is not suitable for some animals, 

 it is sometimes necessary to replace the cover 

 (jP) with a fine screen, but in this case the 

 pebbles of the upper zone are quickly covered 

 with salt precipitations. C is an intermittent 

 capillary siphon. 



the air passes, without stopping, the edges 

 of the cover F which has been simply laid 

 upon \\\^ cylinder. In that way, the air in 

 the breeding area is constantly renewed. 

 It is advisable to provide a sufficient height 

 so that the upper level of the pebbles will 

 be relatively dry. It is also possible to 

 replace the glass plate F by a fine screen. 

 3. We have also made use of the "tidal" 



tank whenever a breeding situation re- 

 quired good irrigation while the topog- 

 raphy of the breeding tank did not aiford 

 it. Thus, when the accumulation of sub- 

 stances of all kinds in a tank is such that 

 the circulation is poor, it is veiy useful to 

 have recourse to this technique which 

 makes it possible to renew the water, in its 

 totality and everywhere, with every tide. 



The substratum 



Generally speaking, the breeding tank 

 does not require any special substratum, 

 and the glass or plastic wall of the tank 

 itself constitutes the breeding ground. 



If it is desired to provide rock animals 

 with a suitable substratum, an adequate 

 piece of rock is placed in the tank; gener- 

 ally, this presents no difficulties. Such, 

 however, is not always the case when it is 

 desired to introduce a sandy or muddy 

 bottom. 



An anaerobic milieu develops very ra- 

 pidly under the first millimeters of the 

 ground ; it immediately begins to dis- 

 charge hydrogen sulphide which may 

 cause the death of the animals and plants 

 to be bred. We have introduced a very 

 simple solution to this problem. 



Quantities of oxygen that were dis- 

 solved in the interstitial water of the 

 coastal sand have sliown us repeatedly that 

 the partial oxygen pressure is weak: ap- 

 proximately 30 to 40 percent of saturation. 

 In certain cases, the oxygen pressure is 

 practically nil. These facts can be ex- 

 plained easily : the oxygen contained in 

 the interstitial water of the sand has its 

 origin in the irrigation of that sediment. 

 When the sand is subject to the swing of 

 the tides, its level also is subject to varia- 

 tions that bring about a circulation of the 

 water. It is a matter of course that this 

 level, for a certain amplitude of the tides, 

 is subject to variations which increase as 

 the elements of that amplitude become 



