412 



STRATIGRAPHIC APPARATUS 



rather consistent stratigraphic numbers; so that we may say, as a result 

 of these two examinations, that probably the number of nemas in each 

 successive layer decreases, rather gradually at first, but rapidly below 

 the third 5 mm. layer; i.e., to a depth of about 15 mm. the sand is 

 thickly populated with nemas; below that level the number falls off 

 rapidly. The deeper layers contain fewer nemas, and fewer species. 



Fig. 1. See also Fig. 2. Apparatus for collecting layers of sand and mud on beaches, 

 mud flats, and at the bottom of ponds, lakes and seas. The unassembled apparatus is 

 shown at the left; its use in collecting is explained in the successive illustrations in Figs. 

 1 and 2, reading from left to right. All the operations except the last must be performed 

 at the time of collecting. The last operation may take place in the laboratory. The 

 rectangular collecting box, with two sides missing, is forced into the sand or mud. The 

 two missing sides (gate and loose bottom) are then inserted, as shown above. To insert 

 the loose bottom the adjacent sand is pawed away. 



Fig. 2. See also Fig. 1. After the sample of sand is removed from the bottom, it is 

 placed in a holder having three thin metal slides (1, 2, 3), that can be forced through the 

 sand parallel to each other, thus cutting it into four layers; in this case each layer is 

 5 mm. thick. After the slides are forced home, as shown at the right, the loose bottom of 

 the box is removed, disclosing the lowest layer of sand, 5 mm. thick. This is washed 

 into a beaker and treated as desired. The top of the box has fine perforations in order to 

 allow the supernatant water to escape without disturbing the top layer of sand or mud. 

 To prevent the possible escape of small organisms, this perforated top may be further 

 covered with the finest bolting silk, stretched on. 



Averaging the figures from the two collections mentioned above, 

 it will be seen that, if they are typical collections, the beach sand 

 examined carried about 3,742,000,000 nemas per hectare in the top 

 20 mm., or about 1,500,000,000 per acre. In 1916, but earlier in the 

 summer, an examination of the same beach showed at least 1,040,000,- 

 000 per acre in the topmost inch of sand. 



