invertebratp: zoology. 73 



Type v.— Turbellaria (any fresh-water form.) 



39. Field-work. Select for investigation a small pool or ditch of water. 

 The water must be stagnant or slightly flowing, but pure and without foul 

 odors. It should contain an abundance of green water plants and perhaps a 

 little floating green slime as indications of the purity of the water. Use for 

 collecting a net of cheese-cloth and a glass jar. Place the jar in the net and 

 fill it with clear water by reaching out from the margin towards the center of 

 the pool. Let the jar down and pass the empty net slowly back and forth 

 through the water, taking care when turning the net not to reverse it. The 

 minute animals contained in the water will thus be caught in the net and may 

 be transferred to the jar by carefully reversing the net over the mouth of the 

 jar and washing it gently. Hold the jar up to the light and examine. Collect 

 material in the same manner from different depths and portions of the pool 

 and finally scoop up a few of the sticks and leaves from the bottom and add 

 a bit of scum and a piece of some aquatic plant. In this way the jar may be 

 made to contain a sample of the different environments the pool affords. To 

 study the gauna of each environment, the different collections must be kept 

 apart and labelled. In this i)a7-ticular case tlie material from the ]x>ttom will 

 be the most important. Exanune the material thus collected and look for 

 soft, very contractile, fiat worms, which vary from a short oval to a long 

 worm-like form. They vary in size from almost microscopic dimensions to 

 1.5™ and in color from slate grey to brown, yellow or green. In an undis- 

 turbed aquarium they frequently crawl in an iiiverted position along the 

 under side of the surface of the water, after the manner of snails. 



40, Find a Turbellarian in the material collected in 89. transfer it to 

 a watch crystal with a pipette or glass tube and study \\nth a dissecting mi- 

 croscope. Notice its change of form and its mode of motion. It moves by 

 muscular contraction aided by cilia, which cover its entire surface. These 

 can be well seen on an edge when the specimen is imder examination with the 

 high powers [41 e]. In one groiip {Dendrocoda) the intestine is dendritic, i. e. 

 branched like the limbs of a tree, and shows very consjiicuously even with the 

 single lens. In others it is a straight rod and not very noticeable {Rhabdo- 

 coela). Notice also several other organs, usually dendritic, on the sides of the 

 alimentary canal. These are the reproductive and yolk glands. The mouth 

 is on the under side, abcjiit central, and rather difficult to see. Two pigment 

 flecks at the anterior end serve as eyes. 



4 I . General amttomy. Several common species of fresh water Turbel- 

 laria (mostly Rhabdocoela) are so transparent that the internal organs may be 



