SCIENCE-GOSSIP. 



151 



PRACTICAL HINTS ON MARINE ZOOLOGY. 



By Professor A. C. Haddon. 



(Continued from page 99.) 

 II. — TOW -NETTING. 



\T O naturalist who has a sea-side holiday should 

 omit to do a little tow-netting, as by this means 

 he comes across organisms which would otherwise 

 be unknown to him except through the medium of 

 books. The necessary paraphernalia are not very 

 extensive : a tow-net or two, wide-mouthed jars, a 

 few tubes and a pipette or narrow glass tube. 



A tow-net merely consists of a ring with a 

 conical bag attached to it. The ring is best made 

 of stout galvanised iron wire or brass ; cane will do, 

 but it is too light. It is best to avoid ordinary iron 

 wire on account of rust. The diameter should not 

 be less than about eighteen inches. To this, three 

 pieces of light cord are tied, say about two feet in 

 length. These, of course, must be of the same 

 length and placed equidistant on the ring, the three 

 ends tied together and lashed to the end of a 

 long cord. The net should be not less than from 

 three to four feet in length and should gently taper, 

 but the apex should not be too pointed. It is best 

 to sew a piece of window-blind cord in the hem at 

 the mouth. Cheese cloth is a suitable material for 

 making a net ; silk-bolting cloth is, however, the 

 best, but it is expensive. The end of the net may 

 be permanently closed or left open with tape sewn 

 on to tie it when in use ; a bottle may in this case 

 be tied to the end with its mouth opening into 

 the net. I think the bottle is somewhat of a pro- 

 tection for the more delicate organisms, but it is 

 apt to get broken. 



The method of collecting is extremely simple, 

 the net is dragged through the water trailing 

 behind a slowly-progressing row-boat. After a 

 short interval it is drawn in and the contents 

 emptied into a wide-mouthed jar full of sea-water, 

 either by everting the apex of the bag or by untying 

 the end and washing it well in the jar, or simply 

 by capsizing the contents of the bottle into the 

 jar. The net is then thrown overboard again and 

 the contents of the jar examined. 



A new world of transparent organism is revealed. 

 Delicate forms glide, pulsate or dart through the 

 water ; some are entirely colourless, others have 

 specks of often brilliant colour. With the excep- 

 tion of the large sea-blubbers, or jelly-fish, the 

 surface animals are quite small, from the size of a 

 hazel - nut downwards to microscopic forms. 

 After some practice it is possible to recognise 

 quite a number of very minute forms by the way 

 they swim through the water. 



It is a good plan, by means of the glass tube, to 

 isolate those forms which are recognised to be rare, 

 especially delicate, or rapacious ; the latter, such 

 as the larvae of the higher Crustacea, may do 

 serious damage in a very short time. The bulk of 

 the sorting is best done at home. Here also the 

 temptation to overcrowding the jars must be 

 avoided. Free-swimming animals can be cap- 

 tured by covering one end of a glass tube with 

 the fore-finger and inserting the other in the 

 water close to the form which it is desired to 

 catch ; on lifting up the finger the water rushes 

 up the tube carrying the animal with it, and on 

 the finger being again placed on the end of the 

 tube the water within can be transferred to 

 another vessel. 



Calm weather is the best for tow-netting, as 

 when the surface-water is rough the majority of 

 the pelagic animals sink to a more quiet level. 

 There is still a good opportunity for original 

 research into the habits of these creatures, and the 

 conditions of the air and water which determine 

 the level in the water at which they may then be 

 most abundant. A great deal of ingenuity has 

 been expended by various naturalists to devise an 

 apparatus by means of which free-swimming 

 organisms can be collected at any definite depth, 

 and from that depth only. One of the best of 

 these is that devised by Mr. W. E. Hoyle, of Owens 

 College, Manchester, but it is a very expensive 

 piece of apparatus, as it requires an electric battery 

 to work it. A rough and ready means for accom- 

 plishing this is to tie a separate piece of cord to 

 the ring of the net and to attach a weight at the 

 opposite point. The net can then be lowered and, 

 if of good length, the body of the net will close the 

 entrance ; when the desired depth has been attained 

 (which can be determined by having previously 

 marked the second cord) the net can be drawn 

 along by the ordinary cord. When it is desired to 

 empty it, the net must be drawn up by the second 

 cord and the mouth will be closed as in the descent. 

 Of course it must be remembered that when the 

 boat is in motion the net will rise above its first 

 level and that in proportion to its speed ; the boat 

 should be at rest when the net is lowered and 

 raised. The results obtained in this way are not 

 thoroughly trustworthy, but they will be found to 

 be interesting and suggestive, and should by no 

 means be neglected. 



