i8 



Marine Invertebrates 



outlet through which the water is drawn from a storage tank is always 

 located several inches above the bottom. A small amount of seawater 

 may be made to last a long time if the investigator regularly filters and 

 aerates it. A very convenient type of filter (Fig. 19) consists of a water- 

 tight wooden box with two 

 pipes passing through its bot- 

 tom. The long pipe serves as 

 an overflow while filtered water 

 runs through a short pipe after 

 having passed through a layer 

 of charcoal (ch), sand (sd), 

 fine gravel (fg) and coarse 

 gravel (eg). From a reservoir 

 (not shown in the figure) the 

 water is delivered to the aqua- 

 ria. When in operation the 

 filter is placed below the aqua- 

 rium from which the water is 

 drawn. (See also p. 540-) 

 Sometimes experimental work requires the use of seawater entirely 

 devoid of any suspended matter, either inorganic or organic. It may be 

 obtained by filtering through collodion membranes (ultrafiltration), 

 using fine Berkefeld filters, or by passing through a thick layer of asbes- 

 tos. Water obtained by the latter method may not be free from bacteria 

 but contains no plankton or micro- 

 plankton. A typical arrangement is 

 shown in figure 20. From a labora- 

 tory faucet seawater runs slowly 

 into a large Buchner funnel, the 

 bottom of which is covered with a 

 layer of asbestos (A) about 1 inch 

 thick. A small watch glass is. 

 placed on its surface to provide a 

 more uniform distribution of water 

 over the entire area. The funnel is 

 inserted into a neck of a 2 liter 



Fig. 19. — -Filter for seawater. After 

 Sachs, eg, coarse gravel; ch, charcoal; 

 fg, fine gravel; sd, sand. 



To suction pump 



Fig. 20. — Filtering of water through as- 

 bestos. A, asbestos; T, glass tube lead- 

 ing to suction pump. 



vacuum flask connected to a 5 gal- 

 lon bottle. A glass tube (T) in- 

 serted in a stopper of the bottle 

 leads to a suction pump. If neces- 

 sary, two 5 gallon bottles may be connected in a series. Filtered sea- 

 water fills up the flask and gradually is sucked into the bottle. After a 

 few days of operation a compact organic film forms on the surface of the 



