THE BRISTLE-WOKMS. 85 



impossible to maintain a proper sequence. A water-colour 

 sketch, the parts being as nearly as possible the colours of 

 life, is the form of sketch most likely to produce permanent 

 satisfaction, but where this is impossible a mapping-pen and 

 Indian ink should be used in preference to pencil; pencil 

 drawings on loose sheets being very apt to get blurred. 

 Annotate your drawings fully at the time that they are 

 made, and mark carefully those points about which you are 

 uncertain; in time light will probably dawn. In addition 

 to the careful drawings of the whole animal, a few entirely 

 diagrammatic sketches of the separate parts should be 

 made. 



As to the points disclosed by your examination, a Nereis 

 is a ringed worm (Annelid), composed of a series of rings or 

 segments, each of which is of similar structure. You may 

 compare it roughly to a railway train, composed of numerous 

 similar carriages linked together. Consider for a moment 

 the railway train as the more familiar object. Its form is 

 obviously an adaptation, as the biologist calls it, to its 

 particular form of movement. As it sweeps gracefully 

 round a curve, you see at once how necessary and suitable 

 its form is, how much the freedom of movement depends 

 upon the yielding linkage. Almost all animals which can 

 move rapidly and gracefully in water, and are of elongated 

 shape, are similarly composed of a series of units. In the 

 language of Biology they are segmented animals, and Nereis 

 and its allies illustrate one of the simplest forms of seg- 

 mentation. A simple form because the component units 

 are similar throughout the body, only the anterior and 

 posterior ends showing slight structural differences. With 

 Nereis should be compared, on the one hand, the Nematodes, 

 with their unsegmented bodies and peculiarly stiff method 

 of locomotion, and, on the other, the more differentiated, 

 segmented animals, such as crab and crayfish, where the 

 body-units are no longer all similar, but are adapted to serve 

 different functions. 



Let us now examine the segments in detail. Any of 

 those from the median part of the body, taken at random, 

 will show the following points : first, the characteristic 

 shape, rounded above and flattened beneath with a central 

 groove ; then the appendages, large lateral outgrowths, form- 



