i9cv 



British Field Zoology 



169 



Fig. 1. 



the bay of the net and tying the edge of the muslin round the neck of 

 the bottle (fig. 1 ). The water filters through the muslin, leaving a con- 

 centrated mass of creatures in the collecting- bottle. This is then 

 emptied into another bottle, when the former is again ready for use. 

 The drag, No. 5, is made by bending two pieces of stout iron wire as in 

 fig. 2, then tying them to- 

 gether at right angles to one . 



another, with a piece of lead ' 



wrapped round the shanks 



(fig. 3). The drag is very useful for drawing to the side 



water-weeds which are beyond reach of the stick. 



I trust the Editor will allow this preliminary digression, 

 since I know from experience what a help it is to be shown 

 how to set about collecting the small inhabitants of our 

 ponds and ditches. 



One word more before leaving the subject of collecting. 

 There is a prevalent idea that microscopic organisms abound 

 chiefly in dirty or putrid water. This is not the case. True, 

 certain forms of life abound in such places, but the most 

 productive ditches and ponds are those in which the water 

 is clear, with a plentiful growth of green weeds, algae, &c. The greatest 

 variety is to be met with in ponds which have a little stream flowing 

 through them. The reason is obvious. 



"Omni vivum ex ovo" — "Every living thing comes from an egg," 

 wrote Harvey. This axiom takes us back to 

 the beginning of every animal — when it ex- 

 isted merely as a tiny speck of organic matter. 

 The word "ovum," usually translated egg, is 

 used here in the restricted scientific sense. 

 As applied to the higher animals it means the 

 minute cell formed in the ovary of the female 

 which, after fertilisation, subdivides — becom- 

 ing first a mass of cells, and then, by a more 

 or less complicated series of changes, a separ- 

 ate individual like the parent. 



In our introductory chapter (F. N. Q., vol. 

 ii. p. 69) we stated that "the simplest forms of life will be studied first 

 — the one-celled animals — then the more complex." This is, indeed, 

 the only way in which the student can hope to attain to a clear idea 

 of that unity of plan which runs through the whole animal world. It 

 may seem a startling assertion to make, but it is nevertheless perfectly 

 true, that at the very first origin — the ovum — all animals are one-celled. 

 The Protozoa remain one-celled throughout life. The Metazoa — i.e., 

 all animals higher in the scale than protozoa — cease to be one-celled 

 as soon as the ovum begins to develop. In the higher forms their 

 cellular character is not discernible except in the early stages of 

 development. 



It would be beyond the province of this paper to pursue further the 



Fi<j. 



