LARVAL OR SWIMMING STAGES 33 



equally spaced pieces of cod-line, the other ends of which were brought 

 together and secured to the towing-line, to which was suspended a short 

 iron sinker. This apparatus was thrown overboard and dragged behind 

 a row-boat, sail-boat, motor-boat or steam-boat under reduced speed, 

 the depth of the net being regulated by the speed of the boat, the length 

 of the tow-line, and the weight of the sinker. 



Water filters through the net, but organisms are kept back, collect 

 on the inside, and tend to be carried into the bottle. This arrangement is 

 rather better than a closed net, where strong filtering currents passing 

 through the point of aggregation of the organisms are liable to damage 

 them. On the other hand the bottle fills with more or less stagnant water, 

 tending to block their entrance to it, but permitting many to fall by their 

 greater weight. Care is necessary, in hauling the net up, to not lose those 

 clinging to its inside. By draining and dipping several times they may 

 be washed down into the bottle, which may then be removed from the net, 

 corked and stood in a pail of cool sea-water in the shade. The net may 

 be cleaned by throwing it out again, open, and a fresh bottle used for a 

 different locality or a different depth on the same excursion. 



Bivalve Larvae of Plankton Collections. — In such a manner may be 

 procured a wealth of plankton material that, when fresh, may at first 

 sight present much of the appearance of pea-soup, but when the bottle 

 is held up to the light it can be seen that each tiny speck is a living, free- 

 swimming or free-moving organism. Being confined in countless num- 

 bers in a small quantity of water, they soon begin to die and drop to the 

 bottom; at the same time the mass becomes more pink in colour from a 

 change in the pigment of its numerous little Crustacea (copepods), similar 

 to that which occurs when fresh lobsters are thrown into a vat of hot 

 water. 



The older stages of bivalve-larvae are compact, heavy objects, that 

 as soon as they are disturbed cease swimming and sink to the bottom, 

 where, protected by their shells and on account of their hardihood, 

 they will keep alive for hours. They can be seen as a darker, 

 granular, more sandlike layer underneath the great fluffy mass of 

 lighter-coloured, pinkish copepods. They may be drawn off by in- 

 serting one end of a glass tube, the other end being closed by the 

 finger, to the bottom of the bottle, when, upon relaxing the finger 

 for an instant, the water will rise rapidly in the tube, carrying some 

 of the bivalve larvse in the current. If the finger is again pressed 

 upon the top and the tube quickly withdrawn and allowed to drain 

 into a deep watch-glass, what lighter objects happen to have been 

 carried over may be removed from the surface by a pipette, leav- 

 ing the bivalve-larvae in thousands almost entirely free from admixture 

 with other animals, and among them, if collected at the right time and 

 place, will occur oyster larvae. 



